A Review of Applications and Experimental Improvements ... - CiteSeerX

Applied Spectroscopy Reviews, 41: 259–303, 2006 Copyright # Taylor & Francis Group, LLC ISSN 0570-4928 print/1520-569X online DOI: 10.1080/05704920600620378

A Review of Applications and Experimental Improvements Related to Diode Laser Atomic Spectroscopy Ga´bor Galbaćs Department of Inorganic and Analytical Chemistry, University of Szeged, Hungary

Abstract: This article attempts to review the major advancements made in the past 12 years, since 1993, in the field of diode laser atomic spectroscopy. The discussion covers experimental improvements (e.g., wavelength stabilization, frequency upconversion, enhancement of tuning characteristics, spectral bandwidth using external cavities, etc.), diagnostic applications in various atomizers, as well as analytical applications (e.g., absorption, fluorescence, and ionization spectroscopy; element-selective detectors for chromatography; etc.). With potential new users of these methods in mind, a detailed overview of the properties relevant to atomic spectroscopy of commercial diode lasers is also given. Keywords: Diode laser, atomic spectroscopy, stabilization, tuning, applications, diagnostics, elemental and isotope analysis

INTRODUCTION AND SCOPE The operation of diode (or semiconductor) lasers was first demonstrated by researchers at General Electric (GE) and International Business Machines (IBM) corporations in 1962 (1, 2). These devices were pulsed GaAs lasers operating at cryogenic temperatures and huge current densities, and thus were not very application friendly. Continuous-wave diode laser operation could only be established at room temperature almost a decade later (3), at Bell Laboratories and in the Soviet Union in 1970. The rate of development Received 18 January 2006, Accepted 22 January 2006 Address correspondence to Ga´bor Galbaćs, Department of Inorganic and Analytical Chemistry, University of Szeged, P.O.B. 440, 6701 Szeged, Hungary. E-mail: galbx@ chem.u-szeged.hu 259

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has accelerated since then, and today diode lasers represent a large family of devices that are sold worldwide in greater numbers in total than any other laser types put together. Along with more convenient operating conditions came a steadily growing range of technical and scientific uses; the range by now has become almost incomprehensively broad, including communication and information technology, material processing, medical procedures, spectroscopy, metrology, and many more.

Overview of Former Reviews on Diode Laser Applications Naturally, several review papers were dedicated to one or another diode laser application area over the years (4 –19). Even more publications—papers, books, and book chapters—dealt with certain branches of laser spectroscopic methods and hence covered diode laser spectroscopy partially (e.g., refs. 20– 27). It is not the intention of the present introduction to account for all publications falling into the second category. Nevertheless, the few reviews dedicated to diode laser applications, either spectroscopic or not, will be attempted to be covered below. Gas phase analytical and monitoring type of applications were discussed by several reviews (4 – 8), among them the first ones ever appear on diode laser applications (4, 5). These applications typically utilize tunable diode laser absorption spectroscopy (TDLAS) at infrared wavelengths for the detection and measurement of various molecules. Not uncommonly, TDLAS is used in combination with some high-frequency modulation and wavelength stabilization schemes, to improve sensitivity, signal-to-noise ratios, and stability. It is also well worth mentioning that there is one international conference series specially dedicated to the molecular spectroscopic applications of diode lasers, and this is the International Conference on Tunable Diode Laser Spectroscopy (the fifth meeting was held in Florence, Italy, in 2005). Collections of the papers presented at this conference are traditionally published in special issues of Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy. The four issues (28 –31) that appeared so far present a valuable source of information to anyone interested in TDLAS. In most medical applications, as Amin projected in a review (9) in 1995, high-power diode lasers should soon replace bulkier Nd:YAG lasers. This was thought due to that experimental and clinical results indicate that the use of 805-nm emission wavelength from diode lasers is preferable to the 1064-nm Nd laser wavelength. Light absorption by the tissue is stronger at the former wavelength and hence necrosis (tissue damage) is greater. In most medical applications, where laser emission is delivered to the tissue via fiber-optic cables, the fact that diode lasers are compact, reliable, and free of maintenance also brings additional benefit. Imasaka wrote at least two reviews on the spectroscopic applications of diode lasers. The first one was devoted to analytical molecular spectroscopy

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with diode lasers (10). The second appeared in 1999 and was a wide scope review on recent diode laser analytical applications (11). In this latter paper, the molecular spectroscopy of compounds of organic and biochemical interest were again the focus, and besides the general characteristics of diode lasers, selected applications from the fields of absorption and fluorescence spectroscopy and the use of diode laser based detectors in separation science were discussed as well. A relatively new area of applications is the surface hardening of metal and alloy products, which was recently rewieved by Kennedy et al (12). The basis of the process is that the thermal treatment of the metal surface results in a transformation, e.g., from austenite to martensite in case of ferrous alloys. This heating can also be done locally using spatially and temporally controlled laser radiation from high-power diode lasers. Diode lasers of optimal emission wavelength can be used so the process will be highly efficient. Due to their very narrow emission linewidth and exceptional stability, diode laser sources can also be fruitfully applied in atomic spectroscopy, which is of direct interest to the present review. The first reviews, by Camparo (13) in 1985 and by Wieman and Hollberg (14) in 1991, could discuss only atomic physics applications. The first analytical results of diode laser atomic spectroscopy were presented in 1988, by Prof. Winefordner and his group (32) at the University of Florida (Gainesville, FL). In 1993, Spectrochimica Acta Reviews devoted a full special issue to diode laser spectroscopy, giving a thorough discussion of a variety of subtopics including the spectroscopic characteristics of laser diodes (15) as well as applications in atomic physics (16) and elemental analysis (17). Among the substantial contributors to that special issue was Professor Niemax and his co-workers. This research group, at the Institute for Spectrochemistry and Applied Spectroscopy (Dortmund, Germany), has done pioneering work on many branches of diode laser atomic spectroscopy, but especially in the area of analytical atomic absorption spectroscopy. Since the end of 1980s, the Niemax group has been and continues to be one of the most productive ones in the field, which is not only reflected by the fact that all three later reviews on diode laser atomic absorption spectroscopy (DLAAS) (18 –20) were written by this group, but also that the most recent one, containing over 50 references, could almost exclusively be based on their own publications.

Definition of Scope As has been outlined above, all recent diode laser spectroscopy reviews, also including the few on diode laser atomic spectroscopy, focused on one or another class of applications or aspects only. The latest publication that encompassed most features and applications was that landmark 1993 issue of Spectrochimica Acta Reviews mentioned earlier. Therefore, it seems

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practical to bind the starting year of coverage for the present review to 1993. It is timely to account for the advancements made in the field in the past 12 years, without limiting the scope to only analytical or absorption spectroscopy. This review will attempt to overview the publications in the area of analytical and diagnostical diode laser atomic spectroscopy as well as related experimental improvements. Only publications developed and tested in direct relation to (potential) use in atomic spectroscopy will be covered. Theoretical and experimental methods that belong mainly to the interest of fundamental atomic physics research, such as cooling and trapping of atoms, absolute frequency measurement of atomic transitions, experiments with atomic beams or other exotic (non-routine) atom sources, etc., also fall outside the scope. A detailed overview of the properties relevant to atomic spectroscopy of commercial diode lasers will be included in consideration of potential new users of these devices. In addition, the point of view of the presentation will be more of an analytical chemist than of an atomic physicist, which is to say that the review is meant to be application oriented. The ultimate goal of the author is to give the reader an updated, multi-aspect overview of the recent achievements and potential of practical diode laser atomic spectroscopy.

OVERVIEW OF THE PROPERTIES OF DIODE LASERS The 1980s and 1990s saw a real explosion of diode lasers; at the time of this writing, there is a wide variety of commercially available diode lasers, whose optical and electrical characteristics differ greatly. The construction and characteristics of the various diode laser types is discussed in detail by optoelectronic and laser books (e.g., refs. 33 – 36). Here it is only beneficial to briefly overview the types and characteristics that are potentially the most useful in atomic spectroscopic applications.

Structure and Principle of Operation Diode lasers are semiconductor devices that have the electrical characteristics of a diode. The passage of electric current through the diode causes a recombination of current carriers (electrons and holes) at the junction between regions of different dopings. This recombination releases energy in the form of light in some specially formulated semiconductors. Si and Ge seem to lack the energy level structure required, so diode lasers are usually made from binary, ternary, or even quaternary composition of elements (see the following subsection). Diode lasers are similar in structure to light emitting diodes (LEDs), and at low current levels they actually function as LEDs as then produce incoherent spontaneous emission. The two key elements of the internal workings that


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make a diode laser different from an LED and allow laser action are the presence of optical feedback and the conditions set for population inversion. Optical feedback comes from reflective structures that form the laser resonator (cavity). In the simplest Fabry-Perot design, the facets of the semiconductor crystal are cut in a way that provides enough reflectivity of them to reflect light back into the light emitting layer. More sophisticated designs, such as distributed feedback (DFB) or distributed Bragg reflection (DBR) diode lasers, include a grating right on the semiconductor chip adjacent to the active (light emitting) layer. The optical feedback stimulates more light emission in the semiconductor, and if the drive current is high enough (reaches the so-called threshold current), population inversion in the active layer occurs, which results in laser oscillation and the emission of highintensity, coherent, highly monochromatic radiation starts. Above the threshold current, the optical power from a diode laser rapidly increases with current, following a more or less linear relationship. The operating voltage is usually as small as 2 to 2.5 volts only. The conversion of electrical energy into optical power is very efficient by laser standards: it is typically between 10 and 50%. A mass-produced solitary laser diode typically has a double heterojunction with stripe geometry. Laser action can be limited to a narrow stripe within the semiconductor junction in two fundamentally different ways. One approach, called gain guiding, restricts the current to a small portion of the junction by applying high-resistivity materials on the surface of the chip, causing population inversion to only be produced in the narrow stripe contacted by the current flow. The other approach, which is called index guiding, confines the light already generated in the active layer by applying refractive index changes next to the stripe. Gain guided lasers are simpler to make and can be used to generate high optical powers, but their beam quality and wavelength stability is relatively poor. Index-guided lasers have superior output quality, but generally can only be produced for lower output power (not exceeding ca. 200 mW), as the inherently tight focusing of the output can degrade the semiconductor surface. The active layer itself typically has a quantum well structure, where the junction is extremely thin (20 nm or less) and is sandwiched between two semiconductor layers with larger bandgaps. This structure reduces the threshold current, thus allowing even higher optical power and higher output modulation speeds. The cavity, provided by the semiconductor chip itself, can be designed according to any of the concepts outlined above; that is Fabry-Perot, DBR, or DFB. The latter two cavity designs produce the highest level of spectral purity (narrowest spectral bandwidth) of the light output. Solitary diode lasers can also be arranged as arrays (bars) on the same semiconductor chip for the purpose of applications requiring more optical power than what can be obtained from single diodes. One of the special types of diode lasers promising for spectroscopic use is the vertical cavity surface-emitting laser (VCSEL). Such a laser has reflective

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surfaces above and below the active layer, typically of quantum well structure, thus forming a very short laser cavity perpendicular to the active layer plane. The reflective surface can be a metallic one, in which case it also provides the electric contact (electrode). In addition to this, a distributed Bragg reflector can also be incorporated in the form of multiple semiconductor layers of alternating composition. This latter construction offers several interesting features for potential use in spectroscopy. For example, such lasers operate with very high wavelength stability (single longitudinal mode) and very narrow spectral bandwidth and can have exceptionally low-threshold currents (1 mA or below). Another possibility is the use of large VCSEL diode laser arrays, in which each individual laser has a slightly different emission wavelength. Using a suitable electrode structure, this can provide a reliable means of wavelength tuning. As the dimensions of individual diode lasers are less than a millimeter, they are packaged in electrical housings equipped with an optical window, which largely simplifies their handling and beam control. General purpose housings come in standard sizes (e.g., TO-18 with a diameter of 9 or 5.6 mm), but some special housings are also available, e.g., from which the light can be directly coupled into optical fibers (F-house, fiber pigtail, etc.). Most laser diode housings incorporate an internal photodiode as well, which can be used for monitoring the power of the optical output.

Emission Wavelength and Spectral Bandwidth The emission wavelength of a diode laser directly depends on the bandgap of the active layer, which in turn depends on the composition of the semiconductor material it is made of. Unfortunately, the composition of the active layer cannot be arbitrarily chosen to produce just any wavelength, but it needs to comply with certain parameters of the substrate on which it is grown. Here, the key parameters to be matched are the bandgap, lattice constant, (interatomic distances) and refractive index. In practice, almost all diode lasers are constructed on substrates of binary compounds (e.g., GaAs), and other elements must be substituted for one or both of the elements in the binary compound to produce the small variations in the refractive index and bandgap needed. Practical difficulties are well illustrated by the fact that rules of successful diode laser fabrication usually dictate that there is no more than 0.1% mismatch between lattice constants of any adjacent layers (with the only exception of the strain layer superlattice lasers, still in the developmental phase). The result of the above constraints is that the emission wavelength of commercially available diode lasers is limited to discontinuous and overlapping sections of the spectrum. Table 1 lists the composition and nominal emission wavelength of mass-produced laser diodes for potential use in atomic spectroscopy at the time of this writing. As can be seen, diode lasers


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Table 1. Composition and nominal wavelength of commercial diode lasers available for potential use in atomic spectroscopy Composition of the active layer InGaN AlGaInP GaInP GaAlAs GaAs InGaAs InGaAsP

Nominal wavelength, nm 390–420 630–680 670 620–895 904 980 1100 –1650

emitting in the UV range, the spectral range of primary interest to atomic spectroscopy, are still not available. Diodes with the shortest available wavelength (blue diode lasers, also referred to as GaN lasers) are the latest member of the family; they were developed by Nichia Chemical Industries (Tokushima, Japan) in 1999. The spectroscopic potentials and characteristics of blue diode lasers were outlined in several recent papers (37 –39). It is to be noted that Table 1, similarly to most manufacturer data sheets accompanying lasers, characterizes the composition only by giving the elements incorporated, but smaller emission wavelength variations are actually achieved by changing the ratio of atoms in the compound (e.g., an In0.73Ga0.27As0.58P0.52 laser emits light at a nominal 1310 nm, whereas a In0.58Ga0.42As0.9P0.1 laser emits at 1550 nm). Due to similar reasons associated with imperfections in the manufacturing process, the emission wavelength of individual laser diodes of the same nominal composition can be expected to fall within ca. +5 –20 nm tolerance range around the nominal wavelength. The currently available emission wavelengths allow the excitation of some species of practically all elements of the periodic table in an atomizer of high enough temperature, although typically not at their resonance lines. For example, illustrative short lists of the accessible elements and strong transitions for diode laser atomic absorption spectroscopy can be found in at least two papers (17, 20). The spectral purity of the optical output from a free-running diode laser is a function of several factors, mostly the internal construction and the operating current. Most commercial diode lasers emit light in a spatially tightly limited pattern (single transverse mode), but at low operating currents their emission may occur at many closely spaced emission wavelengths (multiple longitudinal modes) governed by the dimensions and the gain characteristics of the active layer. The spectral bandwidth of the output then may be as much as several nanometers. As the operation current increases, one of the longitudinal modes becomes dominant, and the bandwidth decreases dramatically. Use as

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an excitation source in atomic spectroscopy in most cases requires emission with high spectral purity: therefore, single longitudinal mode diode lasers operated at currents well above threshold are needed. Diode lasers that oscillate instantaneously in a single longitudinal mode and with a very narrow spectral bandwidth, e.g., DBR of DFB types, are also available from some manufacturers. Typical bandwidths for good quality, free-running single-mode laser diodes are on the order of picometers (or 10– 30 MHz), which is one or two orders of magnitude less than the linewidth of atomic transitions in conventional high-temperature atom sources (e.g., flames, furnaces, plasmas), and hence are usually adequately narrow for even high-resolution (e.g., isotope-selective) atomic spectroscopy. The center emission wavelength of a single longitudinal mode diode laser increases if the laser (housing) is heated, due to the change of the index of refraction and the cavity length induced by thermal expansion. For red laser diodes, the rate of this detuning is about 0.25 nm . K21. Raising the drive current increases current density in the junction and results in the same effect: the emission wavelength also increases. Considering that atomic spectroscopic applications usually require the emission wavelength be finely tuned and stabilized to an atomic transition, it is easy to see why it is customary to use only diode lasers with active temperature and current stabilization. Nevertheless, fine-tuning of the emission wavelength is possible by changing either the temperature or current of the diode in a total range of a few nanometers. The tuning in reality is complicated by the fact that changes in current or temperature can cause a single mode laser to jump to another longitudinal mode (mode hop), thereby changing its emission wavelength abruptly. This results in the discontinuous tuning curve of laser diodes, depicted in Figure 1. Instable (multimode) operation and excess noise is typical at mode-hopping wavelengths, thus making certain wavelengths practically unavailable for a given diode laser even if it lies within the total tuning range. Another practical condition to be considered with relation to tuning with temperature is that the lifetime of a diode decreases dramatically if the diode is operated above room temperature, due to the elevated level of diffusion processes in the semiconductor. On the other hand, the cooling of laser diodes is safe to even cryogenic temperatures; however, in this case, condensation on the optical window needs to be tackled.

Optical Power and Related Matters The highest optical power for solitary laser diodes designed for reliable, continuous operation rarely exceeds 150 – 200 mW. The radiance is already several times higher than that obtainable from any hollow cathode lamps and is more than adequate for most atomic spectroscopic applications, as will be seen below. For brute force applications, where spectral quality is of less concern, diode laser arrays or multiple arrays that can produce


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Figure 1. Typical diode laser current tuning curve (recorded by using an indexguided, quantum well AlGaInP diode laser stabilized at 258C).

hundreds of watts or more optical power are available. If needed, the momentary optical power of a diode laser can always be monitored with the internal photodiode, which is incorporated in most housings. The output from diode lasers has generally very low noise (60 to 80 dB), but it is highly susceptible to back-reflections caused by external sources. Diode lasers are fast electronic devices: optical rise times are typically less than 1 ns, including the time needed for the output fluctuations to settle after a diode is turned on. Hence, digital (on/off) power modulation, or pulsing, at frequencies of GHz or above is usually possible, thus making diode lasers ideal light sources for spectroscopic applications where electronic chopping is needed. Analog modulation with similar top frequencies is also possible by modulating the current to the diode with the use of an arbitrary function generator. However, it needs to be kept in mind that in that case, not only the optical power, but also the emission wavelength, is modulated, as described in the preceding subsection. Consequently, frequency/wavelength modulation spectroscopic methods, which use the current modulation scheme for sweeping across a certain wavelength range, should also in principle correct for the inherently parallel modulation of optical power. Strong electronic transitions can easily become optically saturated by the high-intensity, narrow bandpass radiation from a laser source, especially from a continuous wave (cw) source. This also stands for diode lasers in spite of their seemingly low optical power (17). The attenuation of light intensity can be efficiently achieved by using either a neutral density filter or a polarizer, preferably tilted in either case in order to minimize back-reflection

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of the output light. As will be seen later, as low as 30 – 50 nW optical power is sufficient for the successful application of certain diode laser –based atomic spectroscopic methods. For some spectroscopic methods, the high spectral energy density and continuous operation of diode laser light sources is directly advantageous, such as in diode laser – induced fluorescence spectroscopy (DLIF). The reason is not only that fluorescence signals are proportional, up to a limit, with the intensity of the exciting radiation, but also that the long interaction time of cw laser radiation with atomic species in conventional reservoirs in some favorable cases may even allow the frequent reexcitation of the analyte, thereby efficiently multiplicating the DLIF signal (17, 26).

Beam Characteristics Directionality of the emitted radiation from a diode laser is generally poor, due to the miniature length of the laser cavity. The cross section of the emitted beam is oval and the beam rapidly diverges: divergence can be characterized by ca. 408 in the plane perpendicular, and ca. 108 in the plane parallel to the active layer. Most spectroscopic applications therefore require the use of special collimating optics either internally (micro lenses inside the housing) or externally to improve the beam quality. The collimated beam is still elliptical, but can be circularized by using, e.g., an anamorphic prism pair. The beam can also be circularized and additionally reformed to have a highly Gaussian spatial intensity profile, if needed, by focusing the output beam into a so-called single-mode optical fiber (40, 41). The elliptical beam of diode lasers also exhibit astigmatism; the focal points along the two axes do not coincide, thus making the focusing of the beam to a tight spot difficult. Due to their construction, gain-guided diode lasers have more astigmatism compared to index-guided ones; therefore, they are less suitable for applications needing tight focusing. However, their beam can be corrected to some extent by using a cylindrical lens (40). Polarization is rarely specified by diode laser manufacturers, but the output usually becomes more linearly polarized as the power increases, which also implies a slight dependence on the wavelength. The dominant polarization is parallel to the junction plane. Polarization ratios can exceed 1000 to 1 (16, 34, 40). Coherence lengths of single-mode diode lasers are on the order of several meters. This beam characteristic is rarely an important one in atomic spectroscopy, but may cause undesirable effects. If external objects in the optical setup that randomly reflect the beam back into the diode are less then the coherence length away, the interference can stimulate the emission of more light, thus generating noise (intensity fluctuations) (34).


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Operating Conditions and Potentials in Atomic Spectroscopic Applications The fast response and very low operating voltages render cw laser diodes very susceptible to damage from current and voltage surges, including electrostatic discharges. Additionally, all atomic spectroscopic applications require precise and realizable control of the emission wavelength; that is, the control of the current and temperature of the diode. For these and other reasons, diode lasers need be driven by sophisticated electronic controller circuitry. Using such controllers and by always keeping the operating conditions between the specified limits, laser diodes may reach lifetimes in excess of 50,000 h. Similarly to the case with other lasers, the measurement of the light intensity from a diode laser in any atomic spectroscopy setup sensibly calls for the use of analog detectors with a wide dynamic range, such as photodiodes, photomultiplier tubes, etc. Consequently, charge transfer – based detector devices (e.g., CCDs) generally have limited use only. Emission wavelength measurements in most cases need high-resolution monochromators, with a minimum of picometer resolution, or ideally an interferometric wavemeter. The small electric power consumption and compactness of laser diodes can rarely be actually exploited in atomic spectroscopic experiments (e.g., for creating portable analytical instrumentation), which generally need the use of high-temperature, relatively large atom reservoirs anyway. However, the collimated output beam from several compact laser diodes may be combined in multi-wavelength (multi-elemental) experiments. For analytical atomic spectroscopy, the tunability of the emission wavelength of laser diodes translates into multiple advantages, including the ability of sensitivity (dynamic range) adjustment by tuning to the wings of analyte line profiles, the possibility to perform effective two-point background correction, the possible access of electronic transitions of several analyte species by using the same diode, etc. In atomic absorption spectroscopy, the good directionality and high intensity of collimated diode laser beams allows the elimination of the monochromator from the optical setup to discriminate the useful signal from scattered light or background radiation from the atom reservoir. In diagnostic applications, the small diameter of a collimated diode laser output beam can also help to simplify the spatial filtration of the signal.

Availability of Diode Lasers and Related Instrumentation Laser diodes are mass produced for the industry (for use in telecommunication, medical, scanner, alignment, optical storage systems, etc.) by large electronic companies like Sony, Sanyo, Samsung, etc. Such diodes, which can be purchased from various distributors, are low cost and reliable but are usually manufactured with industrial and not spectroscopic applications in mind.

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For example, wavelengths that are not sought by the industry (e.g., 690 – 760 nm) are hard to find. Many industrial applications do not require single longitudinal mode operation, hence cheap single-mode diodes tend to be single transversal mode ones, at best (industrial manufacturers commonly characterize their diodes simply as single-mode diodes, without actually specifying whether they are single transversal or longitudinal mode). Therefore, careful selection and testing are advised before their use in atomic spectroscopic applications. On the other hand, there are some optronic companies that carry/ manufacture research (spectroscopy)-grade laser diodes, even at custom wavelengths, but for a considerably greater price. Reliable and flexible diode laser controllers are also only offered by a handful of companies. Table 2 gives an illustrative list of diode laser and related optics/instrumentation suppliers with their web addresses.

EXPERIMENTAL IMPROVEMENTS ON DIODE LASER LIGHT SOURCES FOR ATOMIC SPECTROSCOPY Within the covered period of time, research efforts related to the improvement of the characteristics and operation of diode lasers used as excitation light sources in atomic spectroscopy were focusing on a few topics only. Experimental setups providing an extended mode-hop free-tuning range of the emission wavelength (mainly based on an external cavity arrangement) and emission frequency upconversion (mainly based on second harmonic generation) both expand the number of accessible electronic transitions. Spectral bandwidth reduction facilitates high-resolution spectroscopy, whereas wavelength stabilization or locking to a specific transition may equally be important to most atomic spectroscopy applications. The research thrust behind the development of flexible and reliable laser diode driving electronics (e.g., current and temperature controllers), which was also a popular research field earlier, has waned in the past decade, due to the fact that top quality commercial controllers became available in the mid 1990s and onwards.

Tuning of Emission Wavelength and Reduction of Spectral Bandwidth Varying the temperature or current of a diode laser is a simple, but not very efficient, method of tuning. It may provide only a limited mode-hop freetuning range and does not fully exploit the capabilities of the semiconductor material (gain bandwidth). Solitary laser diodes also often exhibit large spectral bandwidth due to the short photon cavity lifetime. A solution toward the improvement of these characteristics is the application of a socalled external cavity diode laser (ECDL) arrangement. This expression


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Table 2. Some companies with their web addresses that provide diode lasers and related optics/instrumentation Company name

Related products

Blue sky research Covega

Diodes Diodes, controller Diodes, optics, controller Diodes Diodes, optics Diodes Controller Diodes Diodes, optics Diodes

http://www.blueskyresearch.com http://www.covega.com

Diodes, optics

http://www.limo-microoptic.de

Diodes, optics, controller Diodes, optics, controller Diodes, optics Diodes Diodes, optics Diodes Diodes, optics Diodes, optics Diodes, controller Diodes Diodes, optics

http://www.newfocus.com

Edmund industrial optics Elliot scientific Frankfurt laser Hamamatsu ILX lightwave Laser diode LaserMate group LG laser technologies Lissotschenko microoptics New focus Newport NVG OpNext Optima precision OSRAM Photonics Power technology Qphotonics ROHM electronics Roithner lasertechnik RPMC lasers Sacher lasertechnik Samsung Sanyo Sharp Sony Thales Thorlabs Toptica photonics Toshiba electronics Truelight World star tech

Diodes Diodes, controller Diodes Diodes Diodes Diodes Diodes Diodes, optics, controller Diodes Diodes Diodes Diodes

Web site

http://www.edmundoptics.com http://www.elliotscientific.com http://www.frlaserco.com/ http://sales.hamamatsu.com http://www.ilxlightwave.com/ http://www.laserdiode.com/ http://www.lasermate.com/diodes.htm http://www.lg-lasertechnologies.com

http://www.newport.com http://www.nvginc.com/ http://www.opnext.com/ http://optima-prec.com/ http://www.osram-os.com/ http://www.photonic-products.com http://www.powertechnology.com http://www.qphotonics.com http://www.rohm.com http://www.roithner-laser.com/ http://www.rpmclasers.com http://www.sacher-laser.com http://www.sem.samsung.com http://www.sanyo.com/ http://www.sharpsma.com http://products.sel.sony.com/semi/ http://www.laser-diodes.thomson-csf.com http://www.thorlabs.com/ http://www.toptica.com http://www.toshiba.com/taec/ http://www.truelight.com.tw/ http://www.worldstartech.com

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refers to an experimental configuration in which a feedback path for the output light is created by using some external optical system that selects a specific wavelength and reflects only this component back into the laser diode. Several configurations (mountings)—e.g., Littrow, Littman-Metcalf—are known, and various electro-optical elements can be applied as wavelength selectors (e.g., grating, filter, etc.); therefore, many varieties are possible. A detailed classification and theoretical treatment of the performance of such systems has been recently given by Zorabedian (42). A highly practical and useful guide on the locking of Fabry-Perot diode lasers to high-finesse cavities has recently been published by Fox et al. (43). In the past few years, a few optical companies (e.g., Toptica, Sacher, New Focus, etc.) also started to manufacture ECDL systems, usually marketed under “tunable laser modules” or similar names. Below, only research arrangements developed and tested in relation with (possible) use in atomic spectroscopy will be discussed. In Littrow-type configurations, which is the most commonly used arrangement, a diffraction grating is used that combines the functions of the wavelength selector and the external mirror. Tuning is provided by rotating the grating, maintaining equal angles of incidence and diffraction. The ECDL system by Hof et al. was set up for 670-nm wavelength using a diffraction grating and produced 0.083-nm (or 56 GHz) mode-hop free-tuning range and 0.037 pm (25 MHz) linewidth (44). Spectroscopic capabilities of the system were demonstrated by performing laser-induced fluorescence spectroscopy of lithium atoms. Georginov et al. experimentally and theoretically investigated the performance of four Toshiba diode lasers emitting at around 670 nm in a Littrow-type ECDL system (45). The best continuous tuning range they achieved was 0.134 nm (90 GHz). Lancaster et al. used a laser diode with an integrated micro lens to build an ECDL that provided a high-quality circular output beam with a continuous tuning range of about 0.01 nm (5 GHz) (46). Andreeva et al. created their ECDL system for the purpose of using it in high-resolution Rb fluorescence spectroscopy (47). Wide continuous tuning range was achieved with the precision mechanical movement of the grating complemented with laser diode current adjustment. A practical nuisance with ECDLs using the Littrow configuration is that the direction of the output beam changes with the turning of the grating; that is, tuning the emission wavelength. Hawthorn et al. (48) have solved this problem by fixing a plane mirror parallel to the grating in the path of the output beam, which was hence reflected twice, thus leaving the cavity in a direction that remained unaltered during tuning. This arrangement was found to provide a steady output direction over a tuning range greater than 10 nm. In one of their publications, Ricci et al. (49) described a compact and economic ECDL system using easily machineable mechanical parts. Linewidths on the order of few hundred KHz, continuous wavelength tuning over 25 GHz, and modulation in the GHz frequency range were reported. Another ECDL system, built by Bathia et al. (50), worked at 895 nm. The tuning range


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was 0.053 nm (20 GHz) and the single-mode emission linewidth was 0.0048 pm (1.8 MHz). This linewidth was shown to be sufficiently narrow for the resolution of hyperfine transitions of Cs. The implementation of a Littrow ECDL system by Laurila et al. was different from others’ in that they used a transmission diffraction grating, made with electron-beam lithography. In this setup a stable, single longitudinal mode operation at 650 nm and a continuous tunability greater than 0.028 nm (20 GHz) were achieved (51). A Littman ECDL configuration, with a grating and a reflector, was developed by Park et al. (52). This configuration was so compact that the whole system could be installed on a standard kinematic mount. Four such systems, working at 852, 894, 780, and 794 nm, were fabricated and tested for use in Cs and Rb spectroscopy. Precise enough movement of the grating was obtained by the use of piezoelectric transducers. A piezo-driven Littman-Metcalf ECDL was also developed and characterized by Mahnke et al. (53) By applying ca. 50-V pulse to the piezo element, a detuning by 13.6 GHz was obtained. If the grating is turned around a carefully selected pivot-point position, the continuous tuning range of the ECDL can be maximized. This concept was investigated and demonstrated by Trutna et al. (54) using a diode laser with a prism beam expander. Maximum tuning range values between 3.4 and 16.9 nm (600 –3000 GHz) for the 1300-nm emission wavelength were obtained. A novel ECDL system was presented by Lan et al. (55). Here, a planarly aligned nematic liquid crystal (NLC) cell was inserted between the grating and the end mirror of the ECDL system. By varying the drive voltage to the NLC cell, the index of refraction of the crystal changes, which was causing a change of the effective cavity length, and also the output wavelength. With the applied 35.5-mm-thick crystal, the diode laser could be tuned over a 4-GHz range. Recently, blue GaN diode lasers have also been incorporated into ECDL arrangements and tested for their performance. In one of the two publications describing such works, Hult et al. (56) found a 90-GHz continuous tuning range for 450- and 410-nm diodes. Leinen et al. (57), who applied their blue ECDL light source to isotope-selective Doppler-free indium spectroscopy, reported about 20-GHz tuning range and 10-MHz emission linewidth.

Wavelength Stabilization It is clearly essential in diode laser atomic spectroscopy, similarly to other laser spectroscopic methods, that the laser emission wavelength (or frequency) is kept as stable as possible, meaning that the magnitude of wavelength fluctuations should be smaller than the linewidths to be resolved. Although in the high-temperature, atmospheric pressure atom reservoirs

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typically used in atomic spectroscopy, where Doppler effect and pressurebased processes cause the spectral linewidths be quite broad by laser terms (it is typically on the order of 0.01 nm), this requirement still demands the use of single longitudinal mode diode lasers. The wavelength stability is equally important for fixed wavelength and controlled (tuned or scanned) wavelength measurements. Naturally, high-resolution atomic spectroscopic methods, such as isotope-selective spectroscopy, set even more stringent requirements against wavelength stability. Wavelength stabilization systems are usually based on the servo loop (or servo lock) concept (36, 42). This concept uses an electronic error signal that is derived in a way that makes it proportional to the magnitude of the detuning (difference between the laser wavelength and a reference wavelength). The electronic control system then tries to tune the laser so that the error signal is minimal. Interferometers, a second laser, or an atomic or molecular transition (e.g. in the form of absorption or fluorescence by the vapors of the analyte kept in a closed, heated cell) can also be used as wavelength reference. Of course, the stability of the laser wavelength can never be better than the linewidth of the reference. Consequently, best results can be achieved using hyperfine transitions under Doppler-free conditions. For good quality single-mode diode lasers, the control part of this general concept can be, in principle, very easily realized, due to their direct tunability by the electronic control of the junction temperature and current. In practice, however, wavelength stabilization systems for diode lasers usually also incorporate some electromechanical components and control the wavelength, at least partially, by controlling the length of an external cavity. One of the added benefits of such a system is that the spectral bandwidth can be reduced as well. The intensity of the scientific interest toward the wavelength (or frequency) stabilization of diode lasers is well illustrated by the fact that nearly two dozen publications appeared on the topic in the reviewed period. Among them are three reviews, by Gawlik and Zachorowski (58), Lopez and Romero (59), and Chawla (60). Doppler-free (or sub-Doppler) saturation spectroscopy of an atomic vapor in a thin cell is the most popular stabilization scheme. Due to their significant vapor pressure at room or only slightly elevated temperatures, mainly alkali metal containing cells are employed. Zhao et al. used Cs vapors in a 150-mm cell and restricted the frequency fluctuation of their ECDL system to 0.8 MHz using the third derivative of the sub-Doppler saturation signal (61). In their similar system, Fukuda et al. (62) also employed a Cs hyperfine line component for the stabilization. Park et al. (63) used Yb atoms for the frequency locking of high-power blue diode lasers and achieved 62 kHz stability. Sukenik et al. (64) developed a modulation-free stabilization scheme based on the use of two acousto-optic modulators with variable offset and Doppler-free saturation spectroscopy. This technique was shown to perform well either with a Rb vapor or a metastable Ar


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discharge reference cell. Tanaka et al. (65) described another modulation-free scheme and demonstrated 100-kHz frequency linewidth with an Rb cell. Two groups disseminated results on the use of sub-Doppler polarization spectroscopy for frequency stabilization of diode lasers. Yoshikawa et al. (66) presented a modulation-free technique with the use of Rb vapor and obtained a bandwidth as good as 100 kHz. The technique of Ratnapala et al. (67) measures two Stokes parameters of the light transmitted through a vapor cell. It was shown that the main advantage of this approach is that it drastically extends the capture range of the servo loop; even detunings many times the linewidth of the reference transition can be controlled. Hyperfine transitions of Rb (around 780 nm) and Cs (around 852 nm) atoms caused by Zeeman splitting were also utilized as references for precise diode laser frequency stabilization in several publications. This approach was used in works by Jiang et al. (68), Nakano et al. (69), Lee et al. (70), Oversteer et al. (71) and Wang et al. (72), In all instances, good stability, characterized by a frequency jitter on the order of a few hundred kHz, was achieved. Some unique stabilization schemes can also be found among the works published. One such work, published by Maerten et al. (73), used a selfadaptive photorefractive filter placed inside the extended cavity of an ECDL, terminated by a partially reflective mirror. Photorefractive crystals are dynamic holographic media which, under the circumstances here, develop a refractive index replica of the illuminating pattern and hence act as a wavelength selective filter. This adaptation of the filter forces the laser to oscillate in a single longitudinal mode. Frequency tuning is not possible, as the wavelength of the single-mode oscillation will be selected automatically around the wavelength of maximum gain. For the presented system working at 810 nm and employing a BaTiO3: Co crystal, a stable operation characterized by only a few hundred MHz frequency jitter was found. Shevy and Deng (74) also described a novel method of stabilization, in which a Doppler-free Faraday resonance in Cs vapor provided optical feedback, whereas FM sideband saturation spectroscopy in a second Cs cell provided electronic feedback. The authors reported the reduction of noise power by more than 6 orders of magnitude and an ultra-narrow laser bandwidth of less than 100 Hz. Resonant optical feedback from a confocal extended cavity stabilized a GaN blue laser diode’s emission wavelength in a system by Hayasaka (75). The observed frequency fluctuations were smaller than 300 kHz.

Frequency Upconversion As was alluded to before, one of the biggest problems that hinders the widespread use of diode lasers as excitation light sources in atomic spectroscopy is the unavailability of diode lasers with UV range emission. This problem can

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be overcome by the use of nonlinear optical methods that help to upconvert the frequency of the emission. These methods are based on a phenomenon known since the 1960s, namely that the propagation of light through weakly nonlinear optical dielectric crystals, such as KNbO3, LiIO3, KTiOPO4 (or KTP), b-BaB2O4 (or BBO), and others, gives rise to vibrations at harmonics of the incoming (pump) light frequency. If one or two sufficiently powerful beams of laser radiation is passed through such a crystal, the laser frequency may be transformed to the second, third, or higher harmonics and to combination (sum and difference) frequencies. These methods are referred to as second and third harmonic generation (SHG and THG for short, respectively), sumfrequency, and difference-frequency generation (SFG and DFG, respectively) in the literature. Detailed theoretical treatment of nonlinear optical processes and a wealth of useful technical information on crystals can be found in an excellent handbook by Dimitriev et al. (76). Another good source of up-todate information is a book chapter by Simon and Tittel (77). A characteristic of nonlinear methods relevant to their application to cw diode lasers is that the intensity of the output is proportional to a certain power of the pump intensity. For example, in the case of SHG, the conversion efficiency is proportional to the square of the pump intensity (36). Thus, nonlinear methods are traditionally used with pulsed laser sources, which can easily provide high peak optical powers. Nevertheless, SHG (and THG) can also be used to upconvert the frequency of cw radiation from diode lasers, at the cost of a considerable loss of optical power. It is reasonable to place the nonlinear crystal inside the cavity of an ECDL, where more optical power is available. In such intracavity SHG setups, the output mirror must have a high reflectance for the fundamental frequency and high transmittance for the second harmonic. To further enhance the conversion efficiency, the fundamental radiation can also be focused into the nonlinear crystal (42). Commercial diode laser SHG systems have recently been made available by selected companies like Toptica. In the covered period of time, intracavity SHG was one of the most frequently used frequency upconversion approach. Hayasaka et al. (78) succeeded in generating 1.8 mW radiation at 397 nm in an LiIO3 crystal by doubling 100 mW GaAlAs laser emission (1.8% conversion efficiency). Lodahl et al. (79) demonstrated the generation of 8-mW optical output at 430 nm with about 40% efficiency by performing a careful analysis of the nonlinear processes in the KNbO3 crystal used. A special extended cavity, in which a dispersing prism and a thin glass plate provided optical feedback with far less power loss than that typically caused by a grating, was employed in a work by Manoel et al. (80). In this system, a KNbO3 crystal generated 20 mW of 423-nm radiation from 77 mW of input from a diode laser. More recently, Klappauf et al. (81) built a powerful but compact standing wave grating tuned ECDL blue laser source, in which 200 mW of second harmonic optical power was generated at 461 nm. The laser was stabilized to an Sr transition.


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Several SHG arrangements made use of a master oscillator power amplifier (MOPA) diode laser system in order to increase the optical power input to the nonlinear crystal. Diode laser MOPA systems usually consist of an ECDL that injects a (tapered) semiconductor amplifier, thus achieving up to 1000 mW optical power (82). MOPA-amplified laser diode radiation was used by Matsubara et al. (83), who produced 100 mW of tunable 214.5-nm cw radiation using a two-stage frequency quadrupling system with KNbO3 and BBO crystals. Using a similar double SHG system, Schwedes et al. (84) reported on the successful generation of 1-mW optical power at 231 nm by quadrupling radiation from a diode laser emitting at 922 nm. A periodically poled KTP (PPKTP) and a BBO crystal was used. In a recent work of Manoel et al. (85), an MOPA-pumped SHG system is described that produced more than 50-mW optical power at 425 nm. Ruseva et al. (86) demonstrated the generation of 150-mW radiation at 457 nm in their compact and inexpensive system, which employed a broad area emitter diode as amplifier and a KNbO3 crystal. Most recently, Le Targat et al. (87) reached an exceptionally high 75% conversion efficiency in their MOPApumped SHG system. The stable 234-mW output power at 461 nm was achieved by choosing a long-cut PPKTP crystal and a looser focusing of the radiation into the crystal, thus minimizing thermal lensing effects. Uhl et al. (88) used SFG of two diode lasers to produce radiation for the measurement of Hg at its 365.119-nm line in a radio frequency discharge (88). An even more complex approach to frequency upconversion was presented by Franzke et al. (89). In their publication, they described a system based on SHG followed by SFG with two diode lasers and two crystals to generate a tunable 283-nm radiation for high-resolution Pb spectroscopy. The SHG of the 850-nm fundamental radiation was performed in a KNbO3 crystal, followed by sum-frequency mixing of the second harmonic to the radiation from an 850-nm DBR diode laser in a BBO crystal. The total process produced 37-nW UV radiation, which was found to be adequate to record the hyperfine absorption spectrum of Pb isotopes in a hollow cathode lamp.

Other Experimental Achievements Non-spectroscopic experimental research areas outside the focus of the above subsections were relatively quiet in the covered period. This is most probably attributable to the formerly mentioned fact that electro-optic instrumentation for the control of diode laser operation went commercial from the mid 1990s. Sophisticated, flexible and powerful current and temperature controllers are now available from several companies (see Availability of Diode Lasers and Related Instrumentation), and almost all major optical companies offer temperature and power-stabilized diode laser modules with built-in collimating optics for various applications. There is one German company, LaserSpec Analytik (now owned by Atomika Instruments), that even offers diode laser

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modules that directly replace conventional hollow cathode lamps in Perkin-Elmer AAS spectrometers (90). This product was brought to the market in 1997 (18), although with little success so far. Modern diode laser control instruments are costly and may be somewhat complicated, and therefore not very suitable for simple atomic spectroscopy experiments and undergraduate laboratories. Consequently, the little research interest alive in this area was mostly devoted to design easy-to-use, inexpensive, but effective control instruments for diode laser atomic spectroscopy. A simple high-speed current controller circuit, which can be easily fabricated in-house and capable of providing a very low-noise (ca. 45 nA rms at 1 MHz bandwidth) and stable current (drift less than 0.25 mA in 3 h), was reported by Libbrecht and Hall (91). Milic et al. (92) constructed a stable power supply in order to reduce the noise, and an electrical setup combining a lock-in amplifier with a PID circuit, in order to reduce the wavelength drift of a diode laser. Galbaćs et al. (93) proposed a simple, digital current controller capable of providing a maximum of 1300-mA current, and equipped with a 10-kHz bi-directional computer interface. Lazar et al. (94) published the constructional and characterizational description of a double-shielded current controller that provides maximum protection for laser diodes by the use of a ripple- and transient-free voltage supply and full galvanic isolation by optocouplers. Andreoni et al. (95) described a temperature controller based on the use of the built-in photodiode of a diode laser as heating element. This approach was claimed to provide long-term frequency stability without the need for a milli-K temperature controller.

DIAGNOSTIC APPLICATIONS RELATED TO VARIOUS ATOM SOURCES Spectroscopic measurements using laser light sources are more suitable for atom source diagnostic experiments than other methods based on the use of conventional devices such as hollow cathode lamps, arc lamps, magnetic probes, etc. This was well demonstrated already in the 1980s and onwards in works by Omenetto, Winefordner and others (e.g., refs. 23, 96 – 99), mainly with the use of pulsed dye lasers and fluorescence spectroscopy. The collimated, high-intensity, and narrow linewidth radiation of laser sources is useful for performing spectrally and spatially highly resolved measurements of line profiles and shifts via both absorption and fluorescence spectroscopy even in the highly luminous environment of plasmas. The experimental data can be used to determine the electron number density, kinetic temperature, species population, flow velocities, and other parameters. Laser diodes provide diagnostic experiments with even more useful features, including short response times (fast modulation), that enable kinetic experiments with high temporal resolution or the study of pulsed atom sources, as well as easy tuning and beam handling. Consequently,


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diode laser sources are more and more often discovered for diagnostic studies in plasmas, flames, discharges, etc. The first thorough demonstration on the use of diode lasers for diagnostic purposes in an atom source (inductively coupled plasma) was given by Baer and Hanson (100) in 1992. The following subsections will overview the results published on the spectroscopic measurement of some atomic or ionic species with diagnostic purposes, classified by the related type of atom source the experiment was carried out in.

Plasma Sources Among other techniques, de Regt et al. (101 – 103) also employed diode laser atomic absorption diagnostic measurements at the 811.53-nm 4s3P2-4p3D3 argon transition in a 100-MHz atmospheric argon inductively coupled plasma (ICP) in three consecutive studies. By fitting the measured line profiles with Voigt function, the radial distribution of electron density and heavy particle temperature was calculated. Although the results agreed well with those obtained using the established Thomson scattering technique, the Lorentzian part of the absorption profiles was found to originate not only from the Stark effect, but also from the Van der Waals effect (the contribution of the latter was estimated to range from 50% to as much as 100%). Therefore, it was concluded that diode laser absorption profile measurements can only be used to obtain electron densities in ICPs, if the Van der Waals effect is properly taken into account. Baer et al. (104) used diode lasers to record O and Ar line shapes at 777.2 nm and 842.5 nm, respectively, in a 27-MHz inductively coupled argon-oxygen plasma. Electron number density and kinetic temperature values were inferred from the Stark- and Doppler-broadening components of the absorption line profiles. The ionization temperature was calculated from the electron number density assuming Saha equilibrium. Timmermans et al. (105) performed diode laser absorption experiments on an argon atmospheric microwave-induced plasma (MIP) in an axial injection torch arrangement. Metastable population densities and electron temperatures were estimated. Trends in the gas temperature and electron density as a function of various parameters (measurement position in the plasma, gas flow rate and gas composition) were also studied. Hancock et al. (106) used a wavelength-modulated diode laser atomic absorption (WM-DLAAS) technique to study the distribution of excited argon atoms in a capacitively coupled plasma (CCP) using frequency doubled radiation from an 860 nm diode laser. Severn et al. (107) found three new diode laser – induced fluorescence (DLIF) schemes that require excitation at wavelengths 664, 669, and 689 nm, which are near industry standard diode laser wavelengths. Two MOPA-driven ECDL systems were used to obtain high-resolution argon-ion velocity distribution and metastable densities in an argon plasma. Diode

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laser absorption and Doppler-shift measurements on argon-ionic transitions were performed in argon plasmas by Beverini et al. (108, 109) for timeresolved diagnostic purposes. High temporal resolution (better than 10 ms) was achieved by fast scanning the laser emission wavelength across four Ar transitions around 801 and 664 nm by regulating (modulating) the injection current at a rate greater than 100 kHz. Temperature and velocity distribution data, as well as optical thickness values, were obtained. Bolshakov et al. (110) used a VCSEL laser diode as tunable emission source for gas temperature measurements inside a low-pressure, low-power ICP. Temperature data were obtained by profiling the absorption of metastable argon atoms at 763.51 nm in argon and Ar/N2 plasmas. A two-dimensional, three-temperature fluid plasma simulation was employed to explain the obtained data. Cervelli et al. (111) presented a diode laser absorption spectroscopy – based method that allows for spatially and time-resolved detection of atomic oxygen generated during laser ablation processes under a molecular oxygen – containing gas environment. It was found that a region exists in the plasma plume where oxygen dissociation is particularly efficient. This region is located some millimeters away from the target, the actual distance slightly affected by experimental conditions. Such laser plasma diagnostic studies are also important for pulsed laser –based atomic spectroscopic methods such as laser-induced plasma spectroscopy (LIPS), which is typically performed under atmospheric conditions and hence is affected by oxygen related interferences (e.g., ref. 112)

Discharges Jung et al. (113) determined excitation temperatures of sputtered Gd and U atoms in an argon hollow cathode discharge using diode laser –excited optogalvanic spectroscopy. Temperature variations with discharge currents ranging from 15 to 50 mA were examined and the values were compared to those determined by emission spectroscopy. Scheibner et al. (114) reported on the use of a GaN laser for the diode laser atomic absorption spectroscopy of Al in a hollow cathode discharge. Ground state density of Al sputtered from the cathode and temperature values were calculated after collecting hyperfine spectra by scanning the wavelength of the laser over an Al transition. Marago et al. (115) investigated Doppler-broadened Ga line profiles at 403 and 417 nm in a hollow cathode discharge. Accounting for hyperfine and isotope structure in the profiles, information on the temperature and density of neutral atoms was extracted. Ohta et al. (116) studied the velocity distribution of argon atoms in a thin glow discharge cell. Wavelength modulation diode laser absorption signal at around the 772.42-nm argon line was recorded and evaluated. Toward low pressures, below 13.33 Pa, the linewidth was found to be narrowing drastically


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and the velocity distribution was calculated to be anisotropic. These findings were explained by the fact that metastable atoms having large velocities are lost upon collisions with the cell wall and their collisions with ground state atoms at low pressures cannot compensate for the loss. Manke et al. (117) examined the production of F and Cl atoms from F2 and Cl2 gas, respectively, by an electrical discharge. Kinetic modeling was performed using concentration and translational temperature values measured. A fiber optic – coupled diode laser spectroscopy system was developed by Zhang et al. (118, 119) for the computer-controlled mapping of temperature and atom density distribution in an argon arc, based on absorption measurements at the 811.531-nm argon transition. Recently, Wolter et al. (120) studied Al atom densities produced by a direct-current magnetron discharge using diode laser absorption spectroscopy. Pure argon as well as Ar/N2 and Ar/O2 working gas mixtures under varying pressures were applied. It was found that the temperature rises with increasing power, but the pressure has little influence on it. In Ar/O2 mixtures, no absorption signal from Al could be detected, which remained unexplained.

Flames and Furnaces Hadgu et al. (121) carried out Rb atom distribution measurements inside and outside a transversally heated graphite furnace atomizer (THGA) using diode laser atomic absorption spectroscopy. It was found that the density of Rb atoms at a position of 1.2 mm outside the end of the tube is 14 –17% of that at the center of the tube. The contribution of outside Rb atoms to the total signal was estimated to be 6%. Measurement results confirmed or supported that (a) there is an axial convective flow toward the center of the tube, (b) non-spectral interferences can be mainly attributed to matrix vapors residing outside the tube, and (c) the atom distribution during the whole atomization cycle is reasonably homogeneous, with about a 25% higher Rb density closer to the platform. Park et al. (122) developed a system and demonstrated the use of it for monitoring the vaporization rate (vapor density) of Yb in a resistively heated atomizer. The system was based on diode laser absorption spectroscopy of Doppler-broadened transitions of Yb at 398.8 nm. The wavelength of the diode laser was simultaneously controlled by a wavemeter and the optogalvanic signal produced by an Yb hollow cathode lamp. The Kaminski group in Cambridge develops laser spectroscopic imaging techniques for various technical and biological flows (123). In some of their works, they also successfully employed diode laser –induced fluorescence (DLIF) spectroscopy to the study of combustion processes in flames. Using their GaN diode – based ECDL system mentioned earlier (56), distribution profiles with high spatial resolution and hyperfine spectrum of In atoms were recorded in atmospheric methanol (124) and methane (125) flames.

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Indium was excited at 410 nm, and the fluorescence signal was detected at 451 nm. Very recently, they also presented a novel thermometric method based on DLIF for the purpose of spatially precise temperature measurements in flames (126). ANALYTICAL APPLICATIONS OF DIODE LASER ATOMIC SPECTROSCOPY A wide variety of analytical works was found in the recent literature. Instrumental and method development in the area of absorption, fluorescence, and ionization spectroscopy, as well as the construction of element-specific detectors for coupling to different chromatographic instrumentation and special uses such as sample evaporation, were all targeted by experiments. Nevertheless, atomic absorption is still by far the most popular branch of analytical diode laser atomic spectroscopy. The domination can be easily pictured by mentioning that DLAAS applications made up nearly 80% of all analytical applications reviewed here. For the sake of clarity, the analytical applications will be discussed through organization into five subsections. Absorption Spectroscopy Direct Absorption In general, direct or classical atomic absorption measurements, as opposed to measurements in modulated arrangements, are less frequently applied due to the much better detection capabilities of modulation techniques (mainly WM-DLAAS). Nevertheless, a few applications can still be found in the recent literature, both in the area of diagnostic (see the former section) and analytical works. The Niemax group studied the possibility of using diode laser atomic absorption spectroscopy in a laser-induced plasma for the selective detection and isotope ratio measurement of 235U and 238U (127 – 129). The goal of the instrumental development project was to provide a useful level of accuracy and precision in a compact, portable instrument that can possibly also applied in remote or hostile environments. The combination of diode lasers as excitation light sources with a modern, small Nd:YAG laser, for plasma generation was seen to be the potential solution. Because of the transient nature of laser-induced plasmas, the probing of any concentrations in the plasma needs to be done within a few milliseconds time window, which prevents the application of the wavelength modulation approach. In the first publication, the group used one diode laser and tuned it sequentially to the respective spectral lines of uranium (682.6736 nm for 235U, and 682.6913 nm for 238U), and the corresponding absorption signals were detected on a shot-to-shot basis (127). The diode laser wavelength was


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controlled by stabilizing the temperature within 0.001 K. The performance of the method was tested by analyzing three standardized graphite pellet samples containing 60% uranium oxide of different isotopic composition. The detection limit was found to be approximately 100 mg . g21 for the minor isotope. The overall accuracy and precision of isotope ratio measurements was about 10%. The above analytical approach was later revisited and improved by the group (128, 129). This time, a detailed study and optimization of the plasma conditions and the use of two diode lasers, one for the detection of each U isotope, to improve analytical performance was the objective. By using the parallel (double beam) detection principle and three homogeneous uranium-oxide pellet samples, a 1.1% relative standard deviation on the U isotope ratio could be achieved. Both the accuracy and the detection limit improved, to 5% and 47 mg . g21 values, respectively. Galbaćs and Geretovszky (130) studied the performance of diode laser atomic absorption spectroscopy in an ICP plasma for lithium analysis. The emission wavelength of the diode laser in this setup was also only stabilized to the 670.78-nm measurement wavelength by tightly controlling the temperature and current of the diode. After careful optimization of the experimental conditions (plasma R.F. power, observation height, carrier gas flow rate), a 150 mg . L21 detection limit and three orders of magnitude dynamic range were obtained for synthetic lithium containing solutions. The main reason for the relatively poor detection limit was seen in the fact that a transition starting from the ground level of atomic Li, which is a poorly populated level in the ICP, could only be used. About 50-fold possible improvement in this figure of merit was projected. The effect of the presence of high salt concentrations (NaCl) on the analytical signals was also studied, and no significant interference was found for up to about 1% (10,000 ppm) salt concentration.

Modulation Techniques Intensity or wavelength modulation techniques are important means of improving signal-to-noise ratios, and hence the analytical performance, in diode laser atomic absorption spectroscopy. The general approach of modulation techniques comprises of modulating the wavelength (or intensity) of the laser light, transmitting the light through the atom/ion source to be probed, and analyzing the attenuated detector signal at the modulation frequency or some harmonics of that frequency using a lock-in amplifier. This process usually significantly reduces noise (primarily 1/f noise) from the laser or the detector by shifting the detection to higher frequencies where the noise is less significant. Depending on the mode of operation, it can also remove spectral baseline, even it is sloping or has some curvature, etc. The result is a great improvement in sensitivity and selectivity compared to direct (classical) absorption measurements.

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Although modulation techniques can be used in many laser spectroscopic arrangements, they are eminently applicable to diode laser atomic absorption spectroscopy, partly due to the convenience with which the emission of diode lasers can be modulated by modulating the injection current. More details on the potential and theory of the approach can be found in dedicated papers written by Silver (131), Axner et al. (26, 132), Liger et al. (133) and others. The area of analytical diode laser modulation atomic absorption spectroscopy in the reviewed period of time was clearly dominated by the activity of two research groups (and their associated partners). One of them was again the Niemax group from Germany, whose work mainly focused on instrumental developments and analytical applications. The other group is Axner’s in Sweden, who introduced the modern theory of signal formalism in WM-DLAAS (132) and perhaps has done most for the theoretical improvement of the technique. Of course, these classifications are not meant to be exclusive. Nevertheless, it seems useful to discuss results of the analysis-oriented modulation DLAAS publications below according to their primary topic, thus dividing them in two subsections (analytical applications and theoretical improvements). In accordance with the scope definition given in the introduction, theoretical works are only covered if they are directly related to the improvement of the analytical performance. Analytical Applications In 1994, Groll et al. (134) performed WM-DLAAS in analytical airpropane and air-acetylene flames for the measurement of Ti, Cs, and Cr applying fundamental and frequency-doubled radiation of diode lasers. For all elements, the achieved limits of detection were comparable to those of hollow cathode lamp GFAAS, even if they were measured at weak transitions, when diode laser availability called for it. The results were achieved with only 50- to 100-nW SHG output power available. Ljung and Axner (135) measured rubidium in five standard reference samples (corn bran, NIST water, riverine water, estuarine water, and seawater) in a graphite furnace. A 1 ng . L21 detection limit and good accuracy, the latter tested in the case of the two reference materials that had certified Rb content, were established. Wizemann and Niemax (136) were the first to apply isotope dilution for calibration in diode laser atomic absorption spectroscopy. Lead isotopes were analyzed by intensity-modulated DLAAS at 405.78 nm, and it was also shown that isotope dilution is an efficient means of the cancellation of physical and chemical matrix effects. The limit of detection for 206Pb and 208Pb isotopes were found to be 60 mg . L21, and it was two times higher for the 207 isotope. The same authors also presented another approach to isotopeselective DLAAS (137). This time, a low-pressure graphite furnace was the atom source, and Doppler-limited as well as Doppler-free absorption spectroscopy of Li and Rb isotopes were performed. The performance of intensity modulation, wavelength modulation, and their combination was also studied. The best detection limits were about 20 ng . L21 for Li and


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130 ng . L21 for Rb. In a work by Krivan et al. (138), WM-DLAAS in a tungsten coil atomizer was described. The performance of the system was demonstrated by the determination of Al and Cr in water, blood serum, powdered graphite, and TiO2 samples (the latter two were introduced as slurries) by using standard addition calibration. Frequency-doubled radiation at 396.15 nm (0.2 mW power) and 427.48 nm (3 mW power) were used. Detection limits in slurry samples were 1 to 2 orders of magnitude higher than in aqueous matrices. It was also found that although high blank values from the coil atomizer seriously limited the performance, but the system was said to be suitable for on-site and on-line analysis, providing lower cost, simpler construction, and similar performance than GFAAS. Wizemann (139, 140) applied the WM technique for the measurement of large isotope ratios of lithium in a low-pressure graphite furnace. Calculated line strengths were used to deconvolute overlapping lines and it enabled the measurement of 7Li/6Li isotope ratios as high as 2000. Isotope dilution was used for calibration, and its advantages in the presence of strong matrix effects were again emphasized and demonstrated in a 1% NaCl matrix. The result of 148 mg . L21 Li concentration compared well to 156 mg . L21 obtained by ICP-AES. Wizemann and his coworkers carried out at least three more studies of the application of WM-DLAAS for isotope selective Doppler-free spectroscopy in graphite furnaces. In the first one, Li isotope ratios were measured by a two-photon excitation scheme under large optical saturation conditions in the co-propagating and counter-propagating arrangement (141). Later, isotopes of rubidium (142) in a standard water sample and rare earth elements (Sm, Eu, Gd, Er, and Lu) (143) in synthetic aqueous solutions were also measured. Uhl et al. (144) used WM-DLAAS for gas leak detection. Trace amounts of argon in helium gas flow were measured in a low-pressure discharge. After the calibration of the resulting absorption signal against the leak rate, it was calculated that the lowest detectable leak rate is 1024 Pa . L . s21. Uhl and his coworkers also measured Hg by WM-DLAAS in a radio frequency discharge (88). Mercury was generated by the cold vapor technique and transported into the discharge by an argon flow. A 100 ng . L21 detection limit was achieved. Koch et al. (145) analyzed chlorine in PVC, polymethylmethacrylate, and Lexan polymers by laser sampling and WM-DLAAS in a low-pressure MIP plasma. Chlorine was detected via one of its metastable species at the 837.824-nm wavelength. Hydrogen and carbon was also measured for the purpose of testing the ability of the technique to accurately measure stoichiometric ratios of the three elements. The detection limit for Cl was found to be 85 ppm. Franzke et al. (146) described two relatively simple methods for the sensitive determination of sulfur in CO2 gas (146). A direct-current discharge plasma, modulated by a 5– 10 kHz square wave function, was used to atomize sulfur compounds. The obtained 0.2 –0.7 ppm detection limits were found to satisfy the requirements for the control of sulfur

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compounds in CO2 used in the food and beverage industry, the envisaged potential user of the methods. Theoretical Improvements A substantial improvement of the modulation technique was presented by Liger et al. (133), who developed an arrangement based on double-beam, double modulation (diode laser wavelength modulation and sample modulation) with logarithmic detection. It was shown that this arrangement with detection at the sum or difference of the frequencies of modulation suppresses a range of interferences, including spurious etalon effects, background absorption, residual diode laser amplitude modulation, laser excess noise and signal variations due to changes in the optical transmittance, etc., thus enabling the measurement of absorbances of about 2 1027 AU. The same set of authors also used their logarithmic signal conversion technique to extend the dynamic range and improve background correction (147). Linearity of the calibration curve up to about 1.7 AU was found in a graphite furnace, even with background absorption at the 1.4 AU level. Rb as pilot element was used in the study. The problems of dynamic range extension and background signals were also addressed by the Axner group. In search of achieving an ultimately wide dynamic range, a new methodology based on the parameterization of the WM-DLAAS signal and calculation of the actual time-integrated sample optical thickness (SOT) curves was developed (148). Based on theoretical considerations, computer simulations, and measurement data for Rb at 780.02-nm wavelength in a graphite furnace, it was shown that the new methodology can extend the dynamic range to the measurement of 20 integrated AU, implying a six orders of magnitude useful analytical range (fg to ng) without the need for any adjustments in laser settings. In the next two-paper set of publications, the group discussed the characteristics of background signals in the case when frequency-doubled diode laser light was used in WM-DLAAS with detection at the second harmonic frequency (149, 150). The detailed theoretical analysis of WM-DLAAS signals was enabled by the former successful development of the formalism for WM spectrometry based on Fourier series (132). It was theoretically proved and experimentally demonstrated (detection of Ca at 422 nm in an acetylene-air flame) that 2-f detection, which is normally used in WM-DLAAS because it provides the largest signal, is plagued by considerably larger amounts of background signals than 4-f and 6-f detection. It was also pointed out that lower modulation amplitudes than normally used are needed for optimal performance in a frequency-doubled WM-DLAAS. Gustaffson and Axner treated the problem of multiple reflections in the optical system of WM-DLAAS instruments, too (151). The problem was identified to be most severe with window-closed graphite furnace atomizers, when the heating of the furnace induces drifts in the thickness of the windows and thereby also of the background signals. The solution


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suggested to the problem was based on the finding that the background signal is reproducible over a considerable period of time. Therefore, a careful choice of the timing of the furnace operation (triggering) can reduce the background drift, thereby allowing the same level of analytical performance than that in an open graphite furnace. In two follow-up publications from the group (152, 153), the detailed characterization of the background signals in graphite furnaces was also given, and it was shown that detection of higher even order frequency components (e.g. 4-f and 6-f) is also boosting the performance in window-closed graphite furnaces. Cavity Ringdown Cavity ringdown spectroscopy (CRDS) is based upon the measurement of the rate of absorption rather than the magnitude of absorption of a light pulse confined in a closed optical cavity. The advantages over normal absorption spectroscopy include: (a) the method is insensitive to light source intensity fluctuations, and (b) extremely long effective path lengths (up to many kilometers) can be realized in stable optical cavities, which is especially useful in trace analysis and/or when measuring at weak transitions. CRDS is now becoming an established technique, mainly in the areas of trace gas analysis and molecular spectroscopy, as is also reflected in CRDS reviews (e.g., refs. 154, 155). Atomic spectroscopic applications typically use a plasma-CRDS experimental setup, which employs an ICP or MIP as the atomization source and a pulsed laser as the light source. These systems showed significant promise for ultra-sensitive elemental measurements. Winstead, Miller, and their coworkers are the pioneers of the area (see, e.g., refs. 156 –158) and they were also the first, and so far the only ones, to study the combination of a diode laser light source with plasma-CRDS (159). The system was developed in an effort to improve the portability and reduce the cost of plasma-CRDS for application purpose and included a compact, low-power microwave-induced plasma and a commercial ECDL diode laser. A technique for controlling the coupling of the cw diode laser to the ringdown cavity has been implemented using a standard power combiner. No acoustooptic modulator or cavity modulation was required. The system performance in the areas of stability and detection sensitivity has been demonstrated by analyzing diluted standard solutions of strontium. The detection limit of Sr at the detection wavelength of 680 nm, chosen to match the wavelength of available diode lasers, was found to be 375 mg/L21 (3 s).

Fluorescence Spectroscopy Smith et al. (160) used a diode laser for the selective excitation of 235U and U in a laser-induced plasma created on the surface of UO2 samples. The

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resonance atomic fluorescence spectrum for both isotopes was obtained on a pulse-to-pulse basis by rapidly scanning the diode laser emission over the transitions. Time-integrated measurements, with the diode laser fixed at either isotope, were also made. Optimum signal-to-noise ratio was obtained at a distance of 0.8 cm from the sample surface. Three samples with 0.204, 0.407, and 0.714% 235U content were measured. Typical accuracy and precision values obtained on a 460 shot basis were 7 and 5%, respectively. Analytical capabilities of this method were suggested to be mainly limited by the continuum emission background from the laser-induced plasma. Recently, Galbaćs et al. (161) developed and successfully applied a diode laser induced fluorescence spectroscopy system for the analysis of Li. The experimental setup was based on an unmodulated, continuous-wave diode laser, exciting neutral Li atoms on their 670.78-nm transition in an ICP atom source. A simple, three-step measurement procedure was devised that corrected for the contribution of lithium thermal emission and scattered laser light in the analytical signal. The DLIF-ICP method has been successfully applied to the determination of lithium in several mineral waters and a thermal salt solution. Despite the facts that lithium was detected on its neutral atom, which was estimated to account for less than 1% of the total concentration of Li in the ICP, and that only about 1 –2% of all atoms could be excited by the laser light at any given time, the limit of detection was still found to be as low as 8 mg . L21. This figure of merit was even suggested to be dramatically improvable by using a more powerful laser diode and detecting the analyte at an ionic line. The linear dynamic range was found to be around three orders of magnitude.

Ionization Spectroscopy Ionization spectroscopic methods utilize the selective ionization of the analyte atoms presented in an atomizer carried out using resonant excitation by laser radiation, also using the assistance of collisions in some schemes. For the purpose of elemental analysis, either the electrons and/or ions generated during the process can be directly detected, like in resonance ionization mass spectroscopy (RIMS) and laser-enhanced ionization (LEI) spectroscopy, or an indirect detection based on the optogalvanic effect can be done (26). Optogalvanic Methods Diode laser optogalvanic spectroscopy is often applied in fundamental atomic physics research to obtain high-resolution spectra of atoms and ions. However, it was only Barshick and his coworkers who published analytical papers in the covered period of time. A hollow cathode glow discharge has been coupled with tunable diode lasers for isotopically selective excitation of gaseous uranium atoms produced by cathodic sputtering. In their first work (162),


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optogalvanic detection of the discharge atom population allowed identification of 235U at depleted, natural, and enriched abundances in uranium metal and uranium oxide samples based on the spectral signatures of two U transitions. Isotope ratio measurements were precise to better than 1% relative standard deviation (at the one s level). In the second work (163), the 235U/(235U þ 238U) isotope ratio was determined in five samples, containing varying amounts of 235U. The best signal-to-noise ratios were obtained using a powerful 150-mW diode laser working at 831.84 nm. Analytical results were compared to those from thermal ionization measurements; 7.8% precision and 3.7% accuracy were achieved even for a sample depleted in 235U. This level of accuracy and precision, although inferior to reference methods of isotope analysis, was declared to be sufficient for screening applications. Resonance Ionization Mass Spectrometry The work of Young and Shaw (164) was one of the first that described the incorporation of a diode laser excitation step into a RIMS optical excitation process to enhance the isotopic selectivity of the technique. Lanthanum isotope ratio enhancements as high as 1000 were achieved by use of a single-frequency cw diode laser tuned to excite the first step of a three-step excitation-ionization optical process; the subsequent steps were excited by use of a pulsed dye laser. Park et al. (165) also used an excitation scheme in the RIMS spectroscopy of samarium, which was initiated by a 677.916-nm diode laser beam for the first transition originating from the lowest metastable energy level. A further two-photon process in the ionization scheme was provided by a pulsed dye laser beam. The obtained mass spectra showed a linewidth of 40 MHz. Based on this data, a 10 improvement of the resolution of the method was estimated. The authors stated that the diode laser – initiated RIMS approach is effective in general for the detection of isotopes with small natural abundance. Bushaw, Wendt, and their coworkers published five papers on diode laser –based RIMS spectroscopy of various isotopes. Among the applied ionization schemes, ones involving only diode laser excitation and ones also using other lasers equally occurred. In an early work, a successful 90Sr determination using crucible atomization and less than pg amounts of samples with 0.8 fg absolute detection limits and better than 1010 selectivity were demonstrated (166). The 41Ca content of concrete samples obtained from the shield nuclear research reactor was also determined by the group (167). In this study, standard procedures were applied for the chemical separation of calcium and the total calcium concentration was determined by X-ray fluorescence (XRF) spectroscopy. The RIMS measurement yielded a detection limit of 5 10210 in terms of the abundance of 41Ca relative to the total calcium content. Reproducibility and accuracy were determined with 41Ca

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spikes and found to be in the range of 15%, limited predominantly by ion counting statistics. Ca isotope determination was again the topic in the next study (168), which demonstrated that diode –laser based RIMS spectroscopy is more practical and powerful than conventional electron-capture radioanalytical methods, the latter being limited in use by the long lifetime of 41 Ca isotopes. Single-, double-, and triple-resonance excitation schemes have been investigated experimentally and theoretically, and it was found that triple-resonance measurements provided the best selectivity and sensitivity with a detection limit of 2 105 atoms. The principal objective of the next work (169) was to attain a selectivity good enough to measure the radioactive nuclides 135Cs and 137Cs against stable 133Cs. Measurements to determine the chronological age of a burn-up sample were performed by using both RIMS and conventional thermal ionization mass spectrometry methods, and the performance with respect to efficiency, selectivity, and isobar suppression was compared. In a more recent study (170), they applied high-resolution diode laser RIMS to the trace determination of gadolinium in biomedical samples (various normal and tumoruos tissue samples taken from laboratory mice, by diode laser – based multi-step resonance ionization mass spectrometry). Utilizing three-step resonant excitation into an auto ionizing level, both isobaric and isotopic selectivity of better than 107 and an absolute detection limit of 1.6 pg (at the main 158Gd isotope) were attained. Linear response has been demonstrated over a dynamic range of six orders of magnitude.

Element-Specific Detectors for Chromatography The construction of element-selective detectors for liquid and gas chromatography (HPLC and GC, respectively) has also been a productive area for diode laser – based atomic spectroscopy. It was exclusively the Niemax group who produced all results in the area in the covered period of time. The detectors described are based on the wavelength modulation diode laser atomic absorption spectroscopy (WM-DLAAS) principle and employed continuous atomizers, such as flames and plasmas. The results were described in detail in two related recent papers (20, 171) by the group, hence here only a brief overview will be given. In the first two works covered here (172, 173), Cl and Br detection in a helium microwave-induced plasma coupled to capillary GC was described. The detection was done on excited, metastable Cl and Br atoms formed in the plasma. Complete dissociation of halocarbon molecules in the plasma was observed. Detection limits were calculated based on the 3s criterion and were found to be less than 100 mg . L21 for Cl in chlorinated species. This figure of merit was about 30 better than the detection limits obtainable using a commercial AES detector. Next, the successful application of GCWM-DLAAS for the determination of (a) volatile chlorinated hydrocarbons


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in oil samples from the recycling process of plastic material and (b) low concentrations of trichlorophenols in extracts prepared from plants loaded with chlorophenols was reported (174). With the use of sample volumes of 1 ml, a detection limit of 30 mg . L21 for trichlorophenols was obtained. In a 1998 GC-WM-DLAAS study (175), the influence of plasma conditions and the fragmentation capability of different types of organic compounds, such as alkanes, alkenes, and aromatics, were studied by measurements of the element ratios of C, H, and Cl. It was shown that the interelemental ratios for the three elements were accurate. It was therefore postulated that calibration and determination of the sum formula of analyte species is possible if an internal standard is used. In connection with element-specific detectors for HPLC, only the use of flame atomizers have been described. The HPLC was coupled to an analytical air/acetylene flame via a hydraulic high-pressure nebulizer. The detection of chromium was done at 425.44 nm, and of manganese at the 403.08-nm wavelength. Both wavelengths were produced by SHG of diode laser emission and were dictated by the availability of suitable laser diodes and nonlinear crystals. Chromium speciation was done in deionized and tap water samples, and by using 30 mW of exciting SHG radiation a detection limit of 0.01 mg . L21, comparable to that by ICP-MS, was achieved (176, 177). Inorganic Mn, toxic cyclopentadienyl manganese tricarbonyl and methyl-cyclopentadienyl manganese tricarbonyl molecules were measured by HPLC-flame-DLAAS in spiked gasoline, human urine, and drinking water samples (178). The detection limit was estimated to be about 1 mg . L21. Further improvement in this figure of merit was projected for when a GaN diode laser in the mW power range would be used, thus reducing the shot noise.

Special Applications Miclea et al. (179) presented a new miniature plasma source, the dielectric barrier discharge (DBD), which can potentially be operated in a microchipbased diode laser atomic absorption spectrometer. The DBD device, originally used in flat panel plasma displays and televisions, is made up of two closely spaced planar electrodes, one of which is covered with an insulating medium (barrier). The space between the electrode is filled with some reduced-pressure noble gas, and the discharge is ignited by applying an alternating high-voltage waveform to the electrodes. A large number of microscopic current filaments of nanosecond duration will be formed between the electrodes, which, due to the high local electron densities, can be utilized for an efficient dissociation of molecules. For example, a gaseous sample can be easily introduced as an admixture to the plasma supporting gas. The model DBD used in the experiment had ca. 50 mm 1 mm 1 mm dimensions and was operated in Ar and He, under a square-wave 750-V peakto-peak voltage of 20 kHz maximum. Under these conditions, the temperature

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of the plasma was found to be 600 –1000 K and the average power was around 0.1 W only. Chlorine and fluorine-containing halogenated hydrocarbons were used as test samples, and WM-DLAAS spectroscopy using two diode lasers (Cl 837 nm, F 685 nm) for the parallel measurement of the Cl and F concentration was performed in the discharge plasma. Detection limits of 400 ppt and 2 ppb, for Cl and F, respectively, were achieved for CCl2F2 in He. The dynamic range exceeded 4 orders of magnitude (180, 181). Michel et al. (182) investigated another novel application for diode lasers. Their work targeted the use of a high-power diode laser array (50 W power, 930 nm emission with 5 nm spectral bandwidth) as a solid sample vaporization device for use in atomic spectroscopy (182). It was found that the ca. 104 W . cm22 power density provided by the focused output beam of the prototype laser array was insufficient for the ablation of metallic samples, but vaporization of biological samples was possible. The analytical performance of the system was tested by the analysis of lead in a certified reference material (dry bovine liver powder) via direct deposition of the vaporized material onto the inner wall of a graphite tube and subsequent graphite furnace atomic absorption spectrometric measurement. In spite of the low recovery (15%) and the resulting low sensitivity, the determination was found to be accurate (ca. 7% accuracy) using calibration by aqueous standards and a matrix modifier, with 12% precision. Based on theoretical considerations, it was concluded that at least an order of magnitude higher power density would be needed to ensure complete vaporization and a well-controlled particle size of the aerosol, thus improving the analytical performance.

CONCLUSIONS This article overviewed the characteristics of diode lasers commercially available at present, and most achievements of the past 12 years in the field of practical diode laser atomic spectroscopy. Diode laser atomic spectroscopy has been and will most probably remain to an rapidly advancing field. Out of the various branches, maybe only DLAAS techniques can be considered to be fully matured, where there is only little place for further improvement. Presently, DLAAS already allows the measurement of ppt range elemental concentrations, with wide dynamic ranges and high, in some instances isotopic, selectivity. For the other analytical and experimental techniques, it would be hard to envision the path of evolution, considering the present rate of development and widening range of applications. Nevertheless, it is probably safe to expect that the applications involving multi-elemental (multi-isotopic) analysis of real-world samples and the development of portable diode laser atomic spectrometers may be among the frequently studied areas in the next decade. Diode laser spectroscopy can now be called an established area of research in the physical sciences, but diode laser atomic spectroscopic


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techniques have not yet really found their way to the commercial market. This is certainly not due to the potentials, which have already been demonstrated many times, but probably a combined effect of the reluctance of routine users to use such novel techniques and the inertia or wariness of the manufacturers to incorporate fundamentally new approaches and devices into instruments, the market of which is so conveniently established. It is clear, however, that when diode lasers operating in the UV range will become available, or at least the costs of good quality DBR/DFB diode lasers will drop, diode laser atomic spectroscopy instrumentation will become significantly simpler and thereby available and useful for a wider range of routine users, such as analytical chemists, geochemists, and other representatives of applied sciences.

ACKNOWLEDGMENTS The author thanks Dr. Zsolt Geretovszky (Department of Optics and Quantum Electronics, University of Szeged, Hungary) for his valuable comments on various topics throughout the manuscript. This work was financially supported by the OTKA agency (Hungary) under grant No. F043213.

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