Surface Plasmon Resonance On A Single Mode ... - Semantic Scholar

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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 47, NO. 1, FEBRUARY 1998

Surface Plasmon Resonance on a Single Mode Optical Fiber Eduardo Fontana, Hector Daniel Dulman, Member, IEEE, David E. Doggett, and Richard Harris Pantell, Life Fellow, IEEE

Abstract—A single mode optical fiber probe employing surface plasmon resonance (SPR) as the transducing mechanism is described. The fiber has one end polished at an angle relative to the longitudinal axis and coated with a thin gold film. Diffraction of the guided mode out of the fiber core, after interaction with the metallized fiber tip, enables visual observation of a spatial SPR, the latter arising as a dark strip within the light distribution of the diffracted beam pattern. Modifications induced on the SPR due to variations in material properties of the gaseous environment next to the fiber probe indicated a detection sensitivity for changes in refractive index down to 1005 . A theoretical model that accounts for the observed diffracted light under SPR as well as the spectral dependence of light reflected back to the fiber input, allows obtaining design parameters for the construction of highly sensitive fiber probes for use in gas sensing applications. Index Terms—Diffraction, fiber, gas detection, gold film, optical, sensor, SPR, surface plasmon.

I. INTRODUCTION

T

HE traditional prism coupling technique for observation of surface plasmon resonance (SPR) [1] has been used extensively in the literature for applications that include optical coating characterization [2], surface roughness determination [3], and chemical sensor development [4]. An optical fiber configuration, having the remote fiber tip polished at an angle relative to the fiber axis with a value close to the SPR angle and coated with a 50 nm thick film of either gold or silver was proposed in the literature [5], [6] as a means of using SPR for the design of highly sensitive fiber probes for remote sensing in a gaseous environment. Although the experimental work described in [6] exhibited evidence of an SPR effect taking place on the metallized fiber tip, from the measured light intensity coupled back to the fiber input, no account was given on possible effects due to coupling of the surface plasmon (SP) oscillation to the guided mode within the fiber.

Manuscript received June 1, 1997; revised April 1, 1998. This work was supported in part by an SBIR grant from the Food and Drug Administration, Department of Health and Human Services, under Grant 1 R43 FD-01 53101-2. Additional support was provided by the following Brazilian agencies: Conselho Nacional de Pesquisa-CNPQ, Funda¸ca¯ o Coordena¸ca¯ o de Aperfei¸coamento de Pessoal de N´ıvel Superior-CAPES, and Funda¸ca¯ o de Amparo a` Ciência do Estado de Pernambuco-FACEPE. E. Fontana is with the Departamento de Eletrônica e Sistemas, Universidade Federal de Pernambuco, Recife-PE 50.740-530, Brazil. H. D. Dulman is with Adelphi Technology, Inc., Palo Alto, CA 94306 USA. D. E. Doggett is with Opto-mystic, Sunnyvale, CA 94086 USA. R. H. Pantell is with the Electrical Engineering Department, Stanford University, Stanford, CA 94305 USA. Publisher Item Identifier S 0018-9456(98)05461-8.

These effects turn out to play an important role in choosing design parameters. The work reported herein describes experimental studies conducted on fiber probes employing SPR as the sensor transducing mechanism. A theoretical model has also been developed to take into account the coupling of the guided mode to the SP oscillation as a route to establishing proper design parameters for the construction of highly sensitive fiber probes for gas sensing applications.

II. EXPERIMENTAL APPARATUS The metallized optical fiber configuration along with the experimental arrangement used to observe SPR effects is illustrated in Fig. 1. A polarizer is used to adjust the polarization state of the laser beam which is focused to a single mode fiber by use of lens L1. Lens L2 focuses a portion of the input beam to photodetector PD1 to allow obtaining a reference signal. The portion of light directed to the single mode fiber, mounted on a thin capillary tube, launches the fundamental mode that propagates along the fiber until reaching the polished end, coated with a 50 nm thick gold film, the latter used to allow coupling to an SP oscillation on the exposed surface. The polished portion of the fiber is illustrated in more detail in the expanded view shown in Fig. 1. The fiber has a numerical aperture of 0.12 and supports a single mode of nm. The core propagation in the wavelength range m and m and cladding have diameters of respectively. The cylindrical cladding surface next to the fiber tip is coated with a partially reflecting aluminum film to enable part of the beam diffracted out of the fiber core to couple back to the fiber input after a secondary reflection on the shown in the inset of gold surface. The polishing angle Fig. 1, has a value close to that required for the onset of SP oscillations which is approximately 45 , for the case of a silica-gold-air multilayer configuration [2]. The diffracted light pattern partially transmitted through the aluminum film is recorded by use of a 512-element photodiode array coupled to a Metrabyte, 12-bit A/D data acquisition board with 100 kHz maximum sampling rate, slotted into a 486-type computer. To carry out the tasks of data analysis and processing, we employ menu- and mouse-driven software with capabilities of on-screen display of the light intensity distribution with added features of data recording and filtering, signal magnification, and averaging with a user defined number of samples.

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FONTANA et al,: SURFACE PLASMON RESONANCE ON A SINGLE MODE OPTICAL FIBER

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Fig. 1. Experimental arrangement to observe SPR on a single mode fiber.

Because excitation of SP oscillations requires an E-field component perpendicular to the gold surface [1], the analyzer shown in Fig. 1 has to be adjusted for proper selection of the polarization state of the diffracted beam. Adjustment of the polarization state of the input beam can also provide the required E-field orientation on the gold surface, and this can be used to identify the resonance effect from the back-coupled light recorded by photodetector PD2 [6]. III. EXPERIMENTAL RESULTS Adjustment of the analyzer axis allowed selecting the transverse magnetic (TM) and transverse electric (TE) portions of the diffracted light out of the optical fiber. In the inset of Fig. 1, a TM guided mode has H- and E-field components perpendicular and parallel to the incidence plane, respectively, the latter containing both the direction along the normal line to the metal surface and that along the fiber longitudinal axis. A TE wave would have field components rotated 90 relative to those of the TM wave. The interesting aspect of early experimental studies with the fiber probe was the existence of a dark strip within the light distribution of the TM diffracted light, an effect that progressively vanished as the analyzer axis was rotated toward the direction corresponding to that of the TE component. This effect could be visually observed in a dark room and was indicative of the onset of an SP oscillation on the metallized core. Fig. 2 represents the TE and TM diffracted light intensity distributions recorded with the PD array aligned along the largest beam dimension, where the higher degree of light absorption due to the onset of SP oscillations for the TM wave is clearly noticeable. This SPR resonance phenomenon can be isolated and the spatial noise present in the data can be minimized by taking the ratio between TM and TE intensities, as represented in the plot of Fig. 3. Changing the wavelength of the light produces changes in the position of the minimum of the resonance, as shown in Fig. 4. In that plot, the spatial noise contained within the diffracted light distribution was filtered out by using fast Fourier

Fig. 2. Light intensity distribution versus PD array pixel coordinates for TM and TE polarized light after diffraction out of the single mode fiber.

Fig. 3. Ratio between measured intensities of TE and TM polarized light after diffraction out of the fiber.


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Fig. 4. Resonance curves observed in the diffracted light out of a single mode fiber for two wavelengths.

processing of the measured data. The observed variation in the resonance minimum correlates with experimental observations of SPR on a planar prism configuration, exhibiting the appropriate trend of an upward shift of the minimum of the SPR resonance position relative to a downward shift in the driving wavelength [1]. Sensitivity to variations of refractive index next to the exposed metal surface was demonstrated by flowing either helium or propane gases through the fiber tip. Under standard atmospheric conditions, these gases have refractive indexes and relative to air of [7], and therefore should produce opposite shifts in the SPR minimum position upon contact with the metal surface. Noticing that very small changes in the SPR resonance curve would be produced under these conditions, the SPR curve in air was first recorded and stored in a computer file. The same procedure was adopted for measurements taken under flowing gas. From the recorded resonance profiles one can define a differential reflectance (1) and are the angle-dependent TM where light intensity functions obtained under exposure of the metal surface to the specific gas and to the air, respectively. This parameter equals approximately the derivative relative to the outer medium refractive index, except for the fact that it is normalized to the resulting SPR curve for air exposed metal. For an increase in refractive index next to the metal surface, the differential reflectance should be positive (negative) below (above) the SPR minimum position. The reverse is true for a decrease in refractive index next to the metal surface. This effect is illustrated in the plot of Fig. 5, which represents the digitally filtered, differential reflectances obtained for the two gases tested in the experiments. Notice that the propane and helium gases indeed produce the expected behavior of the differential reflectance, as described earlier.

Fig. 5. Relative differential intensity of the TM component of diffracted light after exposure of the metal surface to helium and propane gases.

Given the large responses produced for the measured gases, we can expect to detect refractive index changes down to 10 . That corresponds to the lower limit in detection sensitivity of existing optical methods for measurement of refractive index of gases. Attempts to observe the SPR phenomenon from the measurement of the ratio between orthogonal polarizations detected in the back-coupled light, as reported in the literature [6], were not successful, due to an unmatched value of the for the two probing wavelengths used in polishing angle the experiments. The theoretical model described next allows selecting a proper polishing angle and probing wavelength for observation of this effect. IV. THEORETICAL MODEL The fundamental features associated with the coupling of guided light with SP oscillations in a cylindrical fiber can be obtained from a simpler, planar waveguide model. A guided mode in a slab waveguide is generally regarded to consist of a pair of plane waves reflecting back and forth at the core/cladding interface, each with a well defined propagation angle relative to the longitudinal axis. Because the incidence for observation of the SPR minimum intensity is set angle by the glass-gold-air multilayer structure, this interpretation could lead to the conclusion that a slight deviation of the required angle from the optimum value to provide maximum coupling to the SP oscillation, would frustrate observation of the SPR from the light distribution emanating from the metallized fiber tip. The experimental observation of a spatial SPR resonance cannot be accounted for by this interpretation. Instead, one has to take into account that the guided mode is laterally confined in the waveguide. When the longitudinal waveguide structure is terminated at the polished surface, the transverse beam profile has to be described in terms of a Fourier spectrum. Each Fourier component corresponds to a plane wave striking the metal, each having parallel to the metal surface. a wave-vector component The metal reflection coefficient represents the response of



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the surface to each of the plane wave components and is also a -dependent parameter. Therefore, the metal reflection coefficient, represents a filter function from which the reflected beam, transmitted through the aluminum film, can be obtained. Using this principle, and taking into account the coordinate dependence of field amplitudes and the corresponding dispersion relation for the degenerated TE and TM modes in a weakly guiding slab waveguide, the light intensity distribution transmitted through the cladding surface can be calculated from [8]

(2) with the free-space wavelength and where the fiber core refractive index. Parameters and are the radial distance and diffraction angle shown in the inset is the Fourier transform in the wavevector of Fig. 1, and space of the field amplitude profile within the waveguide. The detailed calculation that led to (2) can be found in [8] and was obtained within the far field approximation [9]. The intensity distribution in (2) exhibits the expected cylindrical wave, dependence of light intensity distribution. The parameters and represent the gold reflectivities for TM and TE polarizations, with the former coefficient exhibiting a resonance when Calculation of the light intensity, coupled back to the fiber input, requires determination of the light distribution on the polished surface after partial reflection from the thin aluminum film, illustrated in the expanded view of Fig. 1. This light distribution is then filtered by the gold reflection function, after which it has to couple to the fundamental mode within the slab waveguide. A coefficient can then be defined to account for this mode coupling effect. The amount of coupling is polarization dependent and can be obtained from [8]

Fig. 6. Diffracted intensity distributions for TM and TE waves at two distinct wavelengths, calculated from (2).

the mode coupling effect, the SP resonance condition observed in the backcoupled light intensity, is critically dependent on the angle accuracy obtained during the fiber polishing process. Fig. 6 shows the plot of the calculated diffracted light distribution for TE and TM waves obtained from (2) for the wavelengths considered in this work. Calculations were carried out using values of the optical constants of silica and gold available in the literature [8]. Notice that the resonance effect is clearly observed for TM waves and is absent for the TE guided mode, as obtained from experimental observations. An examination of the data represented in Fig. 4 indicates that the SPR minimum positions are close to those shown in Fig. 6. Fig. 7 shows the wavelength dependence of the ratio between TM and TE mode intensities that would be observed in the back reflection for two values of the polishing angle as calculated from (3). Also shown in Fig. 7 are the expected SPR curves for a standard prism coupling configuration calculated from the ratio [1]

(3) (7) with

representing a polarization independent parameter

(4) with (5) (6) and representing the aluminum reflection and with coefficients for TM and TE waves, respectively [8]. Parameters and are defined in Fig. 1 and are made to correspond to the single mode fiber core and cladding radii respectively. Due to

on a gold coated for two values of the incidence angle silica prism, having the same refractive index as that of the waveguide core. It is worth noticing that the resonance minima calculated for the prism configuration do not correspond to those calculated from (3). For instance for the case and , SPR in a prism configuration occurs for a nm. However the planar waveguide wavelength approach indicates that to be a point of negligible resonance effect. It can also be observed from Fig. 7 that (3) predicts the existence of double resonances, an effect that does not occur in the conventional prism configuration. The observed differences in resonance positions could be attributed to the mode coupling effects described earlier. The calculated curve for the planar waveguide, as obtained from (3), anticipates that optimum operating conditions for


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in the field of SPR fiber sensors are related to the chemical conditioning of the metal surface of the probe with adsorbent layers to enable achieving specificity when using the device for gas detection. REFERENCES

Fig. 7. Calculated spectral dependence of the relative backcoupling coefficient for the planar waveguide model (wg ); as obtained from (3). The plot also includes curves representing the ratio between reflected TM and TE intensities for the standard prism coupling scheme for SPR observation with a fixed incidence angle ; as obtained from (7).

a fiber with similar characteristics would be achieved for a and a wavelength setting polishing angle nm. Under these conditions it would be possible to design a fiber sensor that could be used in remote gas sensing applications. V. CONCLUSIONS In this work we have investigated SPR effects on a specially designed fiber sensor device. We have made the first-time observation of a spatial SPR by allowing the guided light to diffract out of the fiber probe. The resonance effect was characterized by studying the influences of the probing wavelength and refractive index changes next to a metallized fiber tip. A theoretical model that takes into account coupling of the guided mode to the SP oscillation was employed. The model appropriately describes the experimental measurements. For a preset fiber tip design, although a spatial resonance effect can be observed experimentally, this does not imply that the SPR phenomenon can also be detected in the light back coupled to the fiber input end. The latter is difficult to detect because coupling of the guided mode to the SP oscillation and recoupling of diffracted light back to the fiber, with a secondary interaction with the SP oscillation, are mechanisms that shift the resonance condition relative to that observed in the conventional planar prism configuration. A planar waveguide model can be employed to take into account these mechanisms, and from this model setting parameters for the fiber tip polishing angle and operating wavelength of the device can be obtained to allow optimum sensor operation. Future work with this fiber device will include experimental characterization of the wavelength dependence of back-coupled light to verify the prediction of (3) and as a route to determine the feasibility of using this approach for the development of truly remote and highly sensitive fiber probes. Additional important questions that will be addressed

[1] E. Kretschmann, “Die Bestimmung Optischer Konstanten von Metallen durch Anregung von Oberflächenplasmaschwingungen,” Z. Physik, vol. 241, pp. 313–324, 1971. [2] W. P. Chen and J. M. Chen, “Use of surface plasma waves for determination of the thickness and optical constants of thin metallic films,” J. Opt. Soc. Amer., vol. 71, pp. 189–191, Feb. 1981. [3] E. Fontana, “Analysis of optical surfaces by means of surface plasmon spectroscopy,” IEEE Trans. Instrum. Meas., vol. 45, pp. 399–405, Apr. 1996. [4] M. T. Flanagan and R. H. Pantell, “Surface plasmon resonance and immunosensors,” Electron. Lett., vol. 20, no. 23, pp. 968–970, Nov. 1984. [5] E. Fontana, “Surface plasma wave applications,” Ph.D. dissertation, Dep. Elect. Eng., Stanford Univ., Stanford, CA, 1989. [6] L. de Maria, M. Martinelli, and G. Vegetti, “Fiber-optic sensor based on surface plasmon interrogation,” Sens. Actuators B, vol. 12, no. 3, pp. 221–223, Apr. 1993. [7] E. Hecht, Optics, 2nd ed. Reading, MA: Addison-Wesley, 1990, p. 56. [8] E. Fontana, “Diffraction and backcoupling of surface plasmon oscillations in a planar waveguide,” Proc. 1997 SBMO/IEEE MTT-S Int. Microwave and Optoelectronics Conf., vol. 2, Aug. 1997, pp. 477–482. [9] A. E. Siegman, Lasers, 1st ed. Mill Valley, CA: University Science, 1986, p. 665.

Eduardo Fontana was born in Rio de Janeiro, Brazil, in 1957. He received the B.Sc. degree in electrical engineering in 1980, and the M.Sc. degree in physics in 1983, both from Federal University of Pernambuco, Recife, Brazil. He received the Ph.D. degree in electrical engineering in 1989 from Stanford University, Stanford, CA. He is presently a Professor at the Electronics and Systems Department at the Federal University of Pernambuco. His current research activities concern use of surface plasmon spectroscopy in thin-film technology, integrated optics devices, and in the development of optical fiber sensors.

Hector Daniel Dulman (M’91) received the “Ingeniero Industrial-opcion: Electronica” degree and the “opcion: Electrica” degree from the University of Uruguay, Montevideo, in 1982 and 1983, respectively. He received the M.Sc. degree in electrical engineering from the University of Arizona, Tucson, in 1986 and the Ph.D. degree in electrical engineering from Stanford University, Stanford, CA, in 1992. He is currently a Senior Scientist at Adelphi Technology, Palo Alto, CA. He is analyzing sensor designs using surface plasmon spectroscopy and optical fibers and their possible applications, in particular for chemical and physical sensors. His previous research areas have included thin films, dielectric waveguides, silicon light emitting diodes and detectors, strained layer superlattices, interaction of relativistic particles and matter, free electron lasers and high-voltage/power systems. Dr. Dulman is a member of the Optical Society of America.



David E. Doggett received the B.Sc. degree in mathematics (minor in physics) from Iowa State University, Ames, in 1973. He worked for Noctua, Inc., where he participated in the development of a diode laser based, two-axis, backscatter, laser Doppler anemometer. At Benson-Varian, Inc. he participated in the development of a diode laser based, holographically scanned, laser printer. He then participated in a startup company, Synergy Computer Graphics, Inc., to produce a full color, web fed, electrostatic color printer/plotter. At Synergy, he was responsible for the successful creation and implementation of a innovative laser scanner. He also created a paper-web registration system that is presently state of the art. He consulted for Laser Devices, Inc. to produce diode laser aimers and pointers which are currently selling at a volume of 10 000 per year. He consulted for Oxigraf Inc. on a diode laser based oxygen meter where he was responsible for sensor development. Currently, he is consulting on a storage phosphor scanner for X-ray applications. He has 16 patents issued. Mr. Doggett served as president of the Optical Society of Northern California from 1985 to 1986.

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Richard Harris Pantell (LF’93) received the B.Sc. and M.Sc. degrees in electrical engineering from the Massachussets Institute of Technology, Cambridge, in 1950 and the Ph.D. degree in electrical engineering from Stanford University, Stanford, CA, in 1954. He is currently a Professor in the Electrical Engineering Department at Stanford University. His research areas have included network synthesis, highpower traveling-wave tubes, millimeter wave generation, ferroelectrics, lasers, nonlinear optics, and photon-electron effects. In addition to his position at Stanford University, he has been employed by the General Electric Company, the University of Illinois, and University College, London, U.K.. He has been an industrial consultant for a number of companies, including IBM and Compagnie Generale d’Electricite. His current research activities concern free-electron lasers and optical fiber amplifiers.
