Using JPF1 format

JOURNAL OF APPLIED PHYSICS

VOLUME 89, NUMBER 12

15 JUNE 2001

Optical characterization of a laser dye in a solid state host Dhiraj K. Sardar,a) Raylon M. Yow, and Michael L. Mayo Division of Earth and Physical Sciences, The University of Texas at San Antonio, San Antonio, Texas 78249

共Received 11 January 2001; accepted for publication 20 March 2001兲 The optical properties of C18H16N2O2B2F4H2O, an organic laser dye embedded in solid plastic host, have been characterized for a number of laser wavelengths in the visible region. The index of refraction of the dye in plastic host is measured by the conventional method of minimum deviation at these wavelengths. The inverse adding doubling method based on the diffusion approximation and radiative transport theory have been employed to determine the absorption, scattering, and scattering anisotropy coefficients of the dye in plastic host from the measurements of total diffuse transmittance, total diffuse reflectance, and collimated transmittance using an integrating sphere. The total attenuation coefficients obtained by this method have been compared with those determined by the collimated transmission and from the total attenuation measurement. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1371949兴

I. INTRODUCTION

molecules due to their inherent inhomogeneity and irregular shape. Nevertheless, by assuming homogeneity and regular geometry of the medium, an estimate of light intensity distribution can be obtained by solving the radiative transport equation

In recent years, significant advances have been made in the syntheses of a number of new organic dyes that have the potential for tunable solid state laser systems. These dyes are found to possess high quantum fluorescence yield and a reduced triplet–triplet absorption in their fluorescence and laser spectral range.1–4 Owing to numerous advantages, a great deal of effort have been made to synthesize solid state hosts for these dyes. The polymeric plastics have been the primary choice for the host of the laser dyes, because they can readily form concentrated and uniform solutions with dyes. In addition, they are hydrolytically and thermally stable, and also resistant to many solvents such as alcohols and acetone. The high-damage-threshold polymer materials have also played a significant role in the advancement of solid-state dye laser systems. The polymeric plastics have been found to have optical properties suitable for becoming potential laser hosts for the organic dyes. Hermes et al. have shown that several of the pyrromethene-BF2 dyes doped in polymeric solid-state hosts exhibited lasing properties with significantly high slope efficiencies. The laser oscillations have been achieved in various dyes in solid state matrix.5,6 In this article, we present an in-depth characterization of optical properties of an organic laser dye, C18H16N2O2B2F4H2O. The quantum fluorescence yield of this dye is over 90%, which is comparable with those of the majority of conventional laser dyes. Owing to the complex nature of organic dye molecules doped in plastic host, both absorption and scattering must be considered when evaluating laser and optical properties of organic dyes. This information on optical properties can be obtained from the solution of the Chandrasekhar’s radiative transport equation describing the rate of change in the intensity of a narrow incident light beam as a function of the optical properties of the medium involved.7 This is a difficult problem to solve analytically for complex plastic hosts containing organic dye

dI 共 r,s兲 ⫽⫺ 共 ␮ a ⫹ ␮ s 兲 I 共 r,s兲 ds ⫹

冕

4␲

p 共 s,s⬘ 兲 I 共 r,s⬘ 兲 d⍀ ⬘ ,

共1兲

where I(r,s) is the intensity per unit solid angle at target location r in the direction s 共s is the directional unit vector兲, ␮ a and ␮ s are the absorption coefficient and scattering coefficient, respectively, of the medium; p(s,s⬘ ) is the phase function, representing scattering contribution from the direction s⬘ to s, and ⍀⬘ is the solid angle. The first term on the right-hand side of Eq. 共1兲 represents the loss in I(r,s) per unit length in direction s due to absorption and scattering. The second term denotes the gain in I(r,s) per unit length in direction s due to scattering from other scattered light I(r,s⬘ )d⍀ ⬘ 共i.e., light intensity confined in the elemental solid angle d⍀ ⬘ 兲 from direction s⬘ . The functional form of the phase function in biological media is usually unknown. In many practical applications, however, the following Henyey–Greenstein8 formula provides a good approximation of the phase function, and, therefore, is used in all calculations of the inverse adding doubling 共IAD兲 method employed in the present study p共 ␯ 兲⫽

1⫺g 2 1 , 4 ␲ 共 1⫹g 2 ⫺2g ␯ 兲 3/2

共2兲

where v is the cosine of the angle between s and s ⬘ . The Henyey–Greenstein phase function depends only on the scattering anisotropy coefficient g, which is defined as the mean cosine of the scattering angle

a兲

Electronic mail: [email protected]

0021-8979/2001/89(12)/7739/6/$18.00

␮ a⫹ ␮ s 4␲

7739

© 2001 American Institute of Physics

Downloaded 15 May 2003 to 129.115.60.15. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp

7740

g⫽

J. Appl. Phys., Vol. 89, No. 12, 15 June 2001

冕冕

4␲

Sardar, Yow, and Mayo

where t is the physical thickness of the sample and is measured in cm.

p 共 ␯ 兲 ␯ d⍀ ⬘

4␲

共3兲

. p 共 ␯ 兲 d⍀ ⬘

II. MATERIALS AND METHODS

The value of g ranges from ⫺1 for complete backward scattering to zero for the absolute isotropic scattering, to 1 for the complete forward scattering. Although the radiative transport theory gives a more adequate description of the distribution of photon intensity in the turbid medium than does any other model, the general analytical solution is not known yet. Approximate solutions are only available for such restricted conditions as uniform irradiation, or when either absorption or scattering strongly dominates. Although the general solution is not available, it has been possible in recent years to obtain an elaborate computational solution of the transport equation. In order to solve Eq. 共1兲, a knowledge of absorption and scattering coefficients, and scattering phase factor 共or anisotropy coefficient兲, is needed. Therefore, appropriate experimental methods are necessary to measure these optical parameters. As an example, although a single measurement of the total transmission through a sample of known thickness provides an attenuation coefficient for the Lambert–Beer law of exponential decay, it is impossible to separate the loss due to absorption from the loss due to scattering. In this article, therefore, the absorption coefficient measured by the Cary 14 spectrophotometer will be referred to as the total attenuation coefficient. This problem, to some extent, had been resolved by the one-dimensional, two-flux Kubelka–Munk model9 which has been widely used to determine the absorption and scattering coefficients of turbid media, provided the scattering is significantly dominant over the absorption. In the past, researchers have applied the diffusion approximation to the transport equation to study the turbid media.10–12 Most notably, following the Kubelka–Munk model and diffusion approximation, an excellent experimental method has been described by Van Gemert et al. for determining the absorption and scattering coefficients and scattering anisotropy factor.13,14 Unfortunately, the diffusion approximation, coupled with the Kubelka–Munk method, is valid only when the absorption coefficient is negligibly small compared to the scattering coefficient of the turbid medium under investigation. The absorption coefficient of organic dye, particularly in liquid medium, is nonetheless significant in the wavelength range of our interest and cannot be ignored. Therefore, in our study, the IAD method has been employed to determine both the absorption and scattering coefficients. In recent years, the IAD method15 has been used for determining such fundamental optical properties as absorption, scattering, and scattering anisotropy coefficients of biological samples. This method provides by far the most accurate estimates of optical properties than do any other models previously used. Two dimensionless quantities used in the entire process of IAD are albedo, a, and optical depth, ␶, which are defined as a⫽ ␮ s / 共 ␮ s ⫹ ␮ a 兲 ,

and

␶ ⫽t 共 ␮ s ⫹ ␮ a 兲 ,

共4兲

The organic laser dye C18H16N2O2B2F4H2O used in this study has been synthesized by Professor Joseph Boyer of the Chemistry Department at the University of New Orleans. The molecular weight of the dye is 407.6. The dye powder has a blue-violet color and is soluble in dioxane and water. A. Solid state dye material preparation

The dye solution was prepared from a 4⫻10⫺4 M concentration in 2-hydroxyethyl methacrylate 共also known as HEMA兲. To the solution, 100 mg of benzoyl peroxide, an oxidizing agent, was added. The benzoyl peroxide is used to cure the HEMA, which initiates the free radical polymerization. The solution was then poured into a reservoir made from two glass slides and epoxy, sealed and heated in a computer-controlled furnace where the temperature was gradually raised to 120 °C in 120 min. The furnace was turned off and the sample was left in the furnace over night for slow cooling of the sample. The result was a 1-mm-thick blue-violet solid plastic in between slides. B. Index of refraction

The index of refraction of the dye in solid plastic was measured by the method of minimum deviation using a hollow quartz prism in which the dye solution was poured in and solidified by the same procedure described in the previous Sec. A. The 60° hollow prism was made of 1 cm ⫻2 cm quartz slides obtained from the NSG Precision cell, Inc., 共Farmingdale, New York兲. In this procedure, the hollow prism was firmly mounted at the center of an optical spectrometer table. This method was chosen for its simplicity and accuracy. The experimental details for measuring index of refraction can be found in Refs. 16 and 17. C. Scattering anistropy coefficient

Using an independent experimental technique, the scattering anisotropy coefficient g of the dye in plastic host can also be obtained from the measurements of scattered light intensities 共I兲 at various scattering angles 共␪兲 using a goniometer table. The scattering anisotropy coefficient g is given by the average cosine of the scattering angle according to the following expression:

g⫽

兺i 共 cos ␪ i 兲 I i 兺i I i

,

共5兲

where the sums are over all values, i, of the scattering angles and intensities. The anisotropy factor obtained by this measurement was compared with that from the IAD method. The scattering anisotropy factor was determined by irradiating the solid state dye sample placed at the center of a circular goniometer table with the laser beam and a photomultiplier tube 共PMT兲 mounted at the edge of the table as




7741

FIG. 1. Schematic of the experimental setup for the measurement of g.

shown in Fig. 1. The sample holder was affixed to the center of the goniometer table. The laser beam was aligned at a 90° angle with respect to the glass plates containing the sample, and the PMT was attached to an adjustable pointer which could be rotated around the table for measuring the scattered intensities at different angles. The scattered light intensity was measured between 0° and 180° at an increment of 1° from 0° to 10° of scattering angle, and an increment of 5° above 10° of scattering angle. Using these values the scattering anistropic factor, g, was calculated and found to be 0.99. The measurement was repeated three times and the values of g were found to vary less than 5%.

D. Total diffuse reflectance and transmittance

The total diffuse reflectance and total diffuse transmittance measurements were taken using an integrating sphere 共Oriel model 71 400兲. The prepared sample was placed in a holder mounted on one of the ports of the integrating sphere. The measurements were performed on the sample at different wavelengths. The light sources used for these measurements were an argon ion laser 共488 and 515 nm兲, Nd:yttrium– aluminum–garnet 共YAG兲共second harmonic: 532 nm兲, and He–Ne laser 共633 nm兲. The multiline argon ion laser model 2025 from Spectra Physics 共Mountain View, CA兲 was capable of providing wavelengths ranging from 515 to 488 nm with an average output power of 1 to 2 W. The argon ion laser beam diameter at 1/e 2 was 1.25 mm and beam divergence was 0.70 mrad at 515 nm. The diode laser-pumped Nd:YAG laser model LCS-DTL-112A from Laser Compact 共Moscow, Russia兲 provided a second harmonic at 532 nm with the help of a nonlinear frequency doubling crystal. The continuous wave output of the laser at 532 nm had a maximum average power of 20 mW. The TEM00 laser beam diameter at 1/e 2 was less than 1.8 mm and beam divergence was about 1.0 mrad at 532 nm. The average output power of

the He–Ne laser model 125A from Spectra Physics was 50 mW, the beam diameter at 1/e 2 was 2 mm, and the beam divergence was 0.70 mrad at 633 nm. The schematic of the experimental setup employed to measure the total diffuse reflectance and total diffuse transmittance is shown in Fig. 2. The laser was directed into the entrance port A of the integrating sphere, whose exit port is either open or capped with either the sample or a reflective surface identical to that of the interior surface of the integrating sphere depending on the measurement taken. The diameter of the sphere was 6 in. and each port had a diameter of 1 in. Light leaving the sample was reflected multiple times off the inner surfaces of the sphere. A reflecting baffle within the sphere shielded the PMT from direct emission from the sample. The reflected and transmitted light intensities were detected by a PMT 共ORIEL model 7068兲 attached to the measuring port. The PMT was powered by a high voltage power supply 共Bertan, model 215兲. The signal from the PMT was taken to a digital multimeter 共Emco, DMR-2322兲. The measured light intensities were then utilized to determine the total diffuse reflectance R d and total diffuse transmittance T d by the following expressions: R d⫽

X r ⫺Y Z⫺Y

共6兲

T d⫽

X t ⫺Y , Z⫺Y

共7兲

and

where X r is the intensity detected by the PMT with the sample at B, X t is the intensity detected by the PMT with the sample at A and a reflective surface at B, Z is the intensity detected by the PMT with no sample at A and a reflective surface at B, and Y is the correction factor measured by the PMT with no sample at A or reflective surface at B.


7742



FIG. 2. Schematic of the experimental setup for the measurements of T d and R d .

E. Collimated transmittance

The unscattered collimated transmittance T c was measured to determine the total attenuation coefficient. The collimated laser beam intensities were measured by placing the integrating sphere approximately 3 m from the sample so that the photons scattered off the sample would be prevented from entering the small aperture 共approximately 3 mm in diameter兲 at A. The sample was aligned at 90° relative to the incident beam and measurements were taken. The collimated transmittance T c was calculated by the relation T c⫽

Xc , Zc

Prahl, van Gemert, and Welch15 to solve the radiative transport equation to obtain optical properties, has been used to determine the absorption and scattering coefficients. In order to solve the radiative transport equation, the IAD algorithm must be supplied with experimentally determined values of the total diffuse reflectance (R d ) and total diffuse transmittance (T d ). The IAD algorithm now chooses the values for two-dimensionless quantities: albedo 共a兲 and the optical depth 共␶兲 defined in Eq. 共4兲, adjusts the value of the scatter-

共8兲

where X c is the collimated light intensity and Z c is the incident light intensity. From the Beer–Lambert law, the total attenuation coefficient can be determined from the following expression:

␮ t⫽

ln共 1/T c 兲 t

共9兲

where t is the physical thickness of the sample and is measured in cm. F. Inverse adding doubling „IAD… method

A numerical algorithm known as inverse adding doubling 共IAD兲 method, which was originally developed by

FIG. 3. Room-temperature total C18H16N2O2B2F4H2O dye in plastic host.

attenuation

spectrum


of



7743

TABLE II. The wavelength dependent total attenuation coefficient obtained from the IAD method, and collimated transmittance (T c ) and Cary 14 spectrophotometer measurements for C18H16N2O2B2F4H2O dye in plastic host.

␮ t 共cm⫺1兲

Wavelength ␭ 共nm兲

IAD

Tc

Cary 14

488 515 532 633

2.60 4.92 5.44 7.83

1.57 4.48 5.54 7.23

1.83 4.97 6.32 8.33

III. RESULTS AND DISCUSSION

FIG. 4. Room-temperature fluorescence spectrum of C18H16N2O2B2F4H2O dye in plastic host.

ing anisotropy factor 共g兲 related to the phase function, and iteratively solves the radiative transport equation until the final results match with the experimental values of R d and T d . The final values of albedo and optical depth obtained from the IAD method are then used to calculate the absorption and scattering coefficients. G. Total attenuation and fluorescence

The room-temperature total attenuation spectrum ranging from 425 to 700 nm was taken on the solid plastic host containing C18H16N2O2B2F4H2O dye using a Cary 14 spectrophotometer modified by On-Line Instrument Systems. The spectral resolution of all measurements was kept at 0.5 nm. The sample thickness was 0.10 cm. The total attenuation spectrum depicted in Fig. 3 shows two broadband absorption peaking at 570 and 620 nm. The attenuation coefficients of both peaks are approximately 11 cm⫺1. The room-temperature fluorescence spectrum ranging from 590 to 800 nm was taken on the solid-state dye sample using a Photon Technology International fluorometer. The sample was excited with 570 nm and the spectral resolution was 0.25 nm. The computer software, Alphascan, was used to control the fluorometer and collect and analyze the data. The fluorescence spectrum, normalized to an arbitrary value of 10, showing two broadbands peaking at 650 and 700 nm, is presented in Fig. 4.

The indices of refraction of the laser dye in solid plastic host were measured at 488, 515, 532, and 633 nm and were found to be approximately 1.37 at all wavelengths. Measurements were repeated three times at all wavelengths, and the values agreed to within 3%. The anisotropic scattering coefficient, g, of the dye in plastic host was determined to be 0.99 from the goniometric measurement. The total diffuse reflectance and diffuse transmittance were measured on the solid state dye sample at 488, 515, 532, and 633 nm. These measurements were repeated three times and the data were in excellent agreement. These values along with the measured value of the index of refraction were input into the IAD program in order to obtain the values of the absorption, scattering, and the scattering anisotropy coefficients. These values are tabulated in Table I. The attenuation coefficients were also determined from the collimated transmission T c and given in Table II. The total attenuation coefficients at 488, 515, 532 and 633 nm as obtained from the Cary 14 measurement are also given in Table II for comparison. These values are in good agreement. The absorption coefficients for the dye in solid plastic medium were found to be much higher than the scattering coefficients at longer wavelengths employed in this study. The scattering anisotropy factor was determined to be approximately 0.99; this clearly indicates that the scattering in the plastic host containing C18H16N2O2B2F4H2O dye molecules is highly forward scattering. The room-temperature fluorescence spectrum shows two broadbands peaking around 650 and 700 nm. The 650 nm peak is about three times stronger than the 700 nm peak. Because of the strong absorption and broad emission, this dye in plastic host would be an excellent candidate for tunable solid state laser device.

TABLE I. The wavelength 共␭兲 dependent absorption coefficient ( ␮ a ), scattering coefficient ( ␮ s ), total attenuation coefficient ( ␮ t ), albedo 共a兲, and optical depth 共␶兲 obtained from the IAD method using the measured diffuse reflectance (R d ), diffuse transmittance (T d ), and collimated transmittance (T c ) for C18H16N2O2B2F4H2O dye in plastic host. Experimental

IAD

Wavelength ␭ 共nm兲

Rd

Td

Tc

a

␶

␮a 共cm⫺1兲

␮s 共cm⫺1兲

␮t 共cm⫺1兲

488 515 532 633

0.0059 0.0049 0.0036 0.0045

0.7741 0.6308 0.5863 0.5053

0.8550 0.6388 0.5744 0.4851

0.3929 0.2668 0.2018 0.2563

0.2596 0.4926 0.5440 0.7822

1.58 3.61 4.34 5.82

1.02 1.31 1.10 2.01

2.60 4.92 5.44 7.83


7744


The loss due to scattering in laser design is a serious drawback. It is therefore critical to determine the loss of the pump energy by scattering in any laser medium. It is worth mentioning here that, although the absorption coefficients are higher than the scattering coefficients in C18H16N2O2B2F4H2O dye, the scattering is nonetheless not negligible. In fact, owing to the complex nature of the organic molecules as well as the plastic polymetric host medium, the scattering in most cases cannot be ignored. The values of the optical properties reported in this article are of significant importance to the laser community. These values can be significantly important for the design of the tunable solid state dye system. ACKNOWLEDGMENTS

The authors would like to thank Dr. Joseph Boyer of the University of New Orleans for providing them with the sample used in this study. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the ACS, for support of this research, Grant No. ACSPRF#33720-B6.

Sardar, Yow, and Mayo 1

T. G. Pavlopoulos, M. Shah, and J. H. Boyer, Opt. Commun. 70, 425 共1989兲. 2 T. G. Pavlopoulos, J. H. Boyer, M. Shah, K. Thangaraj, and M. L. Soong, Appl. Opt. 29, 3885 共1990兲. 3 M. P. O’Neil, Opt. Lett. 18, 37 共1993兲. 4 S. C. Guggenheimer, J. H. Boyer, K. Thangaraj, M. Shah, M. L. Soong, and T. G. Pavlopoulos, Appl. Opt. 32, 3942 共1993兲. 5 D. J. Gettemy, R. E. Hermes, and N. P. Barnes, OSA Proc. Adv. SolidState Lasers 10, 390 共1991兲. 6 R. E. Hermes, J. D. McGrew, C. E. Wiswall, S. Monroe, and M. Kushina, Appl. Phys. Commun. 11, 1 共1992兲. 7 S. Chandrasekhar, Radiative Transfer 共Oxford University Press London, 1960兲. 8 L. G. Henyey and J. L. Greenstein, Astrophys. 93, 70 共1941兲. 9 P. Kubelka, J. Opt. Soc. Am. 38, 488 共1948兲. 10 A. Ishimaru, Wave Propagation and Scattering in Random Media 共Academic, New York, 1978兲, Vol. 1. 11 L. Reynolds, C. C. Johnson, and A. Ishimaru, Appl. Opt. 15, 2059 共1978兲. 12 R. A. J. Groenhuis, H. A. Ferwerda, and J. J. Ten Bosch, Appl. Opt. 22, 2456 共1983兲. 13 M. J. C. van Gemert, A. J. Welch, W. M. Star, and M. Motamedi, Lasers Med. Sci. 2, 295 共1987兲, and references therein. 14 M. J. C. van Gemert and W. M. Star, Lasers Life Sci. 1, 287 共1987兲. 15 S. A. Prahl, M. J. C. van Gemert, and A. J. Welch, Appl. Opt. 32, 559 共1993兲. 16 B. W. Grange, W. H. Stevenson, and R. Viskanta, Appl. Opt. 15, 858 共1976兲. 17 W. Mahmood bin Mat Yunus, Appl. Opt. 28, 4268 共1989兲.


Using JPF1 format - UTSA Department of Physics & Astronomy

Using JPF1 format - UTSA Department of Physics & Astronomy

Suggest Documents