Optical properties of a laser dye in a solid-state polymeric hostDhiraj K. Sardar and Felipe S. Salinas Citation: Journal of Applied Physics 91, 9598 (2002); doi: 10.1063/1.1476967 View online: http://dx.doi.org/10.1063/1.1476967 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/91/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Investigation of nonlinear optical properties of gold nanograins embedded in indium oxide films by reflection Z-scan using continuous laser J. Appl. Phys. 108, 123109 (2010); 10.1063/1.3524281 Surface-emitting distributed-feedback dye laser of a polymeric multilayer fabricated by spin coating Appl. Phys. Lett. 89, 061123 (2006); 10.1063/1.2336740 Silicon-containing organic matrices as hosts for highly photostable solid-state dye lasers Appl. Phys. Lett. 85, 2160 (2004); 10.1063/1.1793336 High-repetition-rate polymeric solid-state dye lasers pumped by a copper-vapor laser Appl. Phys. Lett. 79, 452 (2001); 10.1063/1.1385585 Optical characterization of a laser dye in a solid state host J. Appl. Phys. 89, 7739 (2001); 10.1063/1.1371949

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Optical properties of a laser dye in a solid-state polymeric hostDhiraj K. Sardar and Felipe S. SalinasDepartment of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas 78249

~Received 21 December 2001; accepted for publication 15 March 2002!

The optical properties of C28H20N2O2B2F4H2O, an organic laser dye embedded in a solid plastichost, have been characterized for a number of laser wavelengths in the visible region. The index ofrefraction of the dye in a plastic host is measured by the conventional method of minimum deviationat these wavelengths. The one-dimensional two flux Kubelka–Munk method based on the diffusionapproximation has been employed to determine the absorption, scattering, and scattering anisotropycoefficients of the dye in a plastic host from the measurements of total diffuse transmittance, totaldiffuse reflectance, and collimated transmittance using double-integrating spheres. The totalattenuation coefficients obtained by this method have been compared with those determined by thecollimated transmission and from the total attenuation measurement on a Cary-14 spectrophotmeter.© 2002 American Institute of Physics.@DOI: 10.1063/1.1476967#

I. INTRODUCTION

In recent years, there have been significant advances inthe syntheses of a number of new organic dyes that havepotential for tunable solid-state laser systems. These dyes arefound to possess high quantum fluorescence yield and a re-duced triplet–triplet absorption in their fluorescence and la-ser spectral range.1–3 Owing to numerous advantages, a greatdeal of effort has been made to synthesize solid-state hostsfor these dyes. Polymeric plastics have been a primarychoice for the host of the laser dyes, because they can readilyform concentrated and uniform solutions with dyes. They arealso hydrolytically and thermally stable, and resistant tomany solvents such as alcohols and acetone. The high-damage-threshold polymer materials have also played a sig-nificant role in the advancement of solid-state dye laser sys-tems. The polymeric plastics have been found to have opticalproperties suitable for becoming potential laser hosts for theorganic dyes. Hermes and co-workers have shown that sev-eral of the pyrromethene-BF2 dyes doped in solid-state poly-meric hosts exhibited lasing properties with significantlyhigh slope efficiencies.4,5

In this article, we present an in-depth characterizationof optical properties of an organic laser dye,C28H20N2O2B2F4H2O. The quantum fluorescence yield ofthis dye is over 90%, which is comparable with those of themajority of conventional laser dyes. Owing to the complexnature of organic dye molecules doped in plastic host, bothabsorption and scattering must be considered when evaluat-ing laser and optical properties of organic dyes.

Although a single measurement of the total transmissionthrough a sample of known thickness provides an attenuationcoefficient based on the Beer’s law of exponential decay, it isimpossible to separate the loss due to absorption from theloss due to scattering. Therefore, the absorption coefficientmeasured by the Cary-14 spectrophotometer will be referredto as the total attenuation coefficient. This problem has beenresolved by the one-dimensional, two-flux Kubelka–Munkmodel6 which has been widely used to determine the absorp-

tion and scattering coefficients of turbid media provided thescattering is significantly dominant over the absorption. Inthe past, researchers have used the diffusion approximationto study turbid media.7–9 Most notably, following theKubelka–Munk model and diffusion approximation, an ex-cellent experimental method has been described by Van Ge-mert et al. for determining the absorption and scattering co-efficients and scattering anisotropy coefficient.10,11

The Kubelka–Munk coefficients,K and S, can be ex-pressed in terms of the absorption coefficient (ma) and thescattering coefficient (ms) in the following forms:

K52ma and S5 34 ~12g!ms2

14 ma , ~1!

where g is the scattering anisotropy coefficient, defined asthe average cosine of the scattering angle. These coefficientscan be expressed in terms of sample thicknesst, diffuse re-flectance (Rd), and diffuse transmittance (Td) as follows.12

S51

btlnF12Rd~a2b!

TdG and K5S~a21!, ~2!

where

a5S 11Rd22Td

2

2RdD and b5Aa221. ~3!

The collimated transmittance (Tc) can be measured byplacing the detector attached to the integrating sphere about2 m from the sample. In this measurement, a narrow colli-mated beam is passed through the sample in such a way thatany photons that are scattered off the sample are not detectedby the detector. The collimated transmittance can be writtenin terms of the absorption and scattering coefficients follow-ing the Beer’s law:

2 ln@Tc#5~ma1ms!t, ~4!

where t is the sample thickness and measured in cm. Bycombining Eqs.~1!–~3!, we can solve for the values ofma ,ms , andg.

JOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 12 15 JUNE 2002

95980021-8979/2002/91(12)/9598/5/$19.00 © 2002 American Institute of Physics

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II. MATERIALS AND METHODS

The organic laser dye C28H20N2O2B2F4H2O used in thisstudy has been synthesized by Professor Joseph Boyer of theChemistry Department at the University of New Orleans.The molecular weight of the dye is 531.6. The dye powderhas a deep violet color and is soluble in dioxane and water.

A. Solid-state dye material preparation

The dye solution was prepared from a 431024 M con-centration in 2-hydroxyethyl methacrylate~also known asHEMA!. A portion of the solution was placed in test tube of1 cm diameter; 100 mg of benzoyl peroxide, an oxidizingagent, was added. The benzoyl peroxide is used to cure theHEMA, which initiates the free radical polymerization. Theprepared solution was heated in a temperature-controlledoven where the temperature was gradually raised to 120 °Cin 120 min. The oven temperature of 120 °C was maintainedfor about 10 min; the oven was then turned off and thesample was left over night for slow cooling. The resultingmaterial was a greenish, solid cylinder. It was then cut intodisks and prisms that were then polished for optical measure-ments.

B. Index of refraction measurement

The index of refraction of the dye doped in solid plasticwas measured by the method of minimum deviation using apolished prism that was cut from a solid cylindrical sample.The apex angle was measured to be 37.5°. In this procedure,the solid prism was firmly mounted at the center of an opticalgoniometer table. By rotating the prism on the optical table,the minimum deviation was observed. Once taking the mea-surements of the distances between the prism and the projec-tion screen, and also the distance between the location of theincident beam and its location at the minimum angle of de-viation, the angle of minimum deviation was determined us-ing simple trigonometry. The values of the apex angle andthe angle of minimum deviation were used to determine theindex of refraction from

n5

sinS a1dm

2 DsinS a

2 D , ~5!

wherea is the apex angle of the prism anddm is the angle ofminimum deviation. This method was chosen for its simplic-ity and accuracy. The index of refraction was measured at anumber of wavelengths. These values are listed in Table I.The experimental details for measuring index of refractioncan be found in Refs. 13 and 14.

These experimental values ofn were subsequently least-squares fitted to the 1 pole Sellmeier’s dispersion equation

n2~l!511Sl2

l22l02. ~6!

The values of the constants,S and l0 , obtained from theleast-squares fit of the experimental data to Eq.~6!, werethen used to recalculate the values of the index of refractionat all wavelengths.

C. Scattering anisotropy coefficient measurement

Using an independent experimental technique, the scat-tering anisotropy coefficientg of the dye in plastic host canalso be obtained from the measurements of scattered lightintensities~I! at various scattering angles~u! using a circulargoniometer table with a diameter of 20 cm. The sampleplaced vertically at the center of the goniometer table wasonly 0.51 cm thick and 1 cm in diameter. Also, the power ofincident laser was kept at less than 1 mW throughout theexperiment. Based on these experimental parameters, thescattering anisotropy coefficientg can be approximated bythe average cosine of the scattering angle according to

g5( i~cosu i !I i

( i I i, ~7!

where the sums are over all values~i! of the scattering anglesand intensities.

The scattering anisotropy factorg was determined byirradiating the disk-shaped sample placed at the center of acircular goniometer table. A He–Ne laser~Uniphase model1101P! with a power of 1 mW and the beam diameter of 2mm was used to excite the sample and an Oriel model 70680photomultiplier tube~PMT! mounted at the edge of the tableas shown in Fig. 1 was used to measure the scattered light.The laser beam was aligned at a 90° angle with respect to theplane of the sample, and the PMT was attached to an adjust-able pointer which could be rotated around the table for mea-suring the scattered intensities at different angles. The scat-tered light intensity was measured between 0° and 180° at anincrement of 1° from 0° to 10° of the scattering angle, and atan increments of 5° from then on. Since the scattered lightintensity can also be in the backward direction, the measure-ments between 170° and 180° were made at 1° increments aswell. Using the values obtained, the scattering anisotropicfactor ~g! was calculated and found to be 0.95. The measure-ment was repeated three times and the values ofg werefound to vary less than 5%.

TABLE I. The wavelength dependent index of refraction ofC28H20N2O2B2F4H2O dye in a plastic host. Sellmeier coefficients:S51.301 andl05152.793 nm.

Wavelength Index of refraction

l ~nm! nexp ncalc

632.8 1.547 1.543575.0 1.549 1.549532.0 1.551 1.555514.5 1.554 1.558501.7 1.559 1.560488.0 1.564 1.563476.5 1.570 1.565

9599J. Appl. Phys., Vol. 91, No. 12, 15 June 2002 D. K. Sardar and F. S. Salinas

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D. Total diffuse reflectance and transmittancemeasurements

The total diffuse reflectance and total diffuse transmit-tance measurements were taken using double-integratingspheres~Oriel model 71400!. The prepared solid sample wasplaced in a sample holder mounted in between the twospheres. The measurements were performed on the sample atdifferent wavelengths. The light sources used for these mea-surements were an argon ion laser~476.5, 488.0, 501.07, and514.5 nm!, Nd:YAG ~second harmonic: 532.0 nm!, andHe–Ne laser~632.8 nm!. The detailed specifications of thelasers employed in this study can be found in Ref. 15.

The schematic of the experimental setup employed tomeasure the total diffuse reflectance and total diffuse trans-mittance is shown in Fig. 2. The laser was directed into theentrance port A of the first integrating sphere whose exit portC is equipped with the sample. The port C is coupled withthe entrance port of the second integrating sphere. The trans-mission from the sample was collected in to the second in-tegrating sphere whose exit port B was capped with a reflec-tive surface identical to that of the interior of the integratingsphere. The reflecting baffles within the spheres shielded thePMT’s from direct photons from the sample. The diametersof the spheres were 6 in. and each of the ports had a diameter

of 1 in. The reflected and transmitted light intensities weredetected by the PMT’s attached to the respective measuringports. The PMT’s were powered by a high voltage powersupply ~Bertan, model 215!. The signals from the PMT’swere taken to digital multimeters~Fluke, Series II!. The mea-sured light intensities were then utilized to determine thetotal diffuse reflectanceRd and total diffuse transmittanceTd

by

Rd5Xr2Y1

Z2Y1and Td5

Xt2Y2

Z2Y2, ~8!

whereXr is the intensity detected by the PMT-1,Xt is theintensity detected by the PMT-2 with a reflective surface atB, Z is the intensity detected by the PMT-2 with no sample atC and a reflective surface at B,Y1 andY2 are the correctionfactors measured by the PMT-1 and PMT-2, respectively,with no sample at C and no reflective surface at B. Thelinearity of the PMT’s was checked and found to be linearover the entire range of visible wavelengths of our interest.

E. Collimated transmittance measurement

The unscattered collimated transmittanceTc was mea-sured to determine the total attenuation coefficient. The col-limated laser beam intensities were measured by placing theintegrating sphere approximately 2 m from the sample sothat the photons scattered off the sample would be preventedfrom entering the small aperture~approximately 3 mm indiameter! at A. The sample was aligned at 90° relative to theincident beam for all measurements taken. The collimatedtransmittanceTc was calculated by

Tc5Xc

Zc, ~9!

whereXc is the collimated light intensity andZc is the inci-dent light intensity.

According to the Beer’s Law, the total attenuation coef-ficient (m t5ma1ms) can be determined from

FIG. 1. Schematic of the experimental setup for the measurement ofg; DMis the digital multimeter, PS is the power supply, and PMT is the photomul-tiplier tube.

FIG. 2. Schematic of the experimental setup for the measurements ofTd andRd .

FIG. 3. Room-temperature total attenuation spectrum ofC28H20N2O2B2F4H2O dye in plastic host, sample thickness was 0.51 cm.

9600 J. Appl. Phys., Vol. 91, No. 12, 15 June 2002 D. K. Sardar and F. S. Salinas

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m t5ln~1/Tc!

t, ~10!

wheret is the physical thickness of the sample and is mea-sured in cm.

F. Total attenuation and fluorescence measurements

The room-temperature total attenuation spectrum rang-ing from 450 to 750 nm was taken on the solid plastic hostcontaining C28H20N2O2B2F4H2O dye using a Cary-14 spec-trophotometer upgraded by On-Line Instrument Systems.The spectral bandwidth was set at 0.5 nm, and the instrumentwas internally calibrated to an accuracy of 0.3 nm. Thesample thickness was 0.51 cm. The total attenuation spec-trum displayed in Fig. 3 shows two strong broad band ab-sorptions peaking at 580 and 630 nm. The attenuation coef-ficients of these peaks are about 6 cm21 and 7 cm21,respectively.

The room-temperature fluorescence spectrum rangingfrom 600 to 800 nm was taken on the solid-state dye sampleusing a Photon Technology International fluorometer and ispresented in Fig. 4. The sample was excited with 560 nm andthe spectral resolution was 0.25 nm. A personal computerequipped with a software~ALPHASCAN! was used to controlthe fluorometer and collect and analyze the data. The spectrawere analyzed and plotted by using the computer softwarepackageSIGMA PLOT.

III. RESULTS AND DISCUSSION

The indices of refraction~n! of the laser dye in solidplastic host were measured at the wavelengths 476.5, 488.0,501.7, 514.5, 532.0, 575.0, and 632.8 nm; the index of re-fraction decreases with increasing wavelength. The index ofrefraction data were least-square fitted to the Sellmeier’sequation. The experimental and theoretical values ofn aregiven in Table I; the experimental values are plotted in Fig.5. The root-mean-square deviation of then values providedin Table I has been found to be less than 1%.

The total diffuse reflectance, diffuse transmittance, andcollimated transmittance were measured on a solid-state dyesample at 476.5, 488.0, 501.7, 514.5, 532.0, and 632.8 nm.These measurements were repeated three times and the datawere in good agreement to within 5%. These values werethen used to calculate the Kubelka–Munk coefficients whichwere used to obtain the absorption and scattering coeffi-cients. These values are tabulated in Table II. The total at-tenuation coefficients are also determined from the colli-mated transmissionTc and given in Table III. The totalattenuation coefficients at these wavelengths were obtainedfrom the Cary-14 measurement are also given in Table III forcomparison. A visual comparison can be seen in Fig. 6.These values were found to be in good agreement. The scat-tering anisotropy factorg was determined to be approxi-mately 0.95; this clearly indicates that the scattering in the

FIG. 4. Room-temperature fluorescence spectrum of C28H20N2O2B2F4H2Odye in plastic host excited at 560 nm.

FIG. 5. Index of refraction at various wavelengths for C28H20N2O2B2F4H2Odye in plastic host.

TABLE II. The wavelength dependent absorption coefficient (ma), scattering coefficient (ms), and total attenuation coefficient (m t) as determined by theKubelka–Munk method using the measured diffuse reflectance (Rd) and diffuse transmittance (Td) for C28H20N2O2B2F4H2O dye in a plastic host.

Wavelength Experimental Kubelka–Munk

l~nm! Rd Td

K~cm21!

S~cm21!

ma

~cm21!ms

~cm21!m t

~cm21!

632.8 0.006 0.064 7.228 0.028 3.614 3.189 6.803532.0 1.402 1.630 2.606 1.030 1.303 3.449 4.752514.5 0.889 3.151 1.266 0.078 0.633 3.014 3.646501.7 0.506 3.470 1.373 0.009 0.686 3.028 3.714488.0 0.679 3.510 1.254 0.162 0.627 3.033 3.660476.5 0.498 3.489 1.429 0.037 0.715 3.020 3.735

9601J. Appl. Phys., Vol. 91, No. 12, 15 June 2002 D. K. Sardar and F. S. Salinas

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plastic host containing C28H20N2O2B2F4H2O dye moleculesis highly forward scattering.

The anisotropy factorg was also determined using theKubelka–Munk method, and was found to vary approxi-mately from 0.7 to 0.9. The measured value is in agreement

with the calculated values within the uncertainty of experi-mental measurements.

The scattering coefficient at all wavelengths below 632.8nm was found to be significantly higher than the absorptioncoefficient. The loss due to scattering in laser design is aserious drawback. It is therefore critical to determine the lossof the pump energy by scattering in any potential laser me-dium. Therefore, further studies on more suitable polymerichosts doped with this dye should be done.

The room-temperature fluorescence spectrum shows twobroad bands peaking at around 645 and 695 nm. Because ofthe strong and broad emission ranging from 600 to 750 nm,this dye in plastic host would be a likely candidate for tun-able solid state laser device. The values of the optical prop-erties reported in this article are of significant importance tothe laser community and can be considerably useful for thedesign of the tunable solid state dye laser system.

ACKNOWLEDGMENTS

The authors would like to thank Dr. Joseph Boyer ofUniversity of New Orleans for providing us with the sampleused in this study. This research was supported by the Na-tional Science Foundation Grant No. DMR 0099479.

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TABLE III. A comparison of the total attenuation coefficients obtained fromthe Kubelka–Munk method, collimated transmittance, and the Cary-14 mea-surements for C28H20N2O2B2F4H2O dye in a plastic host.

Wavelength Kubelka–Munk Collimated transmittance Cary-14

l~nm!

m t

~cm21!m t

~cm21!m t

~cm21!

632.8 6.803 6.900 6.959532.0 4.752 4.262 4.290514.5 3.646 3.833 3.840501.7 3.714 3.664 3.764488.0 3.660 3.733 3.847476.5 3.735 3.820 3.975

FIG. 6. Comparison of obtained total attenuation coefficients forC28H20N2O2B2F4H2O dye in plastic host obtained from Cary-14 spectropho-tometer, collimated transmission measurements, and Kubelka–Munkmethod.

9602 J. Appl. Phys., Vol. 91, No. 12, 15 June 2002 D. K. Sardar and F. S. Salinas

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