Efficient solid-state dye lasers based on polymer-filled microporous glass

Transcription

1 Efficient solid-state dye lasers based on polymer-filled microporous glass H. R. Aldag,a S. M. Dolotov,b M. F. Koldunov,b Ya. V. Kravchenko,c A. A. Manenkov,c D. P. Pacheco,a A. V. Reznichenko b and G. P. Roskova d a Physical Sciences Inc., Andover MA, USA b R&D Enterprise Optronika, Dolgoprudny, Moscow Region, Russia c General Physics Institute, Russian Academy of Sciences, Moscow, Russia d Institute of Silicates Chemistry, Russian Academy of Sciences, St. Petersburg, Russia Solid State Lasers IX, Richard Scheps, ed., Proceedings of SPIE (2000) This paper was published in Solid State Lasers IX (Proceedings of SPIE 3929) and is made available as an electronic reprint with permission of SPIE. Single print or electronic copies for personal use only are allowed. Systematic or multiple reproduction, distribution to multiple locations though an electronic listserver or other electronic means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are all prohibited. By choosing to view or print this document, you agree to all the provisions of the copyright law protecting it.

2 Efficient solid-state dye lasers based on polymer-filled microporous glass H. R. Aldag, a S. M. Dolotov, b M. F. Koldunov, b Ya. V. Kravchenko, c A. A. Manenkov, c D. P. Pacheco, a A. V. Reznichenko b and G. P. Roskova d a Physical Sciences Inc., Andover MA, USA b R&D Enterprise Optronika, Dolgoprudny, Moscow Region, Russia c General Physics Institute, Russian Academy of Sciences, Moscow, Russia d Institute of Silicates Chemistry, Russian Academy of Sciences, St. Petersburg, Russia ABSTRACT This paper reports on the laser emission properties of Pyrromethene 580, Pyrromethene 597, Pyrromethene 650 and Rhodamine 11B in the novel matrix polymer-filled microporous glass (PFMPG). This host material combines the advantages of an organic environment for the dye with the superior thermooptical and mechanical properties of an inorganic glass. Laser efficiency was measured as a function of pump flux for different dye concentrations, resonator feedback, and locations on the sample. Service life, defined as the number of pulses for the output to drop to the 70% point, was recorded at 5 Hz for the higher dye concentrations. The highest efficiencies were observed for Pyrromethene 597 ( 70%), which had a service life of 60,000 shots at 25 MW/cm 2 and 45,000 shots at 50 MW/cm 2. The longest service life was measured for Rhodamine 11B ( 110,000 pulses at 25 MW/cm 2 ), but this dye had somewhat lower efficiency ( 50-55%). Thermal lensing measurements were made for dye-doped PFMPG and MPMMA, and showed that the lensing is much lower in the hybrid matrix. The agreement with the theoretical modeling is very good. Keywords: solid-state dye laser, MPMMA, microporous glass, pyrromethene dyes, rhodamine 11B, thermal lensing 1. INTRODUCTION Dye lasers utilizing a solid host are very attractive for a wide range of applications, including selective photothermolysis in medicine, remote sensing of atmospheric contaminants and underwater communications. The specific advantages of a solid gain medium has been adequately discussed by a number of authors. 1,2,3,4 Various pyrromethene dyes in modified PMMA (MPMMA) have been quite successful in yielding efficient, long-lived performance. 2,5,6 The primary life-limiting issue is the photochemical stability of the dyes, although recent work on identifying the degradation pathways of impregnated dyes has lead to a better understanding of this phenomenon. 2,7,8,9,10 Good beam quality from polymer-host dye lasers has been addressed through appropriate resonator design. 11,12 Finally, short-pulse (ps) output from a solid-state dye laser has recently been demonstrated. 13 The expectation is that the overall capabilities of solid-state dye lasers will continue to expand. A disadvantage of MPMMA is that bulk organic materials typically have relatively poor thermooptical properties. The relevant host-dependent parameters for repped operation are the temperature sensitivity of the refractive index (dn/dt) and the thermal conductivity (K). 14 In Table 1, we list these quantities for a number of laser hosts. Although the thermooptical figure of merit (TFOM = K / [dn/dt]) for PMMA is relatively low, a filled microporous glass (MPG) shows great promise. Table 1. Thermooptical Figure of Merit (TFOM) for Selected Laser Host Materials for Repped Operation Host 10 5 x dn/dt K TFOM ( o C -1 ) (W/m/ o C) (W/m) PMMA x 10 3 Epoxy-Filled MPG x 10 5 Silicate Glass x 10 5 Phosphate Glass x 10 6 Estimate based on material composition. Kigre Q-88

3 There are several approaches to dealing with the thermal issues. One can, for example, rotate and/or translate the polymer medium to distribute the thermal load over a larger volume. 1,15 This strategy, however, leads to a more complex system, requiring attention to a number of optical and mechanical engineering details. An alternative approach is to utilize a host matrix with improved thermooptical and mechanical properties. In this paper, we report on work in this area. We have extensively investigated the laser performance of a rhodamine dye and several pyrromethene dyes in a relatively new matrix, polymer-filled microporous glass (PFMPG). 16,17,18 This novel host consists of two components, a modified PMMA which contains the impregnated dye, and a porous glass structure. The overall porosity (pore volume) of the glass matrix and the average pore size can be varied to optimize the filling of the pores by the monomer mix. The presence of a continuous glass structure, together with the relatively small pores containing the polymer (typically 1 10 nm), allows the efficient dissipation of heat throughout the structure. An added feature is that the porous structure provides effective filtering of the monomer during the impregnation process. This is a significant factor in improving the damage threshold of the polymer component of the matrix. 19 There has been previous work on the laser properties of dyes in dry and wet porous glass structures. 20,21,22 Efendiev et al. 21 compared the performance of Rhodamine 6G in both types of samples. They found that the laser efficiency in the wet material was 50% (similar to that in an alcohol solution), while the value for the dry version was only 30%, with the difference evidently due to the higher scattering losses in the dry material. Dye photobleaching was relatively severe in the dry case. At a pump intensity of only 7.5 MW/cm 2, half the dye molecules are lost after 10 4 shots. The authors do not report similar data for the wet versions. There is very limited previous work on dyes in MPG filled with a solid material. Dolotov et al. 16 measured singleand multiple-shot damage thresholds for PMMA-filled MPG, bulk PMMA, and MPMMA. No laser emission results were reported in this study. Bermas et al. 18 measured the laser efficiency and service life of Rhodamine C in a microporous glass filled with an epoxy. The efficiency was only 20%, with a shot life of 10 4 pulses. Koldunov et al. 17 reported obtaining laser emission from Rhodamine 6G perchlorate in a microporous glass containing PMMA, but no data were included. An alternative hybrid matrix that has been studied recently is polycom glass, 23,24 which is a porous sol-gel glass filled with PMMA. Rahn et al. report an improvement in the multiple-shot damage threshold compared with PMMA. 23 They also, however, observed a progressive microfracturing effect that may be due to differences in the thermal expansion coefficients of the two components. 24 Initial laser efficiencies can nonetheless be very high in this matrix, with a slope efficiency of 72% having been measured for perylene orange. 23 Here, we report for the first time the laser characteristics of a series of important pyrromethene dyes in a porous glass structure filled with modified PMMA. We have also measured the thermally-induced lens in both PFMPG and MPMMA, and have observed significantly lower lensing in the hybrid material. 2. SAMPLE PREPARATION AND CHARACTERISTICS The dye-impregnated PFMPG samples were fabricated in five steps: (1.) preparation of a solution of the appropriate dye in a mix containing the monomer and the other components; (2.) saturation of the microporous glass plate with this dye solution; (3.) free-radical polymerization of the composite for hours at temperatures of o C; (4.) extraction of the PFMPG element from the polymer block on completion of the polymerization; and (5.) optical polishing of the PFMPG elements to obtain samples with high optical quality. The final specimens were in the form of rectangular plates of approximate dimensions 20 mm x 15 mm x 4 mm. Table 2 lists basic information for each sample. The concentrations referred to in this paper represent that for the initial monomer mix. Because of the uncertainty in the efficiency of dye impregnation, the final concentration in the PFMPG plate is not known accurately for most samples. The efficacy of the dye impregnation process depends on the properties of the dye, the glass material, the pore size, and the impregnation technique. As an illustration of this issue, we measured the final dye concentration in selected samples of relatively low dye loading. This was done by measuring the maximum absorbance and sample thickness, and assuming an extinction coefficient equal to that in conventional liquids. The results indicate that the impregnation efficiency is only about 30-35% for PM 580 and PM 650. Similar measurement for a sample containing Rh 11B yielded a value >100%. This might initially seem surprising, but the relative affinity of a given dye might be higher for the pores than for the monomer mix, depending on dye structure and polarity, as well as pore properties. This procedure for calculating the impregnation efficiency cannot be used for most of the samples fabricated, since the very high absorbance required for efficient lasing and long service life precludes accurate measurements of the peak value. Therefore it is not known whether the above impregnation efficiencies can be applied to the highly doped samples as well.

4 Table 2. Basic Characteristics of the PFMPG Samples Dye PM 580 PM 597 Rh 11B PM 650 Concentration (in 10-3 M/l) Matrix Porosity (%) Pore Size (nm) λ max (fluor) (nm) NONLASING CHARACTERIZATION TESTS 3.1. Scattering measurements Because the PFMPG matrix is a heterogeneous one, there is the potential issue of internal scattering losses due to the index mismatch between the polymer and the glass. To evaluate this, we subjected selected samples to a simple, yet quantitative transmission test (Figure 1). The measurement involves propagating an expanded, collimated HeNe beam through the sample, collecting the transmitted light, and passing it through pinholes of various sizes for detection. Light scattering results in a loss of throughput, particularly for the smaller pinhole sizes. The probe beam was 1 cm in diameter in order to nearly fill the PFMPG plates. This was done to yield an average throughput for each sample, and to avoid the problem of site-to-site variation. A ¼ in.-thick sheet of MPMMA containing PM 597 at 2 x 10-4 M/l was included as a benchmark. HeNe (633 nm) 10 x Beam Expander Iris Sample Lens Pinhole Detector Figure 1. Schematic of set-up to evaluate transmission losses. As seen in Figure 2, the PFMPG samples are somewhat lossier than the MPMMA material. The throughput for the bulk polymer ( 91-92%) is essentially independent of pinhole diameter, and is consistent with simple Fresnel losses at the two surfaces. The throughput for the PFMPG materials is nearly as high down to a pinhole size of 150 µ, but drops off more rapidly as the pinhole size is reduced further. These losses will, of course, impact the achievable laser efficiency in the PFMPG matrices. In high-gain scenarios, such as Q-switched, doubled-yag pumping, this difference is less of an issue than, for example, in flashlamp pumping Measurements of the concentration profile Dye profiles were measured by passing a focused HeNe beam of suitable wavelength through the sample, and measuring the transmission as a function of location. Data for the sample containing PM 580 (2.4 x 10-3 M/l) are shown in Figure 3. The measurements were taken across the 15-mm dimension of the sample. A region of 2 mm width was excluded around the perimeter, since this region was not of suitable optical quality for accurate measurements. For this sample, a HeNe wavelength of 543 nm was chosen, since it experienced moderate attenuation (~75%) in passing through the sample. Note that the dye concentration is reasonably flat for each pass, and that the concentrations for different passes are nearly identical.

5 Throughput (%) PM597inMPMMA PM580inPFMPG ( ) Scan B PM597inPFMPG 0.03 Rh11B in PFMPG ( ) Scan A Concentration (Arb. Units) Pinhole Diameter (µ) Position Across Sample (mm) Figure 2. Optical throughput versus pinhole size. Note the increased loss due to scatter in the PFMPG samples. Figure 3. Variation in dye concentration across width of a PFMPG sample containing PM 580 (2.4 x 10-3 M/l). 4. LASER EXPERIMENTS 4.1. Test procedures A multimode, Q-switched, doubled-yag laser was used as the excitation source. The optical layout has been described in detail previously, 6 with the basic test parameters summarized in Table 3. Wavelength measurements were made with a Coherent WaveMate wavelength meter and an Optometrics Model MC1-03 monochromator. Each has an accuracy of ±1 nm, and were in good agreement throughout. Test Parameters Pump Configuration Pump Pulse Duration Pump Beam Diameter (1/e 2 ) Resonator Length Max Reflector Curvature Table 3. Basic Test Parameters Value Longitudinal 5 ns 1.4 mm 7 cm 0.5 m cc Output Coupler Flat; R oc = 20% and 50% Pump Flux (Φ pump ) for Service Life Tests 25; 50 MW/cm 2 Repetition Rate for Service Life Tests In order to have a number of test sites available on each sample, a relatively small beam size ( 1.4 mm dia. to the 1/e 2 point) was used. Each irradiation site was separated from the next by at least one beam diameter to avoid perturbation of neighboring sites. The input energies were then selected to produce pump fluxes Φ pump >10 MW/cm 2. Laser efficiency and service-life tests were performed on the PFMPG samples. The efficiency test involved measurement of energy output versus energy input over a range of energies, from which laser threshold and slope efficiency could be extracted. The test was done on a single-shot basis, with each data point being the average of three shots. This strategy minimized thermooptical distortion and dye degradation, which might have a cumulative effect during the test. The input energy was taken as the energy incident on the front face of the PFMPG plate, and therefore was not corrected for the Fresnel reflection off this surface. In a few cases, the samples were optically thin enough that a significant fraction of the pump beam was transmitted through the sample. In these situations, called out in the sections below, the efficiency was based on the absorbed energy (incident minus transmitted). 5 Hz

6 In the life test, the laser output was monitored as a function of shot number for a fixed pump flux and a fixed irradiation site. The service life was defined as the number of shots to the 70% output point. For a given dye, life tests were performed on the sample with the highest loading, since this usually resulted in the best compromise between high efficiency and long service life. In all the samples studied, the service life was limited by dye photobleaching, and not by optical damage to the matrix. In some cases, laser tests were also run on dye-doped MPMMA samples of the same thickness as the PFMPG plates. Due to the previously mentioned uncertainties in the final dye concentration in the PFMPG matrix, detailed comparisons between the two hosts are difficult to make at this point. The MPMMA samples were used simply as benchmarks for assessing dye performance in PFMPG Test results Pyrromethene 580 PM 580 is an efficient laser dye which has been studied previously in both MPMMA and a methyl methacrylatehydroxypropyl acrylate copolymer. 1,5,13,25 PFMPG plates of concentrations 1.2 and 2.4 x 10-3 M/l were prepared and evaluated for this study. The results for the higher dye loading are included in Tables 4 and 5, with representative efficiency data plotted in Figure 4. This particular sample was optically thin, that is, there was significant transmission of the pump beam through the fresh sample. (The pump wavelength is on the long-wavelength tail of the absorption profile for this dye.) The measured leakage ranged from 12% at an incident pump flux of 10 MW/cm 2 to 27% at 50 MW/cm 2. The efficiencies, pump fluxes, and service-life data reported in Tables 4 and 5 are based on the absorbed, not the incident, pump light. The laser output depended sublinearly on input energy for this sample. This is seen in Figure 4 as a decrease in laser efficiency with increasing pump flux. This behavior was not observed for the other PFMPG plates tested, and is believed to be due to the onset of parasitic oscillations, which start to clamp the output at higher pump fluxes. Evidence for this is the fact that at 50% feedback, the output remained linear up to 30 MW/cm 2. When the feedback was only 20%, the sublinear behavior was seen by about half this value. (The higher feedback more easily dominates over the parasitic.) This phenomenon can generally be avoided or minimized by a combination of frosting and blackening or AR-coating the side surfaces. Figure 4 shows a fairly large site-to-site spread in laser efficiency (also not seen for the other samples). This is not Dye Table 4 Summary of Laser Efficiency Results in PFMPG Concentration (M / liter) R oc (%) Efficiency η max (%) η slope Threshold (µj) (%) PM x 10-3 M/l x 10-3 M/l PM x 10-3 M/l x 10-3 M/l Rh 11B 0.5 x 10-3 M/l x 10-3 M/l x 10-3 M/l PM x 10-3 M/l Efficiency based on the absorbed pump energy, as discussed in the text.

7 Table 5. Service-Life Results in PFMPG Dye PM 580 PM 597 Rh 11B PM650 Concentration (M / liter) 2.4 x 10-3 M/l 3.0 x 10-3 M/l 1.0 x 10-3 M/l 4.3 x 10-3 M/l R oc (%) Pump Flux (MW/cm 2 ) Service Life Rep Rate (Hz) Pulses (70% Point) , , , , , , , ,000 Energy Efficiency 60% 50% 40% 30% 20% 10% Site 1 Site 2 Site 3 Site 10 0% Absorbed Pump Flux (MW/cm 2 ) Figure 4. Efficiency versus absorbed pump flux for PM 580 (2.4 x 10-3 M/I) in PFMPG at a feedback of 20%. attributable to variations in dye concentration, since this was found to be relatively uniform for this sample (recall Figure 3). It is more likely due to either the site dependence of the parasitic oscillation (which depends on geometric factors), or possibly differences in optical quality among the various sites tested. Sites 1, 2, and 3 were adjacent sites near one edge of the sample, while Site 10 was in the center. The service life for this sample was < 4000 shots for the conditions studied (see Table 5). This is at least partly due to the relatively low absorbance at the pump wavelength, as discussed later in Section 6. The photodegradation of this dye is likely due to the presence of molecular oxygen in the MPMMA component of the matrix. Although the samples were prepared under deaerated conditions, there is ample time for atmospheric oxygen to diffuse into the center of these relatively thin samples prior to testing. PM 580 has a very similar structure to PM 567, which is known to be oxygen-sensitive. 7,9,10 The only difference between these two compounds is that the former has n-butyl groups at the 2 and 6 positions, while the latter has ethyl groups at these locations. It is anticipated, therefore, that PM 580 will have a similar susceptibility. PM 580 is the shortest-wavelength dye investigated in this study, lasing at 554 nm under our conditions. The short emission wavelength for this dye presented a minor problem in optimizing the energy output. The dichroic max reflector is tasked to transmit the pump wavelength (532 nm) and reflect the laser wavelength ( 554 nm in this case). The proximity of these two wavelengths complicates the design of a high-damage-threshold optic. In the present work, the dichroic used reflected only 95% of the incident cavity flux at 554 nm, causing it to be somewhat leaky. Consequently, the efficiencies in Table 4 underestimate what can be achieved Pyrromethene 597 PM597 is a very efficient, stable laser dye which has been well-characterized in a variety of solid hosts. 1,2,26 The results obtained for efficiency and service life are summarized in Tables 4 and 5, with selected data appearing in Figures 5 and 6. The transmission data for the sample with the higher loading were reported earlier in Figure 2. The data in Table 4 show that the efficiencies are rather similar for the two dye loadings and the two feedback values. There is, however, a slight advantage at the higher dye loading with R = 20%. For a given feedback and dye concentration, the data were consistent among the different sites tested (see, for example, Figure 5). The fit to a straight line was excellent in all cases, with R 2 values > For comparison, a sample of PM 597 in MPMMA (2 x 10-4 M/l) was evaluated under the same pumping and extraction conditions, with R = 20%. The results are also included in Figure 5, and

8 80% % MW/cm 2 Energy Efficiency 40% 20% Near one edge; R=20% Near center; R = 20% Near one edge; R=50% Near center; R = 50% MPMMA Sample; R=20% Relative Efficiency MW/cm2 0% Pump Flux (MW/cm 2 ) E ,000 40,000 60,000 80,000 Number of Pulses Figure 5. Energy efficiency versus pump flux for PM 597 (3.0 x 10-3 M/l) in PFMPG. Figure 6. Service life of PM 597 (3.0 x 10-3 M/l) in PFMPG at 25 and 50 MW/cm 2. The irradiation site is fixed, and the rep rate is 5 Hz. are essentially identical to those for PFMPG at the same feedback. Given the previously discussed uncertainty in the final concentration in the PFMPG matrix, a rigorous comparison is not possible. The MPMMA data are included as a measure of the rough equality of the performance in the two hosts. The lasing wavelengths at 1.5 x 10-3 and 3.0 x 10-3 M/l in PFMPG were 568 and 571 nm, respectively. During the service-life tests for the higher loading, we observed a small blue shift of 3 nm during the runs. This was generally observed for the longer service-life runs, and is discussed in Section Rhodamine 11B With regard to solid hosts, Rh 11B has previously been studied in MPMMA. 27,28 For the present investigation, PFMPG plates of dye concentrations 2 x 10-4, 5 x 10-4, and 1.0 x 10-3 M/l were prepared and tested (Tables 4 and 5). 60% Figure 7 depicts the laser efficiency as a function of pump flux for a monomer dye concentration of 2 x 10-4 M/l. As seen from the figure, no significant site-dependence was seen for either 20% or 50% 45% feedback, indicating that the optical characteristics are very similar from center to edge. For this dye, there was little difference in output between 20% and 50% feedback for the three loadings. As expected, 30% the laser threshold is lower for the higher feedback, but this is Near edge; R = 20% compensated by a higher slope efficiency for R = 20% (Table 4). The efficiency curves for 2 x 10-4 and 5 x 10-4 Near center; R = 20% M/l are nearly identical, 15% Near edge; R = 50% reaching values of 50-55% at the highest pump fluxes. The values for the most highly doped plate (1 x 10-3 Near center; R = 50% M/l), however, are decidedly lower for all sites tested (peaking at only 35-40%). Looking at the fitting parameters for the various loadings (Table 4), we note a trend to higher laser thresholds as the concentration is increased. The slope efficiencies, on the other hand, although similar for the two lower loadings, drop significantly in going to the highest loading. There are two possible reasons for these observations. As the dye concentration increases, there is the increased possibility of aggregate formation. Dye aggregates are not only nonfluorescing, but they also tend to absorb at the emission wavelength of the dye monomer. Secondly, xanthene dyes have relatively small Stokes shifts. Therefore, self-absorption can be a significant issue as the concentration increases. In the service-life tests, we observed a factor-of-five reduction from 110,000 pulses at 25 MW/cm 2 to 22,000 pulses at 50 MW/cm 2 (Table 5). In general, the emission wavelength shifted to the blue with shot number, in a more or less Energy Efficiency 0% Pump Flux (MW/cm 2 ) Figure 7. Energy efficiency versus pump flux for Rh 11B (2 x 10-4 M) in PFMPG. The data for a concentration of 5 x 10-4 M are very similar.

9 monotonic fashion. Figure 8 shows the data for a pump flux of 25 MW/cm 2, a rep rate of 5 Hz, and a resonator feedback of R = 20%. The overall blue shift during this particular run is 7 nm, and appears to asymptote at 563 nm after 100,000 pulses Pyrromethene 650 Of the commercially available pyrromethenes, PM 650 lases at the longest wavelength ( 625 nm), and has shown good service life in a modified PMMA. 29 This dye, however, has a relatively low quantum yield (0.54 in EtOH) 30 and a large quantum defect compared with PM 580 and PM 597. Consequently, measured energy efficiencies have only been 35% or less. Co-doping the sample with a suitable sensitizer dye has improved efficiencies somewhat, 31 but this apparently utilized simple radiative energy transfer, which is not very efficient. These two photophysical factors (low quantum yield and large quantum defect) result in greater waste heat deposition in the sample, and hence a higher temperature rise for a given pump flux. This is known to adversely impact dye photostability by increasing the mobility of the dye molecules, enhancing the rate of interaction with other reactive species. 32 The greater thermal conductivity of the PFMPG matrix may improve dye stability by reducing the temperature in the irradiated zone. PFMPG plates prepared from monomer mixes of dye concentrations 2.15 x 10-3 and 4.3 x 10-3 M/l were evaluated for laser performance, with the results summarized previously in Tables 4 and 5. For the lower loading, there was some pump-light leakage at 532 nm. This is due partly to the fact that the absorbance of this dye at 532 nm is only 34% of that at the peak, and partly because of the relatively low impregnation efficiency noted earlier. During the efficiency tests, we observed that approximately 3-4% of the incident pump light was transmitted through this sample. The efficiencies in the table are corrected for this. No measurable pump light leaked through the plate with the higher loading for a fresh site. The efficiency results for each concentration were remarkably consistent from site to site. (See, for example, Figure 9). For a concentration of 4.3 x 10-3 M/l and a feedback of 20%, the efficiency asymptotes at 20%. This drops to 15% at the higher feedback of 50%. The same trend is observed at the lower loading, that is, the efficiency is higher at the lower feedback (η 28% at R = 20% and η 18% at R = 50%). Allik et al. 30 measured the laser efficiency of PM 650 in EtOH as a function of concentration, and reported a monotonic decrease in going from 1 x 10-4 to 1 x 10-3 M/l. This is consistent with our observations, and could once again be due to dye self-absorption or aggregation. It should be noted that parasitic oscillations are apparently not responsible for the lower efficiency with increased dye loading, since this would result in higher efficiencies at higher feedback, which is contrary to what is seen. Energy Efficiency In Figure 9, we have included data for PM 650 (2 x 10-4 M/l) in an MPMMA sample of the same thickness as for the PFMPG plates. For the lower dye loading in PFMPG, the values are comparable ( 33% in MPMMA versus 28% in PFMPG). As discussed earlier, comparisons with MPMMA can only be used for rough comparisons at this point. The lower pump fluxes for the MPMMA data are due to the fact that some pump light leaks through this sample. The data were corrected to reflect the actual pump light absorbed. 40% 30% 20% Wavelength (nm) , , ,000 Shot Number Figure 8. Laser wavelength versus shot number for Rh 11B (1 x 10-3 M/l) in PFMPG. The data were taken at Φ pump = 25 MW/cm 2, PRF = 5 Hz and R = 20%. Site 1 Site 2 10% Site 3 Site 4 Site 5 Site 10 MPMMA 0% Pump Flux (MW/cm 2 ) Figure 9. Efficiency versus pump flux for PM 650 (4.3 x 10-3 M/l) in PFMPG. A sample consisting of PM 650 at 2 x 10-4 M/l in MPMMA is included for comparison. The feedback was 20% in all cases.

10 In the service-life measurements, the emission wavelength shifted to the blue with shot number, as was observed for PM 597 and Rh 11B, with an overall shift of 5 nm. 5. THERMAL LENSING MEASUREMENTS The TFOM values in Table 1 imply that, for a given set of conditions, a bulk polymer should exhibit a thermal lens which is between one and two orders of magnitude stronger than that for a polymer-filled porous glass. We investigated this by implementing a set-up where the thermally induced lens can be measured directly (Figure 10). The optical layout is very similar to that utilized by us previously for characterizing polymer rods for flashlamp-pumping, 33 but more closely follows the arrangement of Sumida and co-workers. 34 The technique has the advantage that one can measure the static (unpumped) lens, the thermally induced lens without energy extraction, and the thermally induced lens with energy extraction. (In general, the latter two cases produce different results.) The layout involves setting up a collimated probe beam, which is initially focussed onto the array of a CCD camera without the sample present. The introduction of the gain medium (pumped or unpumped) will, in general, affect the location of this focus. The position of the negative lens (f 1 ) is adjusted by an amount δ to reestablish the original focal position. Using standard ray-tracing, the effective focal length of the gain medium is given by f = (L-f 2 ) f 2 2 / δ. 34 Note that only the lens f 1 is moved during this procedure. f 1 =-25mm + - δ HeNe Probe Beam (633 nm) Array f=1m Output Coupler (optional) f 2 =+300mm L Collimated Section Beam- Combining Optic Computer CCD Camera Attenuators and RG610 Gain Element Pump Beam (532nm) Hard Aperture (5 mm φ) max R (optional) Figure 10. Optical layout for the thermal lens measurements. The pump beam is carefully arranged to be collinear with the probe beam and of the same diameter, so that, during excitation, only the pumped region is investigated. Also, the alignment of the probe beam through the system is such that movement of the negative lens does not walk the beam off the pumped aperture. The 5-mm φ hard aperture allows one to core out a more uniform region of the pump beam and provide a roughly top-hat pump distribution, which is easy to model. 35,36 It is straightforward to separate out the static component to arrive at the pump-dependent (thermal) contribution. In Figure 11, we present data using this technique for PM 597 in MPMMA and in PFMPG without energy extraction. Included in each figure is the result of the theoretical modeling. 36 The experimental data agree quite well with theory, and clearly demonstrates that the thermal lens in PFMPG is indeed much smaller than for MPMMA for the same pumping conditions. 6. DISCUSSION The purpose of this work was twofold: to demonstrate efficient, long-lived dye-laser operation using the novel hybrid matrix PFMPG, and to show that the thermal lens for PFMPG is much lower than that for MPMMA. Although PFMPG is a relatively new host material, the laser results presented here rival those of the benchmark material MPMMA. The superior mechanical and thermooptical properties of PFMPG further supports the promise of this material for practical dye-laser systems. In continuing the development of this matrix, there are two primary areas for improvement: further reduction of internal losses and the achievement of still longer service life.

11 y=3.25e-04x R 2 =4.92E y = x Focal Length (D) -0.2 y= x R 2 = Focal Length (D) Measured Calculated (Top-Hat) -0.3 PFMPG (Measured) PFMPG (Calculated) PMMA (Measured) Heat Deposition Rate (mw) Heat Deposition Rate (mw) Figure 11. The Thermal Lens Measurements for PM 597 in MPMMA (left) and in PFMPG (right). Note the much lower lensing in the hybrid glass matrix. The present level of internal scatter has a relatively small effect on the observed efficiencies under Q-switched, doubled-yag pumping. For PM 597, PM 650, and Rh 11B, the measured values are similar to those achieved in MPMMA. In high-performance applications, however, high raw energy efficiency and good spatial properties are equally important considerations. Consequently, improved index matching between the glass and polymer components is an important area of investigation. As the laser is repped, waste heat is initially deposited in the polymer component. This is followed by thermal diffusion into the glass, with the temperature distributions eventually reaching a steady state. Given the different values of dn/dt for the glass and polymer, the two components should ideally be index-matched for the anticipated thermal loading. Long service life was obtained for optically thick samples. The usual strategy in achieving long-term, stable operation in a solid-state dye laser is to have an OD > 4 at the pump wavelength. As long as the absorption of the photodegradation products is minimal, the output will be more or less constant until the pump beam starts bleaching through the sample. 6 For optically thin samples, the output starts dropping almost immediately, and consequently the service life is relatively short. For Rh 11B, the impregnation efficiency is very high, exceeding 100%. This allows high optical densities to be achieved in PFMPG at moderate monomer concentrations (~10-3 M/l). For the pyrromethenes, on the other hand, the impregnation efficiency is only about 30-35%. Therefore, even at monomer concentrations of several times 10-3 M/l, the absorbance at the pump wavelength is sometimes insufficient to provide a long service life (e.g., for PM 580). For some dyes, simply increasing the dye concentration will provide an acceptable solution. For others, the higher loading results in an efficiency penalty, possibly due to dye aggregation. In such cases, use of thicker samples should result in significant improvements in service life. Alternatively, one could multiplex several samples in the resonator at Brewster s angle. In the longer life tests, there was a relatively small blue shift in the emission wavelength ( 5 nm). It is well-known that the laser wavelength shifts to the red as the dye concentration is increased (due to increased dye self-absorption). As bleaching occurs during the service-life test, the number of dye molecules along the gain length decreases with time, resulting in the observed blue shift. As the pump beam leaks through the sample late in the run, this shift should essentially cease, since there are relatively few dye molecules remaining in the optical path. This is in fact observed, as was seen in Figure. 8. In some applications, this wavelength change is not critical. In those requiring tighter control, a low-loss tuning element, such as a birefringent plate, may be required. 7. CONCLUDING REMARKS We have measured the laser efficiency and service life for three pyrromethene dyes and a rhodamine dye in the novel matrix PFMPG. In most cases, the efficiency values are nearly as high as are observed in the bulk polymer MPMMA. The somewhat higher scattering loss measured in the hybrid matrix is a primary suspect in explaining the observed differences. Near-term improvements in sample preparation are expected to essentially eliminate this issue.

12 Service life is excellent for optically thick samples (e.g., >110,000 pulses at 25 MW/cm 2 and 5 Hz for Rh 11B), and stable output can be achieved until dye photobleaching permits the pump beam to leak through the sample. The use of thicker samples will increase service life more or less proportionately. Measurements of thermal lensing have been made for both MPMMA and PFMPG. The values for the former matrix are between one and two orders of magnitude higher than for the latter, in agreement with theory. In conclusion, we have demonstrated the viability of the PFMPG matrix as a robust solid host for laser dyes. ACKNOWLEDGEMENTS The authors would like to acknowledge the technical assistance of Mr. Suren Shenoy. This work was supported by the U. S. Army AMCOM under Contract No. DAAH01-98-C-R116. REFERENCES 1. D. P. Pacheco, J. G. Burke, H. R. Aldag and J. J. Ehrlich, Proceedings of the International Conference on Lasers 95, STS Press, McLean, VA (1996), pp Mark D. Rahn and Terence A. King, Solid State Lasers VIII, Proc. SPIE, vol. 3613, 1999, pp , and references therein. 3. A. Costela, I. Garcia-Moreno, H. Tian, J. Su, K. Chen, F. Amat-Guerri, M. Carrascoso, J. Barroso and R. Sastre, Chem. Phys. Lett. 277, 392 (1997). 4. K. M. Dyumaev, A. A. Manenkov, A. P. Maslyukov, G. A. Matyushin, V. S. Nechitailo, and A. M. Prokhorov, J. Opt. Soc. Am. B9, 143 (1992). 5. Ya. V. Kravchenko, A. A. Manenkov, G. A. Matyushin, V. M. Mizin, D. P. Pacheco and H. R. Aldag, Proc. SPIE, vol. 2986, 1997, pp D. P. Pacheco and H. R. Aldag, Solid State Lasers VII, Proc. SPIE, vol. 3265, 1998, pp M. D. Rahn, T. A. King, A. A. Gorman, and I. Hamblett, Appl. Opt. 36, 5862 (1997). 8. M. Faloss, M. Canva, P. Georges, A. Brun, F. Chaput, and J.-P. Boilot, Appl. Opt. 36, 6760 (1997). 9. T. Suratwala, K. Davidson, Z. Garlund, and D. R. Uhlmann, in Solid State Lasers VI, SPIE Proc. Series, Vol. 2986, 1997, pp M. Ahmad, M. D. Rahn and T. A. King, Appl. Opt. 38, 6337 (1999). 11. S. Chandra, T. H. Allik and J. A. Hutchinson, Opt. Lett. 20, 2387 (1995). 12. A. Mandl, A. Zavriyev and D. E. Klimek, IEEE J. Quantum Electron. 32, 1723 (1996). 13. S. V. Garnov, A. A. Manenkov, D. P. Pacheco and H. R. Aldag, Proc. SPIE 3265, 306 (1998). 14. W. Koechner, Solid-State Laser Engineering, 4 th edition, Springer-Verlag (1996), pp A. S. Bank, D. L. Donskoy and V. S. Nechitailo, in UV and Visible Lasers and Laser Crystal Growth, SPIE Proc. Series, Vol. 2380, 1995, pp S. M. Dolotov, M. F. Koldunov, A. A. Manenkov, G. P. Roskova, N. M. Sitnikov, N. E. Khaplanova and T. S. Tsekhomskaya, Sov. J. Quantum Electron. 22, 1060 (1992). 17. M. F. Koldunov, A. A. Manenkov, N. M. Sitnikov and S. M. Dolotov, Proceedings of the 25 th Annual Boulder Damage Symposium, Proc. SPIE, vol. 2114, 1994, p T. B. Bermas, A. V. Bortkevich, Y. V. Kostenich, S. M. Lan kova, Y. M. Paramonov, A. N. Rubinov and T. S. Tsekhomskaya, Quantum Electron. 24, 27 (1994). 19. A. A. Manenkov and V. S. Nechitailo, Sov. J. Quantum Electron. 10, 347 (1980). 20. G. B. Altshuler, E. G. Dul neva, K. I. Krylov, I. K. Meshkovskii and V. S. Urbanovich, Quantum Electron. 10, 784 (1983). 21. T. S. Efendiev, Y. V. Kostenich, A. N. Rubinov, G. B. Altshuler, E. G. Dul neva and I. K. Meshkovskii, Appl. Phys. B33, 167 (1984). 22. G. B. Altshuler, E. G. Dul neva and A. V. Erofeev, Sov. Phys.-Tech. Phys. 30, 941 (1985). 23. M. D. Rahn and T. A. King, J. Mod. Opt. 45, 1259 (1998). 24. M. D. Rahn and T. A. King, Appl. Opt. 34, 8260 (1995), and references therein. 25. R. E. Hermes, T. H. Allik, S. Chandra and J. A. Hutchinson, Appl. Phys. Lett. 63, 877 (1993). 26. R. E. Hermes, in Visible and UV Lasers, SPIE Proc. Series, Vol. 2115, 1994, pp D. A. Gromov, K. M. Dyumaev, A. A. Manenkov, A. P. Maslyukov, G. A. Matyushin, V. S. Nechitailo and A. M. Prokhorov, J. Opt. Soc. Am. B2, 1028 (1985). 28. Ya. V. Kravchenko, A. A. Manenkov and G. A. Matyushin, Quantum Electron. 26, 1045 (1996).

13 29. R. S. Anderson, R. E. Hermes, G. A. Matyushin, V. S. Nechitailo and S. C. Picarello, Solid State Lasers VII, Proc. SPIE, vol. 3265, 1998, pp T. H. Allik, R. E. Hermes, G. Sathyamoorthi and J. H. Boyer, Visible and UV Lasers, SPIE Proc. Series, Vol. 2115, 1994, pp T. H. Allik, S. Chandra, R. Utano, T. R. Robinson and J. H. Boyer, Solid State Lasers V, Proc. SPIE, vol. 2698, 1996, pp S. Popov, Appl. Opt. 37, 6449 (1998). 33. D. P. Pacheco, H. R. Aldag, I. Itzkan and P. S. Rostler, Proceedings of the International Conference on Lasers 87, 34. STS Press, McLean, VA (1988), pp D. S. Sumida, D. A. Rockwell, and M. S. Mangir, IEEE J. Quantum Electron. 24, 985 (1988). 35. A. K. Cousins, IEEE J. Quantum Electron. 28, 1057 (1992). 36. D. P. Pacheco and H. R. Aldag, manuscript in preparation.