Singlet oxygen and dye-triplet-state quenching in solid-state dye lasers consisting of Pyrromethene 567–doped poly共methyl methacrylate兲 Mohammad Ahmad, Mark D. Rahn, and Terence A. King
Solid-state dye lasers based on poly共methyl methacrylate兲 共PMMA兲 doped with Pyrromethene 567 dye 共P567兲 have been investigated. The preparation techniques employed provided high photostability and laser damage threshold for P567 in pure PMMA with 270,000 pulses emitted before the conversion efficiency fell to half its initial value for a pump fluence of 0.16 J cm⫺2. When PMMA was modified with 1,4-diazobicyclo 关2,2,2兴 octane singlet oxygen quencher, the longevity increased to 550,000 pulses, corresponding to a normalized photostability of 270 GJ mol⫺1. Modification of PMMA with a triplet quencher 共perylene兲 yielded no improvement, but in ethanol solutions both additives enhanced photostability. It is possible that in PMMA, stabilization by means of triplet quenching that depends on dye diffusion is prevented but that stabilization by means of singlet oxygen quenching that depends on the faster oxygen diffusion rate will succeed. © 1999 Optical Society of America OCIS codes: 140.3460, 140.3580, 140.3380, 140.2050.
1. Introduction
Solid-state dye lasers 共SSDL’s兲 could potentially find many applications in various fields including medicine, remote sensing, and spectroscopy or as inexpensive general tunable laser sources. The main features of SSDL’s are tunability over a wide range of wavelengths from the near ultraviolet to the near infrared, a low microjoule-per-pulse threshold, a compact and maintenance-free system, and a low-cost gain medium. SSDL’s are usually based on either glass or polymer hosts doped with laser dye molecules,1 and by modifying the components of the host one can make a variety of host media with different optical and physical properties. These media include solgel glasses, organically modified silicon alkoxide glass 共ormosil glass兲,2 porous solgel glass impregnated with poly共methyl methacrylate兲3–5 共PMMA兲, and various modifications of acrylic polymers that include modified PMMA.6,7 The most important issue concerning the develop-
The authors are with the Laser Photonics Research Group, Department of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK. The e-mail address for M. D. Rahn is
[email protected]. Received 4 May 1999; revised manuscript received 14 July 1999. 0003-6935兾99兾306337-06$15.00兾0 © 1999 Optical Society of America
ment of SSDL’s is the photostability of the dopant dye molecules. Photodegradation causes a gradual loss of output power with use because of a reduction in the available dye molecule concentration along with possibly an increase in optical loss caused by products of degradation. Physical parameters of the host material that affect the photostability include the material’s thermal conductivity, which relates to the ability to radiate heat quickly out of the sample and a reduction in diffusion controlled reaction rates. A host with high thermal conductivity may make it possible to work with a high repetition rate and to maintain a long service life in the host. Another important issue is the host’s damage threshold. Glass hosts generally have better thermal properties and a higher damage threshold than plastics,5 which have simple fabrication procedures and excellent optical quality. Modification of PMMA with low-molecular-weight additives can, however, improve the damage threshold,6 and excellent laser performance has been achieved in both classes of host material.1 It has been suggested that the most important factor to affect the performance of a SSDL is the laser dye dopant,1 and many dyes have been incorporated into solid host media to form SSDL’s. Coumarin dyes8 with absorption from 400 to 550 nm and rhodamine dyes6,7 with absorption from 550 to 750 nm have both been used. Greater success, however, has 20 October 1999 兾 Vol. 38, No. 30 兾 APPLIED OPTICS
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been achieved with the perylene9 and pyrromethene10 dye families. The pyrromethene laser dyes have received particular attention because they show high efficiency owing to their low triplet absorption losses at the fluorescence emission wavelengths.11,12 Moreover, the pyrromethene family of dyes has been shown to exhibit excellent photostability in both glass13 and plastic14 –16 host materials. Although the pyrromethene family of dyes provides the best SSDL performance,1 photodegradation still occurs, which does depend on the host’s composition. Previous studies have established that the predominant photodegradation mechanism of Pyrromethene 567 共P567兲 is self-sensitized photo-oxidation.12,17 In this process, energy transfer from the triplet state of P567 forms singlet oxygen, which then reacts with the ground state of P567, resulting in permanent degradation. In this study we have attempted to improve the photostability of P567 by using two additives in the PMMA host material to block this process, 1,4-diazobicyclo 关2,2,2兴 octane 共DABCO兲 and perylene. DABCO acts by quenching singlet oxygen and was shown previously to enhance the photostability of pyrromethene dyes in solution18 and in epoxy host media.14 Perylene’s triplet-state energy 共1.52 eV兲 is below that of P567 共1.63 eV兲,12 and therefore perylene is able to quench the triplet state of P567, which it was hoped would enhance photostability by preventing formation of singlet oxygen. To our knowledge, the use of neither this triplet quencher nor any other triplet quencher as an additive in a SSDL was reported previously. It is shown in this study that the singlet oxygen quencher DABCO enhances the photostability of P567 both in ethanol and in a solid PMMA matrix. Although the P567 triplet quencher perylene improved the photostability of P567 in ethanol, no significant improvement was realized in PMMA. 2. Host Preparation
Commercially available methyl methacrylate monomer 共Aldrich Chemical Company兲 was distilled to remove the initiator 共hydroquinone monomethyl ether兲. P567 共3.36 ⫻ 10⫺4 M兲 was dissolved into the monomer, and the mixture was placed in a water-filled ultrasonic bath until the dye was completely dissolved. The singlet oxygen quencher DABCO or the triplet quencher perylene was added according to the required concentration along with 1 mg兾mL of 2,2azobis 共2-methylpropionitrile兲 polymerization initiator. Finally the mixture was replaced in an ultrasonic bath to completely dissolve the solid constituents. Air-tight glass tubes containing the dye mixtures were placed in a water bath at a temperature of 40 °C for a week until a viscous liquid was formed. The tubes were then transferred to an oven where the temperature was increased stepwise at 5 °C兾day until it reached 90 °C. Then the temperature was reduced over two days to room temperature. The glass tubes were broken to remove the polymerized samples, which were then cut into disks and hand polished. 6338
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3. Experimental Arrangement for Laser Performance Tests
The laser cavity was a compact plane–plane cavity, such as that used in the study reported in Ref. 5. The input mirror was dichroic, with 90% transmission at 532 nm and 95% reflectivity from 560 to 600 nm. The output mirror was a 70% broadband reflector whose reflectivity was not optimized. The cavity length 共15 mm兲 was chosen small to reduce the cavity losses that result from a highly divergent output. The pump source was a Nd:YAG laser operating in the second harmonic at 532 nm. It could deliver as much energy as 60 mJ兾pulse in 6 ns repetitively or in single shots. Two lenses in a telescope arrangement focused the pump beam onto the sample, where the diameter was 2 mm for all measurements. The pump beam was aligned 16° off axis so any transmitted pump light was not collinear with the output beam and did not fall onto the volume absorbing powermeter. The maximum pump fluence was 1.9 J cm⫺2, but all the photostability experiments performed on solid samples were performed with a pump fluence of 0.16 J cm⫺2 incident upon the surface of the samples and at a repetition rate of 2 Hz. The slope efficiency of each solid sample was measured on a single-pulse basis, and averaging five pulses provided each data point. Performing singleshot measurements minimized thermal effects and photodegradation. For efficiency measurements the input power was measured as the power incident upon the front face of the sample. The output beam’s divergence from solid samples was typically 2.9 mrad at a pump fluence of 0.16 J cm⫺2 and increased with the increase of the pump fluence to 6.7 mrad at 0.38 J cm⫺2. The output beam from either solid samples or cuvettes containing dye solution had a wavelength of 565 nm and comprised interference rings with a strong mode in the center. Experiments performed on dye solutions used a small known volume of solution in a cuvette with a path length of either 10 or 2 mm. Slope efficiencies were measured with a 2-Hz repetition rate. For photostability measurements of solutions, the pump fluence varied from 0.28 to 0.36 J cm⫺2. 4. Results
Table 1 contains a summary of the results for P567 in ethanol containing various concentrations of DABCO or perylene additive. The slope efficiencies of ethanolic solutions of P567 with both additives in various concentrations are shown in Fig. 1. It can be seen that the slope efficiency is very high, 70 –90%, and is not affected by either additive in moderate concentrations. No saturation in efficiency was observed up to a maximum pump fluence of 0.36 J cm⫺2. The addition of high concentrations of perylene 共ⱖ2 ⫻ 10⫺3 M兲, however, tended to reduce the slope efficiency to 37%. The dependence of the photostability of P567 in ethanol on the additive concentration is illustrated in Figs. 2 and 3 for DABCO and perylene additives,
Table 1. Summary of Laser Performance Data of P567 in Ethanol with Various Concentrations of DABCO and Perylene Additives
P567 Concentration 共⫻10⫺4 M兲
DABCO Concentration 共⫻10⫺4 M兲
Perylene Concentration 共⫻10⫺4 M兲
Path Length 共mm兲
Slope Efficiency 共%兲
Photostability 共GJ mol⫺1兲
Operation Longevity 共Number of Pulses兲
0.1 0.1 0.1 0.1 1.66 1.66 1.66 1.66 1.66
0 0.1 1.0 10 0 0 0 0 0
0 0 0 0 0 1.66 3.32 16.6 33.2
10 10 10 10 2 2 2 2 2
88a 90a 76a 72a 72 76 74 73 37
3.0 5.9 8.0 5.3 3.0 3.9 4.4 5.9 2.7
11,520 23,040 31,200 20,640 31,920 36,000 40,800 54,000 25,000
P567 concentration, 3.36 ⫻ 10⫺4 M.
a
respectively. Increases in the photostability by factors of 3 and 2 to 8 and 6 GJ mol⫺1 were achieved with DABCO and perylene, respectively. It was also found that an optimum concentration exists for both additives and that an increase beyond this concentration reduces the photostability back toward values observed with no additive. Table 2 lists the laser performance data obtained from 3.36 ⫻ 10⫺4 M P567 doped into PMMA. The slope efficiency of P567 in PMMA was as high as 65%, which is ⬃10% less than in ethanol, least in part because of a more divergent laser output from the solid than from the solution. Other factors that affect the slope efficiency include more severe thermal effects in the solid. Figures 4 and 5 show the longevity of laser operation of dye-doped PMMA samples codoped with DABCO and perylene, respectively. High photostability is evident in all cases with operation half-lives; the number of pump pulses required for reducing the conversion efficiency by 50% exceeded 190,000 pulses in all cases. P567 doped into pure PMMA provided a half-life of 270,000 pulses, which equates to a normalized photostability 共the accumulated pump en-
Fig. 1. Slope efficiencies of 3.36 ⫻ 10⫺4 M P567 in ethanol versus DABCO additive concentration for a 3.5-mL solution in a 10-mm path-length cuvette, and 1.66 ⫻ 10⫺4 M P567 in ethanol versus perylene additive concentration for a 0.65-mL solution in a 2-mm path-length cuvette.
ergy on the sample during the period of half-life, per mole of dye molecules contributing to laser action兲 of 130 GJ mol⫺1. We achieved this high value in pure PMMA by ensuring that the pump fluence was lower than the laser damage threshold. P567 is, therefore, more photostable in PMMA than in ethanol, which is consistent with previous reports.17 Apart from thermal processes, no extra degradation channels are expected in PMMA compared with ethanol, so this photostability increase may be understood in terms of reduced diffusion processes in PMMA. It can be seen from Fig. 4 that the DABCO additive doubles the photostability of P567 in PMMA. As in ethanol, an optimum concentration of DABCO in PMMA for best efficiency and photostability exists and is close to 3.36 ⫻ 10⫺3 M. Although Table 2 suggests an im-
Fig. 2. Conversion efficiency versus the normalized energy input 共accumulated pump energy onto the sample per mole of dye molecules in the gain region兲 for 1 ⫻ 10⫺5 M P567 in ethanol with no DABCO and with 1 ⫻ 10⫺4 M DABCO. Tests were performed in a 10-mm path-length cuvette containing 3.5 mL of solution. The pulse energy was 9 mJ. Inset, normalized photostability of P567 in ethanol versus DABCO concentration. 20 October 1999 兾 Vol. 38, No. 30 兾 APPLIED OPTICS
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Fig. 3. Conversion efficiency versus the normalized energy input for 1.66 ⫻ 10⫺4 M P567 in ethanol with various concentrations of perylene additive. Tests were performed in a 2-mm path-length cuvette containing 0.65 mL of solution. The pulse energy was 11.7 mJ. Inset, normalized photostability of P567 in ethanol versus perylene concentration.
Fig. 4. Conversion efficiency versus the number of excitation pulses for 3.36 ⫻ 10⫺4 M P567 in PMMA with various concentrations of DABCO additive. The pump pulse energy was 5 mJ. Inset, slope efficiency of the same samples versus concentration of DABCO.
provement in photostability when 3.36 ⫻ 10⫺3 M perylene is added to the solid host, inspection of Fig. 5 provides no clear indication that perylene stabilizes P567 in PMMA. A high concentration of perylene in PMMA reduces the slope efficiency.
repetition rate. If one extrapolates these data one can estimate a normalized photostability of 212 GJ mol⫺1 for P567 in their modified PMMA. The performance that they observed was seriously reduced without modification of the PMMA because of laser damage to the host material. No such damage was observed in this study when we used pure PMMA, presumably because of our use of preparation conditions that resulted in a polymer structure that is resistant to laser damage or because of the slower repetition rate used in this study. It can be concluded that the benefits from modification of the host matrix to make modified PMMA are greatly to increase the damage threshold and slightly to increase the photostability. Improvements in photostability by a few hundred percent can be achieved by the use of some additives. DABCO was used previously to improve the longevity of laser operation in epoxy-based SSDL’s.14 DABCO
5. Discussion and Conclusions
An important conclusion that arises from this study is that the efficiency and particularly the photostability of P567 in pure PMMA are very high. It may be noted that at the pump fluence used, 0.16 J cm⫺2, excellent performance can be achieved without any modifications to the PMMA host material. The photostability of P567 in pure PMMA was found to be 270,000 pulses, or 130 GJ mol⫺1. The study most nearly comparable to this one is that conducted by Pacheco and Aldag,14 who achieved 100,000 pulses before the conversion efficiency fell to 80% of its original value for a 0.176-J cm⫺2 pump fluence at a 5-Hz
Table 2. Summary of Laser Performance Data of P567 in PMMA with Various Concentrations of DABCO and Perylene Additives
P567 Concentration 共⫻10⫺4 M兲
DABCO Concentration 共⫻10⫺4 M兲
Perylene Concentration 共⫻10⫺4 M兲
Path Length 共mm兲
Slope Efficiency 共%兲
Photostability 共GJ mol⫺1兲
Operation Longevity 共Number of Pulses兲
3.36 3.36 3.36 3.36 3.36 3.36
0 33.6 336 0 0 0
0 0 0 3.36 33.6 336
9.56 9.35 9.36 8.96 9.06 9.48
65 64 38 56 71 5
130 270 150 140 160 100
270,000 550,000 290,000 220,000 310,000 190,000
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Fig. 5. Conversion efficiency versus the number of excitation pulses for 3.36 ⫻ 10⫺4 M P567 in PMMA with various concentrations of perylene additive. The pump pulse energy was 5 mJ. Inset, slope efficiency of the same samples versus concentration of perylene.
stabilization mechanism of DABCO. Singlet oxygen quenching depends on the diffusion constants of oxygen and DABCO, whereas triplet quenching depends on the diffusion constants of perylene and P567. Of these four species, oxygen has the fastest diffusion rate in PMMA, so it is reasonable to conclude that singlet oxygen quenching occurs in PMMA and triplet quenching does not. The concentration of DABCO required for improving the photostability of P567 is higher in PMMA than in ethanol, a result that is also consistent with the effect of reduced diffusion. Also, concentrations of DABCO and perylene in PMMA greater than 3 ⫻ 10⫺3 M reduced the laser efficiency as a result of quenching of the excited singlet state of the dye.22 In brief conclusion, we have described the preparation of one of the best SSDL materials found to date. Although triplet quenching improves photostability in liquids, it is ineffective in solid host materials, presumably because of reduced dye diffusion. A molecule that can act as a singlet oxygen quencher is effective. Future studies may concentrate on the search for even better host materials, operation in the blue– green spectral region, and a fundamental study of excitons in solid host materials. References and Notes
is known to be a singlet oxygen quencher, a reducing agent,21 and possibly a free-radical scavenger18 in various systems. The other additive used in this study was perylene, which is known to be an effective quencher of the triplet state of P567.12 In ethanol, DABCO 共10⫺4 M兲 and perylene 共16.6 ⫻ 10⫺4 M兲 additives increased the photostability of P567 from 3 to 8 GJ mol⫺1 and from 3 to 6 GJ mol⫺1, respectively. Here, perylene is quenching the triplet state of P567, thus blocking the formation of singlet oxygen, which is known to destroy P567 molecules.12,17 Correspondingly, DABCO is blocking the degradation process either by quenching singlet oxygen or by intercepting free radicals that are likely to compete with singlet oxygen in the photodegradation process. Additionally, because DABCO is a reducing agent, it may reduce photodegradation products, thus reversing oxidation to re-form active dye molecules. Modification of the PMMA host with a DABCO additive increased the longevity of operation from 270,000 to 550,000 pulses, a property that, combined with a slope efficiency of 64%, makes this material one of the best reported. Unlike in ethanol, perylene doped into PMMA has no stabilization effect. One of two possibilities can be concluded: that the photodegradation mechanisms in PMMA are entirely different from those in ethanol or that reduced diffusion severely affects the triplet quenching process. DABCO additive in PMMA, therefore, improved photostability, whereas perylene did not. Based on previous research12,17 and because DABCO has been shown to quench singlet oxygen in PMMA,19 singlet oxygen quenching is the favored explanation for the 19,20
1. M. D. Rahn and T. A. King, “Comparison of solid-state dye laser performance in various host media,” in Solid State Lasers VIII, R. Scheps, ed., Proc. SPIE 3613, 94 –105 共1999兲. 2. H. Schmidt, “Organic modification of glass structure, new glass or new polymer?” J. Non-Cryst. Solids 112, 419 – 423 共1989兲. 3. G. Gall, X. Li, and T. A. King, “Optical characterisation of sol-gel optical composites,” J. Sol-Gel Sci. Technol. 2, 775–778 共1994兲. 4. X. Li, T. A. King, and F. Pallikari-Viras, “Characteristics of composites based on PMMA modified gel-silica glasses,” J. Non Cryst. Solids 170, 243–249 共1994兲. 5. M. D. Rahn and T. A. King, “High-performance solid-state dye laser based on Perylene-Orange doped polycom glass,” J. Mod. Opt. 45, 1259 –1267 共1998兲. 6. D. A. Gromov, K. M. Dyumaev, A. A. Manenkov, A. P. Maslukov, G. A. Matyushin, V. S. Nechitailo, and A. M. Prokhorov, “Efficient plastic-host dye lasers,” J. Opt. Soc. Am. B 27, 1028 –1031 共1985兲. 7. S. M. Giffin, I. T. McKinnie, W. J. Wadsworth, A. D. Woolhouse, G. J. Smith, and T. G. Haskell, “Solid state dye lasers based on 2-hydroxyethyl methacrylate and methyl methacrylate co-polymers,” Opt. Commun. 161, 163–170 共1999兲. 8. A. Costela, I. Garcia-Moreno, J. Barroso, and R. Sastre “Studies on laser action from polymeric matrices doped with Coumarin 503,” Appl. Phys. B 67, 167–173 共1998兲. 9. G. Seybold and G. Wagenblast, “New perylene and violanthrone dyestuffs for fluorescent collectors,” Dyes Pigm. 11, 303–317 共1989兲. 10. M. Shah, K. Thangaraj, M. L. Soong, L. T. Wolford, J. H. Boyer, I. R. Politzer, and T. G. Pavlopoulos, “Pyrromethene-BF2 complexes as laser dyes. 1,” Heteroatom. Chem. 1, 389 –399 共1990兲. 11. M. P. Oneil, “Synchronously pumped visible laser dye with twice the efficiency of Rhodamine 6G,” Opt. Lett. 18, 37–38 共1993兲. 12. A. A. Gorman, I. Hamblett, T. A. King 共Laser Photonics Research Group, Department of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK兲, and M. D. 20 October 1999 兾 Vol. 38, No. 30 兾 APPLIED OPTICS
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13.
14.
15.
16.
17.
Rahn, are preparing a manuscript with the title “A pulse radiolysis and pulsed laser study of the Pyrromethene 567 triplet state.” M. Faloss, M. Canva, P. Georges, A. Brun, F. Chaput, and J.-P. Boilot “Toward millions of laser pulses with pyrrometheneand perylene-doped xerogels,” Appl. Opt. 36, 6760 – 6763 共1997兲. D. Pacheco and H. R. Aldag, “Comparison of the laser performance of solid state dye media under long- and short-pulse excitation,” in Solid-State Lasers VII, R. Scheps, ed., Proc. SPIE 3265, 2–12 共1998兲. R. S. Anderson, R. E. Hermes, G. A. Matyushin, V. S. Nechitailo, and S. C. Picarello, “Photostability of dye doped modified polymer at extremely high intensities: Medlite™ laser system,” in Solid-State Lasers VII, R. Scheps, ed., Proc. SPIE 3265, 13–20 共1998兲. R. E. Hermes, T. H. Allik, S. Chandra and J. A. Hutchinson, “High efficiency pyrromethene doped solid state dye laser,” Appl. Phys. Lett. 63, 877– 879 共1993兲. M. D. Rahn, T. A. King, A. A. Gorman, and I. Hamblett, “Photostability enhancement of Pyrromethene 567 and Perylene Orange in oxygen free liquid and solid dye lasers,” Appl. Opt. 36, 5862–5871 共1997兲.
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18. T. I. Suratwala, K. Davidson, Z. Gardlund, D. R. Uhlmann, S. Bonilla, and N. Peyghambarian, “Molecular engineering and photostability of laser dyes within sol-gel hosts,” in Solid-State Lasers VI, R. Scheps, ed., Proc. SPIE 2986, 141– 152 共1997兲. 19. P. R. Ogilby, M. P. Dillon, M. Kristiansen, and R. L. Clough, “Quenching of singlet oxygen in solid organic polymers,” Macromolecules 25, 3399 –3405 共1992兲. 20. A. A. Gorman, I. Hamblett, C. Lambert, B. Spencer, and M. C. Standern, “Identification of both preequilibrium and diffusion limits for reaction of singlet oxygen, O2共1⌬g兲, with both physical and chemical quenchers—variable temperature, time resolved infrared luminescence studies,” J. Am. Chem. Soc. 110, 8053– 8059 共1988兲. 21. G. Buntinx, P. Valat, V. Wintgens, and O. Poizat, “Photoreduction of 4,4⬘-bipyridine by triethylamine and by 1,4diazabicylclo关2.2.2兴octane in acetonitrile as studied by nanosecond absorption and Raman spectroscopies,” J. Phys. Chem. 95, 9347–9352 共1991兲. 22. K. I. Priyadarsini and J. P. Mittal, “Effect of 1,4-diazabicyclo关2,2,2兴-octane on the laser properties of 7-amino coumarin dyes,” J. Photochem. Photobiol. A 61, 381–388 共1991兲.
