Amplified spontaneous emission and recoverable ... - OSA Publishing

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J. Opt. Soc. Am. B / Vol. 19, No. 8 / August 2002

B. F. Howell and M. G. Kuzyk

Amplified spontaneous emission and recoverable photodegradation in polymer doped with Disperse Orange 11 Brent F. Howell and Mark G. Kuzyk Department of Physics, Washington State University, Pullman, Washington 99164-2814 Received December 17, 2001; revised manuscript received February 22, 2002 We report optically pumped lasing action at the attractive wavelength of 650 nm in the common organic dye 1-amino-2-methylanthraquinone (Disperse Orange 11). The dye was incorporated into poly(methyl methacrylate) rods, and amplified spontaneous emission was studied under second-harmonic Nd:YAG laser excitation in a transverse pumping configuration. Gain and conversion efficiency were found to be comparable with those for other laser dyes. Dye photodegradation was found to be superior and reversible. © 2002 Optical Society of America OCIS codes: 160.3380, 140.6630, 350.3390, 160.5470, 140.3440, 160.4890.

Organic laser dyes in solution are widely used in tunable lasers as amplifying media. A solid amplifying material made of dye-doped polymer has many advantages. It is safer, easier to use, and more robust, thus expanding the possible applications. Furthermore, laser-dye-doped polymers can be incorporated into the core of polymer fibers, making single-mode fiber lasers possible. However, solid-state polymer devices have not yet become a practical technology because of the poor photostability of chromophores. During optical pumping, the dye molecule experiences irreversible photodegradation.1–7 Lasing efficiency continuously and permanently decreases over a relatively short period of time, rendering the material useless as a lasing medium. Attempts have been made to delay the photodegradation process by use of additives in the polymer matrix6,8 or by rotating the pumped medium,9–11 thereby continuously changing the location of excitation. But to our knowledge, no laser-dye-doped polymer has been found that can perform with the reliability and simplicity needed for long-term use in a commercial system. In this paper we report lasing action in Disperse Orange 11 (DO11) pumped with 532 nm. Amplified-spontaneous-emission (ASE) efficiency is good with a center wavelength of ⬃650 nm. This wavelength is particularly attractive because it lies at an absorption minimum for poly(methyl methacrylate) (PMMA), which is commonly used in fabricating polymer optical fibers. (All hydrocarbon-based polymers have a window at 650 nm due to a dip in the CuH overtone spectrum.) Because of its ease of fabrication, excellent optical quality, and compatibility with laser dyes, PMMA was used in our studies as the solid polymer host for DO11. Gain was determined and conversion efficiency was found to be comparable with other popular laser dyes such as rhodamines and courmarins in solid polymer matrices. DO11 photostability studies showed that it has superior resistance to photodegradation. We also found the novel result that the photodegraded dye molecules in the dark recover their original ASE efficiency. This nonpermanent photo0740-3224/2002/081790-04$15.00

degradation, as well as its attractive lasing wavelength, makes DO11 (and other cycloquinones) an excellent candidate for solid-state and fiber-based laser systems. DO11 and methyl methacrylate were purchased from Sigma-Aldrich and were used as received. The molecular structure of DO11 is shown in the inset of Fig. 1. DO11doped PMMA rods were thermally polymerized in test tubes. Tert-butyl peroxide was used as an initiator and butanethiol was used as a chain transfer agent in the amount of 33 ␮l per 10 ml of methyl methacrylate for each. Polymerization took place in a 95 °C oven for 48 h. Samples were made with concentrations of 3, 6, and 9 g of DO11 per liter of methyl methacrylate. Dye saturation resulted in slight aggregation for the 6-g/l sample, while substantial aggregation was observed in the 9-g/l sample. Dye-doped polymer rods of diameter 0.5 in. and length 2 in. were cut and polished on one end while leaving the other end rough. The outer surface of the cylindrical rods was optically smooth from contact with the test tubes’ inner smooth surface during polymerization. The absorption spectrum shown in Fig. 1 (curve a) was obtained by using a thin-film sample and an Ocean Optics Inc. spectrometer. A xenon arc lamp provided white light illumination. The absorption maximum of DO11 in PMMA was found to be at 470 nm. A fluorescence spectrum, also shown in Fig. 1 (curve b), was obtained with the same sample and spectrometer under low-intensity 532-nm Nd:YAG second-harmonic excitation. The fluorescence peaks at 595 nm with a shoulder at ⬃630 nm. The 532-nm pump can be seen as a small peak in the fluorescence spectrum. The large wavelength difference (⌬␭ ⫽ 180 nm) between the absorption peak at 470 nm and the ASE at 650 nm is larger than for most other laser dyes, in which it is typically ⬍100 nm.12 Because of this large Stokes shift, very little reabsorption of ASE by the bulk material is expected. This also suggests phototautomerization as one possible underlying process responsible for the fluorescence and ASE. The 532-nm, 35-ps pump pulses were generated by a mode-locked Con© 2002 Optical Society of America


Fig. 1. (a) Absorption spectrum of DO11-doped PMMA, (b) Fluorescence spectrum of DO11-doped PMMA when pumped with 532 nm. Inset, molecular diagram of DO11.

tinuum laser operating at 10 Hz. This laser was used in all ASE and photodegradation studies. Photodiodes were used in conjunction with a laser-triggered charge integration system, analog-to-digital converter, and LabVIEW software to measure pump and ASE radiation in all nonspectrometer-based experiments. Absolute pump and ASE power were measured with a Laser Probe Inc. radiometer. The dye-doped polymer rods are pumped in a transverse configuration, resulting in ASE radiation perpendicular to the incident pump. The polarization of the incident pump determines whether the ASE radiation is generated and emitted horizontally or vertically. In all measurements pump polarization is vertical relative to the optical table with resulting ASE radiation propagating parallel to the table. Using a concave and cylindrical lens, the resulting pump beam profile is formed into a line of uniform intensity with dimensions of 14 mm ⫻ 0.1 mm at the sample as determined by BeamStar beam diagnostic hardware and software. With an iris after the cylindrical lens and before the sample, the pump profile is apertured to make it of uniform intensity along the length of the profile. The dye-doped polymer rod is pumped along the side and at the polished end of the sample to ensure that no ASE radiation travels through unpumped material before exiting. Figure 2 shows emission spectra from the 6-g/l sample as a function of pump power. The transition can be seen from normal fluorescence, through threshold, to complete ASE. The FWHM of ASE radiation is 11 nm centered at ⬃ 649 nm. The inset of Fig. 2 shows the corresponding fluorescence and ASE absolute signal versus pump power, with a strong nonlinear dependence through threshold as expected. ASE threshold is reached with a pump power of ⬃40 ␮J, corresponding to a peak intensity of ⬃1 ⫻ 108 W/cm2 . Conversion efficiency was measured with the same experimental setup as previously discussed. ASE output power from one end of the sample was measured as a function of the 532-nm pump power. The results for the 3, 6, and 9-g/l dye concentration samples are shown in Fig. 3. Saturation onset is evident at pump powers above ⬃500 ␮J. Although their absolute efficiencies increase with increasing dye concentration, their slope efficiencies

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are quite similar. A maximum ASE conversion efficiency of 5% is found in the 9-g/l sample. This value is comparable with and in some cases better than that of other commonly used laser dyes in a polymer matrix pumped in a similar fashion.1–4,8,13,14 Because the ASE radiation is generated and radiated equally in two opposite directions along the rod sample, the ASE power that is measured out one end of the sample is only half of the total generated power. Therefore the true conversion efficiency for the total generated ASE is ⬃10%. A single-pass gain measurement of DO11 in PMMA was determined by the ASE method proposed by Shank.15 The same pumping and 9-g/l sample arrangement was used as previously described, except the length of the pumped region of the rod was varied from 12 to 6 mm, using a beam block. Gain G was determined by using the equation G ⫽ 共 2/L 兲 ln共 I L /I L/2 ⫺ 1 兲 ,

(1)

where L is 12 mm and I L and I L/2 are the resulting ASE intensities for the 12- and 6-mm pump lengths, respectively. Figure 4 shows the computed gain versus pump peak power. For the unsaturated region below 3 MW, the calculated gain per unit length is found to be ⬃1 cm⫺1 MW⫺1. This ASE gain is 20% of the value obtained in a similar manner for Rhodamine 6G.9 It should be noted

Fig. 2. Transition of DO11 fluorescence to complete ASE dominance centered at 649 nm with increasing 532-nm pump power. Inset, corresponding fluorescence and ASE intensity versus 532-nm pump power through ASE threshold.

Fig. 3. DO11-doped PMMA conversion efficiencies for 3, 6, 9-g/l dye concentrations displayed as ASE power versus 532-nm pump power.

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Fig. 4. Calculated gain versus 532-nm pump peak power for DO11-doped PMMA.

Fig. 5. Photodegradation of DO11-doped PMMA ASE intensity as a function of time. (a) First degradation run, sample area not previously degraded; (b) third degradation run, twice previously degraded and recovered sample area. Excitation source is continuously on 10-Hz, 0.2-mJ/pulse, 35-ps, 532-nm pump radiation.

that during the efficiency and gain measurements no attempt was made to limit optical feedback from the inner front surface of the rod. This may have slightly reduced the measured values. To determine the ability of DO11 to resist photodegradation in PMMA and recover once degraded, the same experimental setup was used as previously described. As constant pump radiation was incident on the sample, the resulting ASE output was detected and recorded as a function of time. Photodegradation of the sample is evident in the detected decrease in ASE output while the pump intensity remained constant. Because the Nd:YAG laser used for pumping is inherently unstable over long time periods, the required pump stability was maintained with computer-controlled ultrafine rotation of a ␭/2 waveplate before a polarizer. All three concentration samples were studied, but the 9-g/l sample gave the best results. Figure 5(a) shows the ASE output from the 9-g/l sample over a period of 9 h while it was being constantly excited with 10-Hz, 0.2-mJ/pulse, 532-nm pump. This results in an ⬃62% drop in ASE efficiency after 324,000 pump pulses at a constant peak intensity of 5.7 ⫻ 108 W/cm2 . This is, to our knowledge, the best photodegradation resistance yet to be found for an organic laser dye in a polymer matrix pumped in a similar fashion. A slowing in


the rate of photodegradation is evident over time. Approximately half of the total degradation occurs during the first 3.5 h, while the remaining half occurs during the final 5.5. An explanation for the saturation of degradation is that the photodegradation process is reversible and the saturation level arises from the offsetting effects of degradation and recovery. In this recovery effect, the photodegraded state of the dye molecule is unstable, allowing molecules to relax thermally back to their original form and contribute to ASE. At long exposures, the rate of photodegradation approaches the rate of recovery. Because the rate of photodegradation is dependent on pump intensity and dye concentration, and because the pumpenergy threshold for ASE in this sample is ⬃40 ␮J, less pump power could be used on a higher-concentration sample, resulting in even better recovery from photodegradation than shown here. The possibility of complete recoverability of a photodegraded sample was studied. After having been photodegraded to 50% of original ASE output, the sample was allowed to rest unpumped for approximately 26 h at room temperature. Subsequently the sample was pumped under identical conditions, and the ASE intensity was measured. We observed a 95%–105% recovery of the ASE efficiency. Care was taken to make sure the experimental conditions were identical the following day and that the sample was pumped at the same (previously degraded) location with the same intensity. To observe this recovery effect in real time, a recently degraded sample was pumped for 20 s once every hour for 24 h. With the pump beam blocked the vast majority of the time, the degraded area of the sample was allowed to recover essentially unpumped at room temperature. The ASE signal was intermittently checked by using a beam block attached to a motor and controlled by the data acquisition software. Figure 6 shows the results for the 9-g/l sample pumped with 5.7 ⫻ 108 W/cm2 peak intensity. Curves (a) and (c) correspond to degradation while (b) and (d) show recovery. A function including a rising and decaying exponential, written generally as I ⫽ P 1 exp共 ⫺t/ ␶ 1 兲 ⫹ P 2 关 1 ⫺ exp共 ⫺t/ ␶ 2 兲兴 ⫹ I 0 ,

(2)

Fig. 6. Two successive photodegradation and recovery cycles of a single DO11-doped PMMA sample area. (a) First degradation, (b) corresponding first recovery, (c) second degradation, (d) second recovery. Excitation source is 10-Hz, 0.2-mJ/pulse, 35-ps, 532-nm pump radiation continuously on for degradation runs and intermittent for recovery runs.


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where I is the ASE intensity and t is time, was fitted to the data. Although the function nicely matched the data, more work is needed to extract physical meaning from the parameters. It was found that for a 50% photodegraded sample the time required for total ASE efficiency recovery at room temperature is approximately 30 h. This recovery process is accelerated if done at elevated temperatures. Furthermore, the sample’s resistance to photodegradation becomes better with successive cycling. The approximate half-lives (time required for 50% decrease in ASE) represented by curves (a) and (c) are 210 and 290 min, respectively. Figure 5(b) shows the photodegradation of a sample area after having gone fully through two previous degradation–recovery cycles, i.e., twice previously degraded to 50% of its initial ASE intensity and allowed to recover each time. While a fresh sample area has a half-life of 210 min, [Fig. 5(a)], the half-life of the sample twice previously degraded is almost doubled to 400 min. To our knowledge this effect has never been previously observed and implies that the photodegradation resistance of laser dye in polymers may be increased through optical processing. In conclusion, we have observed ASE in DO11 in PMMA with good efficiency. The ASE peaks at the important telecommunication wavelength of 650 nm. The material has very good stability and photodegradation shows reversibility. Furthermore, the degradation process is made slower by subjecting the material to several degradation–recovery cycles. This material system is therefore scientifically interesting and may have applications in which stable organic-solid laser sources are required. Experiments in progress include the investigation of the lasing properties of DO11 with liquid dye cells and dye-doped fibers.

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ACKNOWLEDGMENTS

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The authors thank SpectraLux Corporation and The Washington Technology Center for supporting this work.

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B. Howell’s e-mail address is [email protected].

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