Broad band solid state dye lasers based on LDS 698 doped in modified polymethyl methacrylate (MPMMA) with laser wavelength about 650nm were demonstrated. It was demonstrated that the fluorescence spectra of LDS 698 in solid host MPMMA displays an obvious blue shift about 50nm comparing with that in ethanol solution. The dye concentration has great effect on the laser’s performance including laser slope efficiency and lifetime. The lifetime increased dramatically with the increase of the LDS698 concentration. With pump repetition rate of 10Hz and intensity of 0.1J/cm2, the maximum lifetime 300,000 shots corresponding normalized photostability 102GJ/mol was obtained with LDS 698 at 1.5×10-4mol/L.
© 2008 Optical Society of America
Organic dye lasers are a kind of very important laser sources for many applications where tunable high power or high pulse energy beams are required, such as nonlinear optics, medicine and industry . Usually, liquid dye solution is used as gain medium of dye lasers. However, these liquid lasers cannot reach the commercial marketplace widely because of their inherent defects such as large and complicated dye circulatory systems and sometimes toxic and flammable solvents. Because solid state dye lasers can overcome the disadvantages of liquid dye lasers while keep the advantages of broad tuning range and high efficiency of laser dye, the research on such a laser has attracted much attention and made great progress in past years [2–6]. Rhodamine dyes, coumarine dyes, perylene dyes and pyrromethene dyes solid state action has been reported in sol-gel glasses, organically modified silicates (ORMOSILs), methyl methacrylate based polymers and copolymers [7–12]. But most of these works are focus on the dyes with their tuning range less than 620nm such as PM567, PM597 and Rhrodamine 590 and so on. In order to extend the solid state dye laser’s tuning range more to the red, large Stokes shift dye DCM has been doped ORMOSIL and the peak wavelength of DCM doped ORMOSIL is about 620nm . Trying to go further, in our work LDS 698 which can be tuned between 645–730nm in Ethanol was used to extend solid state dye laser’s wavelength more to red.
In this paper, LDS 698 with different concentration were doped in modified polymethyl methacrylate (MPMMA) and their laser characterizations were studied. The MPMMA was used as host matrix because of its excellent optical homogeneity with large volume which is very important for a laser medium.
2.1 Preparation of dyes doped MPMMA
Methyl methacrylate (MMA) was washed three times with 10% (weight per volume) aqueous sodium hydroxide to remove the inhibitor and then two times with distilled water. After drying over anhydrous CaCl2, MMA was vacuum distilled . Laser dye LDS 698 was used as received (Exciton Inc.). The ethanol was chosen as modifying additives to PMMA because ethanol is good solvent for LDS 698. It has been proved that the ethanol can increase the performance of laser medium such as lasing efficiency and damage threshold [14, 15]. The modified PMMA samples were fabricated by dissolving the required amount of dye to the ethanol, and then the dye solution was mixed into the purified MMA with volume ratio Ethanol : MMA=1:3. Polymerisation was carried out in silanised glass tubes using azoisobutypronitrile (AIBN) as the radical initator. After the curing process, the samples were cut into cylindrical rods 15mm in diameter and 20mm in length, and their ends were polished to obtain reasonably flat, smooth, glass-like surface.
The absorption spectra of LDS 698 in MPMMA and ethanol were measured with spectrophotometer UV-3010PC (Shimadzu Company) and the fluorescence and laser spectra were measured at room temperature with a spectrometer HR4000 (Ocean Optics, 200– 1100nm). The pump laser source in this work was a second-harmonic of Q-switched Nd: YAG laser with a 10ns full-width at half-maximum (FWHM) at 532nm and beam diameter of 5mm. Longitudinal pump was chosen for laser experiments.
The solid state dye laser cavity is comprised a dichroic plane mirror Mb with high reflectivity (R>95%) in 580–680nm and high transmission (T>95%) at 532nm and a plane output coupler Mo with transmission 45–55% in 580–680nm, as shown in Fig. 2. The length of the laser cavity is about 12cm. M1 and M2 are high reflectivity at 532nm and high transmission at 1064nm. They are used couple the pump beam into the dye laser cavity. After M2 a beam splitter (BS) was used to monitor the input energy. Before the energy sensor1, a beam filter was used to reflect the residual 532nm pump laser. The output energy and pump energy were all detected by energy sensors J25-MB (Coherent Inc) and the outputs of energy sensors were recorded by computer through EPM 2000 dual-channel energy meter.
3. Results and discussion
3.1 Absorption and fluorescence properties
Figure 3 shows the absorption and fluorescence spectra of LDS 698 in ethanol and MPMMA. It can be seen that the absorption band of LDS 698 in MPMMA and ethanol is about between 400–550nm and the difference is not very significant between 450–800nm. The bandwidth of LDS698 fluorescence spectra in MPMMA and ethanol is also not obvious different and both are approximately 80nm (FWHM). But it can be seen LDS 698 in solid host MPMMA displays an obvious blue shift of fluorescence spectra. The peak wavelength of LDS 698 in MPMMA is about 632nm and 50nm shorter than that of the dye in ethanol 682nm. For comparison the fluorescence spectra of LDS 698 in liquid MMA monomer was also obtained and it is similar to ethanol as shown in Fig. 3.
To understand the large blue shift of LDS 698 in solid MPMMA, it is necessary to pay some attention to the behavior of the dye in media with different viscosity and polarity. As shown in Fig. 1 just like DCM the LDS 698 molecule includes a dimethylamino group and an aromatic ring. In the ground state the molecule is almost planar, which corresponds to the maximum conjugation between the dimethylamino group and the phenyl ring. According to the Franck– Condon principle, the locally excited state is still planar, but solvent relaxation takes place with a concomitant rotation of the dimethylamino group forming twisted intramolecular charge transfer (TICT) state, stabilized by the polar solvent molecules. The formation of the TICT state is strongly dependent on the viscosity and polarity of the medium. In polar solvents such as ethanol and liquid MMA monomer the transition to TICT state completed very quickly, no fluorescence from the locally excited state is observed and only the large red-shifted emission from the TICT state is detected [16–18]. But in highly viscous medium such as MPMMA, the rotation around the nitrogen-aromatic carbon is hindered and the TICT state that caused red shift of fluorescence is not formed. Thus fluorescence mainly comes from the locally excited state and the maximum of the fluorescence emission from LDS 698 in MPMMA is located at 632nm, a large blue shift about 50nm compared with that in ethanol.
3.2 Slope efficiencies and laser lifetimes
Dye laser efficiency measurements were obtained by averaging several single-shot data points at each input energy. The slope efficiency was calculated by a simple linear least squares fit to the input versus output data and shown in Fig. 4, as summarized in Table 1. It can be seen that the dye concentration has influence on the slope efficiency. In our laser configuration the slope highest slope efficiency is 17.2% when the dye concentration is 0.75mol/L. We believe the efficiency of the dye laser can be improved by using a more optimized cavity and eliminating the samples surface reflection that leads the cavity loss increase. The output wavelength was also monitored by spectrometer as shown in Fig. 5 and collected in Table 1. The Ep and Eo in Fig. 5 are pump energy and output energy when the laser spectra was detected. The laser spectra all have a wide band-width of about 11–14nm. The maxima of the lasing spectrum is 649–653nm. It can be seen that the laser spectra is not smooth enough. That may be caused by that the lasing material is big in our work and slightly differences in the local polymer density or local dye concentration may cause the emission of several slightly different laser lines.
The laser lifetime is a key performance parameter of a dye laser, which limits the applications. In this work, the normalized output energy (normalized by initial output energy) as function of number of laser pulses was obtained, as shown in Figs. 6(a) and 6(b) and also listed in Table 1. The input energy is 20mJ with its diameter 5mm, corresponding 0.1J/cm2 pump intensity, at the repetition rate of 10Hz. It should be pointed out that in Fig. 6(b) the increase of the conversion efficiency in the beginning of the experiment is probably caused by the gain medium achieving a more optimal dye concentration by means of photodegradation of a part of dye moclues. The laser lifetime of the sample is defined as the number of pump pulses when the output energy decreases to 50% initial value. It can be seen that the concentration has great influence on the solid dye laser’s lifetime. We obtain about 300,000 shots with concentration of 1.5×10-4mol/L, while only 2,700 shots with dye at 0.5×10-4mol/L. In our experiments, higher concentration sample has a longer lifetime. A possible explanation is given as follow. As indicated in Ref. 19 one of main mechanisms by which the dye degrades is the photochemical reaction of dye molecule with surrounding chemically active molecules, thus when the dye concentration is high enough and a fraction of dyes may exhaust most of the active molecules, then the photodegradation rate of the other dyes will decrease. In other words a part of LDS 698 molecules act as stabilizer preventing the other dye molecules photodestruction when the concentration is high. However, the increase in concentration also leads the dye hard to be dissolved in MMA uniformly. When take this into account, we find that concentration at 1.5×10-4mol/L is suitable.
For comparison with the results obtained by different authors with various experimental arrangements, we use the normalized photostability which is defined as the accumulated pump energy absorbed by the system per mole of dye molecules before the output energy falls to one half of its initial value.
Normalized photostability Where Epump is the pump pulse energy (in joules), N 1/2 is the number of pulses to half initial output energy, r is the radius of the pumpbeam on the surface of the sample and l is the length with their units in centimeters and C is the concentration (molar per liter). The units of normalized photostability are gigajoules per mole. The results were collected in Table 1. It can been seen that the normalized photostability increases dramatically with the increase of dye concentration. The maximum normalized photostability 102GJ/mol was obtained with LDS 698 at 1.5×10-4mol/L.
In conclusions, solid state dye lasers based on LDS 698 doped in MPMMA with wavelength about 650nm are demonstrated. The dye concentration has great effect on the laser’s performance including laser slope efficiency and lifetime. Pumped by a second harmonic Q-switched Nd:YAG laser with 10Hz repetition and intensity 0.1J/cm2, the maximum lifetime about 300,000 shots corresponding normalized photostability 102GJ/mol was obtained with LDS 698 concentration at 5×10-4mol/L.
The authors gratefully acknowledge the financial support from the Program for New Century Excellent Talents in University (NCET) and Program of excellent team in Harbin Institute of Technology.
References and Links
1. F. J. Duarte, “Organic dye lasers: brief history and recent developments,” Opt. Photon. News 14, 20–25 (2003). [CrossRef]
2. I. G. Kytina, V. G. Kytin, and K. Lips, “High power polymer dye laser with improved stability,” Appl. Phys. Lett. 84, 4902–4904 (2004). [CrossRef]
3. A. Mandl, A. Zavriyev, and E. Klimek, “Energy scaling and beam quality studies of a zigzag solid-state plastic dye laser,” IEEE J. Quantum Electron. 32, 1723–1726 (1996). [CrossRef]
5. F. J. Duarte, A. Costela, I. Garcia-Moreno, R. Saster, J. J. Ehrlich, and T. S. Taylor, “Dispersive solid-state dye laser oscillators,” Opt. Quantum Electron. 29, 461–472 (1997). [CrossRef]
7. A. Costela, I. Garcia-Moreno, and R. Sastre, “Polymeric solid-state dye lasers: Recent developments,” Phys. Chem. Chem. Phys. 5, 4745–4763 (2003). [CrossRef]
8. G. Somasundaram and A. Ramalingam, “Gain studies of Coumarin 1 dye-doped polymer laser,” J. Lumin. 90, 1–5(2000). [CrossRef]
11. E. Yariv, S. Schultheiss, T. Saraidarov, and R. Reisfeld, “Efficiency and photostability of dye-doped solid - state lasers in different hosts,” Opt. Mater. 16, 29–38 (2001). [CrossRef]
12. Y. Yang, M. Wang, G. Qian, Z. Wang, and X. Fan, “Laser properties and photostabilities of laser dyes doped in ORMOSILs,” Opt. Mater. 24, 621–628 (2004). [CrossRef]
14. K. M. Dyumaev, A. A. Manenkov, A. P. Maslyukov, G. A. Matyushin, V. S. Nechitailo, and A. M. Prokhorov, “Dyes in modified polymers: problems of photostability and conversion efficiency at high intensities,” J. Opt. Soc. Am. B 9, 143–151 (1992). [CrossRef]
15. W. J. Wadsworth, S. M. Giffin, I. T. McKinnie, J. C. Sharpe, A. D. Woolhouse, T. G. Haskell, and G. J. Smith, “Thermal and optical properties of polymer hosts for solid-state dye lasers,” Appl. Opt. 38, 2504–2509 (1999). [CrossRef]
16. M. Alvarez, F. Amat-Guerri, A. Costela, I. Gareia-Moreno, M. Liras, and R. Sastre, “Laser emission from mixtures of dipyrromethene dyes in liquid solution and in solid polymeric matrics,” Opt. Commun. 267, 469–479 (2006). [CrossRef]
17. B. Valeur, Molecular fluorescence: principles and applications (Wiley-VCH, Boschstrasse,2001), Chap. 3.
18. L. C. Zhou, J. Y. Liu, G. J. Zhao, Y. Shi, X. J. Peng, and K. L. Han, “The ultrafast dynamics of near - infrared heptamethine cyanine dye in alcoholic and aprotic solvents,” Chem. Phys. 333,179–185 (2007). [CrossRef]
19. S. Popov, “Dye photodestruction in a solid-state dye laser with a polymeric gain medium,” Appl. Opt. 37, 6449–6455 (1998). [CrossRef]
20. M. D. Rahn, T. A. King, A. A. Gorman, and I. Hamblett, “Photostability enhancement of Pyrromethene 567 and Perlene Orange in oxygen-free liquid and solid dye lasers,” Appl. Opt. 36, 5862–5871 (1995). [CrossRef]