Pyrromethene dyes doped polymeric channeled waveguide lasers with permanent DFB structures were fabricated via a novel pen-drawing technique with the patterned polydimethylsiloxane (PDMS) chips fabricated through a casting process as the substrates. With the high resolution dispensers, dye doped high viscosity pre-polymers were written into the PDMS grooves and the cross-section of the channeled waveguides could be controlled by both the polymer composition and the pen-drawing parameters. Highly stable laser output with 4.8 × 106 pulses of laser lifetime at 500 Hz of pump repetition rate has been obtained, which is suggested to be among one of the best results of pyrromethene 567 (PM567) up to date.
© 2010 OSA
Due to their advantages such as low cost, high efficiency and wide spectrum range on other tunable laser sources in the visible and the capability to be toxic- and maintenance-free, in comparison with the traditional liquid counterparts, solid-state dye lasers (SSDLs) based on polymeric materials [1–3], sol-gel derived glasses [4,5], and composite materials [6,7] have generated extensive research interests during the last 2 decades. In recent years, great progresses have been made on the performances of SSDLs especially the longevity, by applying various host matrices. For instance, the utilization of fluorinated polymers [8,9], organic-inorganic hybrid materials [10–12], and nano-particles dispersed polymers [13,14] as solid hosts have improved the laser performances significantly. The studies on the photo-degradation mechanisms [15,16], energy transfer process between dye molecules [17,18] and other researches such as miniaturized cavity design and surface emitting lasers [19–22] also contributed to the efforts for highly efficient, long lifetime and broadly tunable organic lasers for practical use.
Due to their compatibility to integrated optics, optical waveguides activated by organic chromophores with DFB structures have been investigated intensively in recent years, which had extensive applications [23,24]. For instance, as one of the latest application of SSDLs in our lab, we proposed recently the multi-color laser arrays on the micro-flowcytometry chips, which were fabricated and mounted to couple the laser output into a micro-fluid channel . However, most of the dye-doped DFB waveguide lasers reported currently were made by spin-coating technique and stacking of layers [13,23–29], which were materials consuming and neither not suitable for the fabrication of complicated structures. For example, in the case of the flowcytometry chip, it was almost infeasible to integrate various colors of waveguide lasers doped with various organic dyes on one single chip with a high positioning accuracy by spin-coating or dip-coating in a cost-effective way without contamination.
Recently, a novel pen-drawing fabrication technique has been reported and developed in our group, which facilitated multiple-layered waveguides with extremely low material consumption in the limited area on any surface, no matter flat or curved. The trapezoid cross-sections by direct writing on the flat surface of poly(methyl methacrylate) (PMMA) substrates  and rectangle cross-section by post-etching treatment  have been reported. However, there were still some severe difficulties on the fabrication of methacrylate copolymers (MA-cop) waveguides on the PDMS substrate, which was more common as flowcytometry chips, due to the poor adhesion between MA-cop and the substrate, and the high contact angle of MA-cop on the PDMS substrates during the pen-drawing process. The poor cross-section profile of the drawn waveguide should also be improved for DFB lasing.
Herein, we reported the improvement of our pen-drawing technique on the fabrication of channeled waveguide lasers on the patterned PDMS integrated chips. We found that the silyl-methacrylate copolymer (Si-MA-cop) and deep-UV treatment could solve the problems such as poor adhesion, poor cross-section profiles and also provide longer durability of laser waveguide in comparison with our previous works.
2. Experimental details
The PDMS on Polyester (PET)/Indium Tin Oxide (ITO) chips were made by a casting process, as shown in Fig. 1 . First, the commercially available silicone, known as SIM-260 (Shinetsu Chem. Co.) mixed with its curing agent, was casted onto the master pattern made by the photolithography of photo-resist SU-8 3000 (Microchem Co.) on quartz substrate. An ITO coated PET film with the PET surface pre-coated to improve the adhesion was put onto the PDMS pre-polymer as the cover and then pilled off from the SU-8 master after the polymerization of PDMS. The thickness of the PDMS layer was 150 μm, and the size of the channel for laser drawing on the PDMS chip was 7 mm(L) × 200 μm(W) × 60 μm(D). The refractive index of PDMS was ~1.41.
The highly viscous pre-polymer of Si-MA-cop was prepared as follows: First, laser dyes PM567 and coumarin 540A (C540A) (laser grade from Exciton) were mixed with methyl methacrylate (MMA) monomer and 3-(trimethoxysilyl)propyl methacrylate (TMSPMA, Sigma-Aldrich). The ratio between MMA:TMSPMA was kept at 40:60 in volume if not mentioned specifically, and the molar ratio between PM567 and C540A was 1:1. The initial concentration of each dye in the mixed solution was 2 mM. 0.166 wt% of 2,2’-azobis(2-methylpropionitrile) (AIBN) was also introduced as the initiator for polymerization. After the dissolution of the dye, the mixed solution was filtered by a 0.2 μm pore size filter (WhatmanLaboratory, PTFE disposable filters). Polymerization was conducted at 72°C under vigorous stirring, and was stopped at the viscosity of ~700 mPa·s at room temperature. PM567, pyrromethene 597 (PM597) and the other dyes such as LDS 722 and LDS 798 solely doped PMMA or P(MMA-TMSPMA) pre-polymers, were also prepared in the similar process.
A high-resolution air-pressured dispenser (ML-5000XII, Musashi Engineering) was adopted for the pen-drawing process. As shown in Fig. 2 , by synchronizing the air pressure control and the nozzle movement, the pre-polymer was injected into the channel on PDMS/PET/ITO chips. It should also be mentioned that the adhesion between the deep-UV pre-exposed PDMS substrates (172 nm, 9 J/cm2, Ushio UER20-172) and Si-MA-cop was improved to the practical level for the pen-drawing. The thickness and cross-section of the waveguides depended on the fabrication parameters such as the nozzle size, air pressure, moving speed, viscosity of the pre-polymer, and polymer composition. The typical drawn parameters adopted in this work, including the air pressure, moving speed and nozzle size were 300 kPa, 3 mm/s, and 100 μm, respectively, if not mentioned specifically. The thickness and cross-section profiles of the waveguides were measured by a micro-scope (Nikon TE 2000-U) and surface profiler (SE-1100, Kosaka Laboratory Ltd.). The waveguides were then post-baked at 72°C to complete the polymerization. Subsequently, an index-type (or developed surface relief) first order DFB structure on the waveguides was made by an interfered deep-UV-exposure (244nm, cw, Wave-train & model 2060, Spectra Physics) , as shown in Fig. 1. The periodicity of the DFB structures varied according to the laser emission wavelength expected. The refractive index of P(MMA-TMSPMA) and PMMA waveguides were 1.476 and 1.498 (Metricon Model 2010), respectively.
The laser performances of the DFB waveguides were evaluated by transversely pumping with a passively-Q-switched and frequency-doubled Nd:YAG laser (0.5 ns, 532 nm) in a beam size of 7 × 0.3 mm2. The pump energy and repetition rate were 30 μJ/pulse and 500 Hz, respectively. The output from the waveguide laser was collected by a multi-channel spectroscope (HR4000, Ocean Optics) and a photodiode.
As considering the real use of the tunable DFB waveguide lasers on the integrated flowcytometry chip, for instance, the laser-induced fluorescence measurement [25,30,34], it was suggested that the amplified spontaneous emission (ASE) content in the longer wavelength region should be depressed. Previously, it has been demonstrated that by further optimizing the UV doze amount for the DFB recording and the dye concentration, the ASE ratio might be reduced .
According these requirements, The ASE suppression and spectral profile were focused in this work. As the most common structure, rectangular waveguides were expected for the tunable waveguide lasers. However, even during the pen-drawing process, shrinkage of the pre-polymer injected into the PDMS grooves was observed. Without etching process, the rectangular cross-section was hard to obtain due to the surface tension force and shrinkage during the solidification.
Figure 3 showed the schematic of two approaches of the pen-drawing and solidifications of the integrated lasers. To align the laser output and the flow-channel, laser waveguides were drawn on the bottom of the PDMS grooves. In scheme (a), vertical multi-mode lasing was obtained from the thick waveguide and DFB didn’t work at the thicker parts beside the sidewalls. So, the ASE content couldn’t be suppressed in this case. In scheme (b), it was suggested that by injecting the pre-polymer onto the bottom of PDMS groove with less injection amount followed by the control on the evaporation behavior, single-mode waveguide lasers with rectangular cross-section and less ASE content might be obtained.
3. Results and discussions
In the following discussions, the influence of pen-drawing parameters such as the air pressure, and the polymer composition on the ASE content in the output laser was investigated with the waveguides written in the two schemes, as shown in Fig. 3.
Figure 4 showed the microscope images of the waveguides wrote in scheme (a) of Fig. 3.The cross-section profiles of the waveguides were determined by the shrinkage of the pre-polymer injected into the PDMS grooves. In case of the PMMA pre-polymer, the cross-section profile of its channel waveguides were far from rectangle, as shown in Fig. 4(a). In contrast, a less shrinkage could be observed from that of the P(MMA-TMSPMA) channel waveguides, as shown in Fig. 4(b). The center surface was almost flat and ~25 μm of thickness was obtained. The much less shrinkage of P(MMA-TMSPMA) waveguides in comparison with that of PMMA waveguides might be ascribed to the higher boiling point of TMSPMA (190°C) than MMA (100°C). It should be noted that previously the LDS dye-doped sol-gel derived channeled waveguides made by spin-coating on wet-etched slide glass substrates have been reported . However, the cross section was half-round without mentioning the infeasibility of integrating different colors of spin-coated channeled waveguide lasers on one single substrate. And it should also be mentioned that more efficient DFB structure formation might be expected with the much flatter surface of the P(MMA-TMSPMA) waveguides under the exposure of the interfered 244 nm laser beams for DFB recording.
Although it was multi-mode lasing due to the relatively large thickness and the cross-section profile was still not exactly rectangular, narrow line-width laser output under the 532 nm laser pumping have been observed after recording the DFB structure , as shown in Fig. 5 . With the PM567 and C540A as the dopants, the laser output could be tuned from 541 nm to 570 nm by varying the DFB pitch. The line-width of the output laser was ~0.14 nm and the ASE ratio was ~3%. Similar narrow line-width tunable laser output could also be observed with the other channeled waveguides, for instance, those doped with PM597 or other dyes such as LDS 722 and LDS 798 on the same chip, as shown in Fig. 2(b) and Fig. 5. By selecting different channeled waveguides with different dopants or DFB structures on the same chip, tunable narrow line-width laser oscillation in various colors could be realized easily. Compared with other reports on the organic tunable lasers with permanent DFB structures made by spin-coating and stacking of layers, it was suggested that the tunable DFB channeled waveguide lasers made by the pen-drawing technique and integrated on one single PDMS chip were more practical in use, without mentioning the other advantages such as cost-effectiveness and compatibility with complicated surface and patterns.
By using the scheme (b) in Fig. 3, at the fixed moving speed of 10 mm/s, P(MMA-TMSPMA) waveguides directly wrote on the PDMS bulk surface at various air pressures were obtained. As shown in Fig. 6 , with the increase of air pressure from 50 to 300 kPa, the cross-section profile of the waveguide was improved with a flatter top surface, which might improve the quality the permanent DFB grating structure formed in the waveguide under the two 244nm laser beam interference . It was found that at the air pressure of 300 kPa, the ASE ratio of the DFB waveguide laser doped with PM 597 on the PDMS bulk surface was dropped to the level of 10−3, almost 5 times lower than that in Fig. 5, as shown in Fig. 7 .
However, the cross-section profiles of the P(MMA0.4-TMSPMA0.6) waveguides were still far from the shape of rectangular. In this work, it was found that the cross-section profiles might also be influenced by the polymer composition, i.e. the MMA/TMSPMA ratio. As shown in Fig. 8 , under the fixed pen-drawing parameters, the P(MMA1-x-TMSPMAx) waveguides in various TMSPMA content wrote on the PDMS bulk surface were obtained, with different cross-section profiles. It was found that with the decrease of TMSPMA content from 0.4 to 0.3, the cross-section profile closer to rectangular was observed with a flatter top surface, which might be ascribed to the drying up behavior resulted from the relatively faster evaporation at lower TMSPMA content. At the TMSPMA content of 0.3, the thickness of the waveguide was ~5 μm, which still resulted in multi-mode lasing. However, DFB laser output with ASE ratio of less than 1% was observed with the PM 567 doped P(MMA0.7-TMSPMA0.3) waveguide, as shown in Fig. 9 , which was lower than that shown in Fig. 5. It was believed that by further optimizing the pen-drawing parameters, DFB recording process, dye concentrations and so on, lower ASE ratio might be obtained. And it should also be noted that to make the assessment on the 2 schemes of waveguides shown in Fig. 3, the channel waveguides as shown in Fig. 4(b) had less limitation on the dye concentration due to the relatively larger thickness, which might avoid the high concentration doping and dimmers as the result, and improve the absorption of the pumping beam. On the other hand, the waveguides wrote onto just the bottom of PDMS surface, were more flexible and more likely to get the single-mode DFB laser output. It was suggested that by applying the dual-layered waveguide structure, i.e. writing a thin blank layer stacked on the top of the dye doped P(MMA-TMSPMA) as the DFB layer, while using the relatively thick dye doped P(MMA-TMSPMA) as the active layer, single mode DFB laser output with depressed ASE content might be obtained . This work is currently underway.
The laser efficiency and threshold energy of the PM 567 and C540A co-doped P(MMA-TMSPMA) channeled waveguides were also measured. At the output wavelength of 554 nm, as shown in Fig. 10 , the laser threshold was 1.2 μJ and the slope efficiency was estimated to be 1.7%. Here, it should be mentioned that due to the larger waveguide thickness, multi-mode laser output was obtained, which worsened the laser efficiency. It was suggested that by applying the dual-layered waveguide structure, i.e. writing another thin blank layer stacked on the top of the dye doped P(MMA-TMSPMA) as the DFB layer, while using the dye doped P(MMA-TMSPMA) as the active layer, single mode DFB laser output might be obtained . As also shown in Fig. 10, under the same conditions, the slope efficiency and threshold of PM 567 solely doped PMMA waveguide were merely 1.3% and 1.8 μJ, respectively.
Finally, the laser lifetime of the DFB waveguide lasers was investigated by measuring the laser output after various periods of pumping. As shown in Fig. 11 , at the pump condition mentioned above, the laser output showed no sign of degradation during the first 3.9 × 106 pulses and then it decreased to half of its initial value after 4.8 × 106 pulses. The normalized photostability (NP) of the PM567 co-doped with C540A in P(MMA-TMSPMA) channel waveguide laser, as defined by others , was 1334 GJ/mol, which to the best of our knowledge, was comparable to the best results of PM567 ever reported, considering the relatively high pump repetition rate at 500 Hz adopted in this work, while normally the pump frequency of several tens of Hz was adopted by other authors [11,12]. By decreasing the pump repetition rate, longer laser lifetime and higher NP was expected. The high photostability of PM567 in this work might be ascribed to both the polymer composition and interaction between dye molecules, i.e. the PM567 and C540A. Previously, long laser lifetime has been reported by A. Costela et al in the bulk sample with the same composition . And it should also be noted that the energy transfer from the triplet-state of PM567 to the ground-state of C540A, which later might transfer the singlet-state energy to the ground-state of PM567, could improve not only the efficiency but also the stability of PM567 by depressing photo-degradation, for instance, the photo-chemical reaction of triplet-state PM567 with singlet-state oxygen, which was thought to be one of the main photo-degradation path of PM567 [15,17]. Recently, excellent laser performances were obtained with pyrromethene dyes doped organic-inorganic hybrid bulk materials based on the modification of nanometer-sized polyhedral oligomeric silsesquioxanes [12,35], which was expected to have good adhesion with the PDMS substrates. With the high efficient and long lifetime tunable waveguide lasers, the practical integrated laser chips might be expected.
In conclusion, in this work, laser dye doped polymeric channeled waveguide lasers with permanent DFB structures on the PDMS chips were fabricated by a novel pen-drawing technique. With the high resolution dispensers, dye doped high viscosity pre-polymers were written into PDMS grooves and the cross-section of the DFB channel laser waveguides could be controlled by the polymer composition and pen-drawing parameters. Highly stable laser output has been observed with the lifetime exceeding 4.8 × 106 pulses, which is suggested to be one of the best results of PM567 up to date.
This study was supported by Industrial Technology Research Grant Program in 2007 from the New Energy and Industrial Technology Development Organization in Japan.
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