The paper presents polymeric waveguide dye laser with distributed Bragg reflector (DBR) grating or distributed feedback (DFB) resonator. DBR grating and DFB resonator were fabricated on a surface of SU-8 2002 photoresist polymer using interference of laser beams. Polystyrene (PS) and poly(vinyl butyral) (PVB) waveguides dispersed with laser dye of 3-(2-benzothiazolyl)-7-(diethylamino)-coumarin (Coumarin 6) and tris(8-quinolinolate)aluminum (Alq) as a host were used. Single mode of lasing with TE mode was measured from the polymeric waveguide with DBR grating and DFB resonator. Threshold of lasing with DBR grating is lower than that with DFB resonator. For PVB/Alq/Coumarin 6 waveguide, 0.1 mJ cm-2 pulse-1 of threshold was measured with DBR grating and 0.3 mJ cm-2 pulse-1 with DFB resonator. Slope efficiency between 0.06 and 0.09 % was measured for PS/Alq/Coumarin 6 waveguide and that between 0.07 and 0.15 % for PVB/Alq/Coumarin 6 waveguide.
©2009 Optical Society of America
Since organic materials have flexibility for materials selection and fabrication of devices, compared with inorganic devices, molecular electronics and optics have been widely studied for the next generation key devices. In last two decades, there have been extensive studies of organic light emitting diodes (OLEDs) which are of practical usage as a light source and a thin flat panel display. Organic solid-state lasers (OSSLs) have also been investigated for on-chip laser sources on an integrated circuit. The studies of OSSLs in the past have been well summarized in recent review . Among them, there have been widely investigated OSSLs with distributed feedback (DFB) resonator. High slope efficiency of 12 % was achieved using nanoparticle dispersed dual-layered waveguide organic laser device . Surface emission oraganic dye laser was fabricated using nanoimprinted circular grating DFB resonator . Reduction of threshold and increase of slope efficiency was achieved using two dimensional DFB resonator [4,5]. Using holographic gratings, tunable OSSLs was demonstrated [6-9]. With assistance of cascade energy migration of excited state, we have achieved the reduction of lasing threshold . All-plastic organic dye laser with DFB resonator was reported . Ti-sapphire femtosecond pulse pumped organic DFB laser emission was also demonstrated .
Recently, we succeeded in reducing the threshold of lasing using an organic waveguide device with distributed Bragg reflector (DBR) grating compared to that with DFB resonator. In this paper, we present laser emission from polymer/dye waveguide with DBR grating and DFB resonator which were fabricated by photoresisit polymer and compare the laser performance of waveguide devices between DBR grating and DFB resonator.
After spinning thin film of NANO™ SU-8 2002 photoresist polymer (MicroChem Corp.) on a quartz substrate, we prebaked the thin film at 65 °C for 7 min and soft baked at 95 °C for 14 min. Then DFB resonator and DBR grating were fabricated on a surface of SU-8 2002 photoresist polymer by illuminating the interference beams of a frequency-tripled Nd:YAG pulse laser emitted at 355 nm with a pulse duration of 30 ps and repetition rate of 10 Hz, and periodic gratings were formed on a surface of SU-8 2002. After illumination of laser shots of 130 µJ pulse-1 for 24 s, the thin film was developed in SU-8 Developer for 3 min in an ultrasonic bath, followed by rinsing in 2-propanol and drying, and then hard baked at 175 °C for 1 h. Rippled structure of the surface was evaluated using an atomic force microscope (AFM) on contact mode (Nano Scope III, Degital Instruments). The mixture of 3 wt % of laser dye of 3-(2-benzothiazolyl)-7-(diethylamino)-coumarin (Coumarin 6), 30 wt % of tris(8-quinolinolate)aluminum (Alq) as a host, and 67 wt % of polystyrene (PS) or poly(vinyl butyral) (PVB) as a matrix was dissolved in chloroform and then it was spin-coated on SU-8 2002 DFB resonator or DBR grating to fabricate the polymer waveguide. The thickness of polymer waveguide was ca. 600 nm. The thickness of the rippled SU-8 2002 layer was ca. 400 nm.
Stripe-shape of exciting beam of a frequency-tripled Nd:YAG pulse laser (λp=355 nm) with a pulse duration of 30 ps at a repetition rate of 10 Hz was used as a pumping source. Lasing emission from the waveguide was monitored with a Tokyo Instruments multi-channel analyzer equipped with 1200 lines/500 nm gratings and an Andor iDus charge coupled device. UV-Vis absorption spectra were recorded with a Shimadzu UV-2101PC spectrophotometer and the photoluminescence spectrum with Shimadzu RF-1500 fluorophotometer. Index of refraction (n) of waveguide was determined with a critical angle using a prism coupling method, in which the complete reflection of light turned into a partial one, i.e., part of light penetrates into the bulk film. The laser sources were a polarized He-Ne laser (632.8 nm) and a laser diode (830 nm). Waveguide and the prism of Hoya TaFD21 with n=1.926 at 632.8 nm and n=1.909 at 830 nm were coupled with an air gap. Index of refraction at lasing peak n(λ) was evaluated using a one-oscillator Sellmeier-dispersion formula of
where q is a measure of the oscillator strength and A is a constant containing the sum of all the other oscillators which were determined using refractive indices measured at 830 and 632 nm and the absorption wavelength of dominant oscillator λ 0 of 392.5 nm.
3. Results and Discussion
Figure 1 shows the cross-section of rippled periodic structure fabricated on surface of SU-8 2002 photoresist polymer by illumination of the interference beams with incidence angle of 30.5 °. The periodic structure works as DFB resonator and DBR grating. Periodic pitch of 342.4 nm (73 pitches in 25 µm) was measured as shown in Fig. 1 and the periodic pitch Λ th of 349.7 nm was calculated using an equation of
with incidence angle of 30.5 °, where λ p is wavelength of pumping laser, in this case, 355 nm, and θ is the incidence angle. Measured pitch is in good agreement with calculated one within 2 % of experimental error. Amplitude of pitch was ca. 6 nm for both DFB resonator and DBR grating.
Figure 2 shows the schematics of polymer waveguide laser with DFB, DBR-DFB and DBR. In the present case, we changed the distance between the waveguide edge (the emission edge) and the edge of the rippled structure (grating) on SU-8 2002 photoresist polymer from 0 to 10 mm (the distance is shown by a double-ended arrow in the figure). Diameter of pump beam is 6.5 mm and length of SU-8 2002 layer is 3.25 mm. For the geometry shown in Fig. 2(a), the emission edge is overlapped with the rippled edge and pump beam hits only the waveguide on SU-8 2002 grating. In that case, the laser emission from the DFB resonator is measured. For the geometry shown in Fig. 2(b), there is enough distance between the emission edge and the edge of the rippled structure to activate DBR lasing. In that case, pump beam covers waveguide with SU-8 2002 grating structures, and laser emission for DBR-DFB is measured. For the geometry shown in Fig. 2(c), pump beam is activated the waveguide with no SU-8 2002 grating, and laser emission from DBR grating only is measured.
Typical laser emission of Coumarin 6/Alq in PS and PVB waveguide with DFB resonator is shown in Fig. 3(a), that with DBR-DFB in Fig. 3(b), and those with DBR in Figs. 3(c) and 3(d). Laser emission from the waveguide with DFB resonator only follows large stimulated emission from amplified spontaneous emission (ASE). In contrast, DBR-DFB and DBR provide the laser emission only from the waveguide as shown in Figs. 3(b) and 3(c). In the case of Fig. 3(d), lasing with large ASE is measured for PS/Alq/Coumarin 6 and only ASE for PVB/Alq/Coumarin 6. The distance between DBR edge and the edge of the active layer is too far to get effective feedback for lasing.
For PS/Alq/Coumarin 6 waveguide, peak wavelength of laser emission for DFB resonator in Fig. 3(a) is 540.0 nm and those for DBR-DFB and DBR are 542.2 and 542.4 nm, respectively. Peak shift of ca. 2 nm for DBR-DFB and DBR are ascribed to the difference of effective index of refraction due to the difference of thickness of waveguide. For DFB resonator and DBR, wavelength of laser emission λ L is calculated as
where n eff is effective index of refraction, Λ th is the grating pitch on the surface of SU-8 2002 photoresist polymer by the interference, and m is the number of mode (m=1,2,3, ⋯). Using m=2 and Λ th of 338.75 nm calculated from θ=31.6°, n eff of 1.594 is calculated for DFB resonator and n eff of 1.601 for DBR-DFB and DBR for PS/Alq/Coumarin 6. For PVB/Alq/Coumarin 6 waveguide, with Λ th of 343.6 nm calculated from θ=31.1°, n eff of 1.573 is calculated for DFB resonator, n eff of 1.574 for DBR-DFB, and n eff of 1.575 for DBR.
Effective indices of refraction in four layers waveguide of 600 nm active layer, 400 nm SU-8 2002 layer on quartz substrate were calculated using NL guide waveguide calculator. Effective index of refraction was calculated to be 1.5724 for PVB/Alq/Coumarin 6 (n=1.570 @ 540 nm) and 1.6006 for PS/Alq/Coumarin 6 (n=1.622 @ 540 nm) with SU-8 2002 (n=1.605 @ 540 nm). These effective indices of refraction are in good agreement with those measured above.
Grating position also affects the threshold of lasing. Threshold of lasing is shown in Fig. 4. As shown in Fig. 4(a), for PVB/Alq/Coumarin 6 waveguide, it is clearly shown that the threshold of lasing for grating position less than 3.5 mm with DFB resonator is ca. 0.3 mJ cm-2 pulse-1, whereas that for grating position more than 5 mm is ca. 0.1 mJ cm-2 pulse-1 with DBR grating. In the case of PS/Alq/Coumarin 6 waveguide, as shown in Fig. 4(b), the threshold between 0.17 and 0.30 mJ cm-2 pulse-1 was measured for DFB resonator and that between 0.10 and 0.20 mJ cm-2 pulse-1 for DBR grating. Threshold of lasing from DBR grating is lower than that from DFB resonator. As shown in Fig. 3, DFB lasing follows larger ASE with increasing pump energy but DBR lasing does not. Effective feedback is obtained for DBR grating compared with that for DFB resonator, and thus threshold of lasing for DBR grating is lowered compared with that for DFB resonator.
PS waveguide has the slope efficiency between 0.06 and 0.09 %, whereas PVB waveguide has that between 0.07 and 0.15 %. Low slope efficiency is due to the low quantum efficiency of Alq/Coumarin 6 host-guest system.
Further reduction of threshold will be achieved using DBR grating with large amplitude and drastic increase of slope efficiency using for example, Rhodamine 6G with higher quantum efficiency.
We demonstrated laser emission from Coumarin 6/Alq in polymeric waveguide with DBR grating and DFB resonator fabricated on SU-8 2002 photoresist polymer. We succeeded in reducing the threshold of lasing using waveguide laser with DBR grating fabricated on surface of SU-8 2002 photoresist polymer.
We acknowledge discussion with Dr. W. Sakai.
References and links
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