We report distributed Bragg reflector (DBR) polymer lasers fabricated using dot matrix holography. Pairs of distributed Bragg reflector mirrors with variable mirror separations are fabricated and a novel energy transfer blend consisting of a blue-emitting conjugated polymer and a red-emitting one is spin-coated onto the patterned substrate to complete the device. Under optical pumping, the device emits sing-mode lasing around 622 nm with a bandwidth of 0.41 nm. The working threshold is as low as 13.5 μJ/cm2 (~1.68 kW/cm2) and the measured slope efficiency reaches 5.2%. The distributed feedback (DFB) cavity and the DBR cavity resonate at the same lasing wavelength while the DFB laser shows a much higher threshold. We further show that flexible DBR lasers can be conveniently fabricated through the UV-imprinting technique by using the patterned silica substrate as the mold. Dot matrix holography represents a versatile approach to control the number, the size, the location and the orientation of DBR mirrors, thus providing great flexibility in designing DBR lasers.
© 2015 Optical Society of America
Organic semiconductors have well established themselves as an attractive technology in modern optoelectronics, with an increasing penetration into the commercial market of displays (in both mobile-phones and televisions), and followed by a rapid development of organic solar cells and transistors . Additionally, their properties of strong absorption, large optical gain, broad spectral emission, simple processing and being free from concentration quenching make them appealing as laser gain media [2–4]. In this respect, the aim is to realize broadly tunable, light-weight, low-cost and even flexible coherent sources. With a significant progress in improving device performance of organic semiconductor lasers (OSLs) in the last decade, some breakthroughs have been achieved in the last few years. On one hand, the successful demonstration of cheap LEDs pump removes the requirement for a complex, expensive laser to pump OSLs, leading to devices with significantly reduced complexity and cost [5,6]. On the other hand, OSLs have been exploited as explosive vapor sensors , integrated optical sources for spectroscopy [8–10], biomolecule detectors [11,12] and refractive index monitors [13,14] with outstanding performance. These successful device prototypes open a door for OSLs to enter the market without competing with inorganic lasers in their successful fields. These advances make it timely to develop simple-fabricated, high performance and newly designed organic lasers.
Distributed feedback (DFB) polymer lasers have been extensively investigated and are shown to exhibit excellent performance in terms of low working thresholds and directional output. The active film is either over-coated onto a corrugated substrate or is directly patterned , in this way a standing wave is formed due to the interference effect originating from the periodic thickness modulation of the active film. This thickness modulation however could bring some disadvantages: (1) The DFB cavity provides diffractive output coupling which has been identified as a major channel of loss . (2) Incoherent scattering arises due to the inevitable surface nonuniformity of the active film. (3) Problems such as interface nonradiative recombination and inhomogeneous carrier injection are present when electrical pumping is applied. The first two deteriorate device performance while the last one makes the realization of electrically pumped OSLs even more difficult. In distributed Bragg reflector (DBR) lasers, the gain medium located between two Bragg mirrors is uniform. As the amplification area is separated from the feedback components, the aforementioned shortcomings can be overcome. Such polymer lasers have been previously realized using electron beam lithography [16–18] or conventional interferometric lithography [19, 20]. However, the electron beam lithography technique needs sophisticated equipment and lacks efficiency and fabrication of DBR lasers with conventional interferometric lithography is tricky, as it is difficult to precisely control the location, the size, the orientation and the spacing of Bragg mirrors. As a result, there are very few papers on polymer DBR lasers in literature and their potential in OSLs is far from being fully exploited.
Dot-matrix holography is used to write dot-matrix holograms for anticounterfeiting or decoration of commercial products . Generally speaking, a dot-matrix hologram consists of thousands of fine diffraction grating dots to display the required image. In this paper, dot-matrix holography is developed to fabricate DBR polymer lasers with good performance. Pairs of distributed Bragg reflector mirrors with variable mirror separations are first defined in the photo-resist and then transferred onto the silica substrate. The active film is spin-coated onto the cavity from a blended conjugated polymer solution. Pairs of Bragg mirrors serve as the reflectors while the uniform active film between them provides optical gain. The location, the number, the size, the orientation and the spacing of Bragg mirrors could be controlled by a computer in dot-matrix holography and the single Bragg mirror is written within one single laser pulse (~15 ns). Thus, Dot-matrix holography provides both great flexibility and efficiency for fabricating DBR polymer lasers. We also describe operating characteristics and principles of DBR polymer lasers fabricated from dot-matrix holography. Finally, mechanically flexible DBR polymer lasers are demonstrated by UV-nanoimprint lithography where the patterned silica substrate is used as the mold.
2. Experimental details
2.1 Device fabrication
Figure 1(a) depicts the setup of the dot-matrix holography printer developed in the SVG Corporation and this system was named as HoloMaker. It utilizes a compact diode-pumped solid-state laser Awave351-0.5W@1k (Advanced Optowave Corporation. Wavelength: 351 nm. Repetition rate: 1 kHz. Pulse duration: less than 15 ns), whose expanded beam illuminates a reflective spatial light modulator (Digital micromirror device, DMD, 1024 × 768 pixels) located before two Fourier lens. The two Fourier lens constitute a 4f coherent imaging system and a 1:1 image is formed. The laser beam image was then transmitted through the diffraction grating and only ± 1 orders of diffracted beams are selected to pass through the objective lens. The focused spot is finally used to write dot gratings into the photoresist. The orientation of the diffraction grating and thus the grating vector in each dot can be controlled via a rotation motor ( ± 0.05°) controlled by the computer. The 2D stage is driven by a programmable micro-stepping controller and advances in the X-Y plane ( ± 0.5 μm). The grating period in the exposed dot can be calculated using , where is the period of the diffraction grating and M is magnification of the objective lens. Figure 1(b) shows a photograph of HoloMaker. Several unique techniques are adopted in this equipment. First, the DMD (0.7 inch with a pixel size of 13.68 μm) is used as an aperture to control the size and the shape of each grating dot. If an objective lens with magnification of 20 is employed, the length and width of the grating dot could be set within (2 μm, 700 μm) and (2 μm, 500 μm), respectively. Second, an automatic focusing servo system is implanted to correct defocusing caused by the nonuniformity of the substrate in real time. Third, the technique of flying exposure is applied in which a grating dot requires a single laser pulse and recording takes place in accordance with stage positioning. In this way, grating dots (Bragg mirrors in DBR lasers) with controllable number, size, orientation, location and period can be efficiently fabricated.
Figure 2(a) shows the working configuration of lasers investigated in this study. When the pump beam is placed between the two Bragg mirrors (1), DBR lasing is obtained and when it is placed onto the single Bragg mirror (2 or 3), DFB lasing is observed. For device fabrication here, a diffraction grating with a period of 16 μm was inserted and magnification of the chosen objective lens was 20. The resulted grating period in recorded dots is 400 nm. To perform lithography, a film of the positive photo-resist (RZJ390, Suzhou Ruihong Co., Ltd.) with a thickness of 100 nm was first spin-coated onto the silica substrate, and four pairs of Bragg mirrors with separations of 0.2 mm, 0.5 mm, 1 mm and 2 mm were then defined by Holomaker. The size of each Bragg mirror was set to be 160 μm. The grating vector of mirrors in the same pair was parallel and along the mirror spacing. After being exposed under suitable conditions, the photo-resist was developed in the developer for 8 s and relief gratings are obtained. Figure 2(c) shows the atomic force microscope (AFM) image of the dot grating in the photo-resist and Fig. 2(e) gives its surface profile. The grating period is around 400 nm and the grating depth is between 70 nm and 90 nm. The substrate was then etched using a plasma etcher (Tegal, 901e) for 1 min where the photo-resist grating served as the mask. Figure 2(d) shows the AFM image of the etched grating in the silica substrate and Fig. 2(f) gives its surface profile. The transferred grating in the silica substrate shows a depth of ~60 nm. After that, a conjugated polymer blend consisting of blue-emitting poly(9,9-dioctylfluorene) (PFO, Sigma-Aldrich) and red-emitting Poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenyl-enevinylene) (MEH-PPV, Jilin OLED Co., Ltd.) (90:10 wt. %) was dissolved in toluene at a concentration of 12 mg/g. The solution was then spin-coated onto the patterned silica substrate at a spin speed of 2800 rpm. The thickness of the conjugated polymer film is 160 ± 10 nm as measured in areas without grating dots. The sample was then purged in nitrogen for several minutes to eliminate the solvent. Finally, the UV-curable adhesive was drop-cast onto the active film and UV-cured for 2 s (UV lamp, 2W, 365 nm). The transparent polymer layer (~5 μm) serves as the encapsulation layer to protect the active film from oxygen and moisture. The laser device could then be tested under ambient conditions and stored for months . If the recently reported self-healing organic gain medium can be utilized, the encapsulation layer may not be necessary [23,24]. Figure 2(b) shows a completed DBR laser with a mirror separation of 200 μm. As for the fabrication of mechanically flexible DBR lasers, a drop of the UV-curable adhesive was first placed onto a PET substrate (with a thickness of 0.3 mm) and then the patterned silica substrate was pressed against it. The sandwiched structure was UV-cured for 2 s and the PET film with grating dots in the solidified adhesive was peeled off from the silica substrate. The active film was then spin-coated onto it and the encapsulation layer was also applied.
2.2 Optical characterization
The schematic setup for optical pumping is shown in Fig. 3. A diode-pumped, Q-switched and frequency-tripled Nd:YLF laser (Spectral Physics) at the wavelength of 349 nm was employed. The pump pulses had duration of 8 ns and the repetition rate was set to be 50 Hz. Pump pulse energy was displayed by the laser itself and was further adjusted with a set of neutral density filters. A quartz cylindrical lens was used to focus it into a strip with a width of 0.2 mm. The length of the strip was controlled by an adjustable slit, and only the central part was selected to ensure the intensity uniformity. The pump beam was then directed upon the sample at 20° with respect to the surface normal. The length of the pump beam is placed parallel to the grating vector of the Bragg mirror. The sample was also mounted on a motorized precision stage that could be moved in all three dimensions to vary the pump positions. Emission from the device was coupled into a multimode optical fiber and analyzed using a spectrometer with a resolution of 0.29 nm (Avaspec-ULS2048-USB2). Output energy was measured using a sensitive energy-meter (Coherent, J-10SI-HE) where the detector with a diameter of 10 mm was placed near the sample surface to let all lasing energy in.
3. Results and discussions
3.1 DBR lasing performance
Figure 4(a) shows the emission spectra to the normal of the sample surface at various pump energies. In this case, the pump strip was located onto the uniform active film between the two mirrors. For pump energies below the threshold (around 5 nJ), normal emission consists of low intensity photoluminescence (PL) from MEH-PPV. The PL spectrum was strongly modulated by peaks that have been out-coupled by the Bragg mirrors. A dip located at around 622 nm is thus observed. This dip is often referred to as the photonic stop-band in organic DFB lasers  and lasing occurs in the gap for gain modulated gratings while on the band edge for index modulated gratings. For the DBR laser, that naming for the dip is not suitable as the symmetry of the periodic structure has been broken by the insertion of the defect layer. Considering the propagation of PL photons around the dip is inhibited in these two Bragg mirrors, these photos would be reflected between them. Those obey the interference equation could form standing waves (shown in Fig. 6(a)) and stand out as defect modes. The interference equation is , where is the wavelength of the supported standing wave, m is the order, is the mode effective refractive index and is the propagating distance (slightly longer than the mirror separation due to the penetration of the light field into Bragg mirrors). Such defect modes have been found in polymer microcavities and DBR lasers below laser thresholds . However, we did not observe such defect modes in these four DBR samples. This may be due to the following reasons. First, the mirror distances are too long and the calculated spacing between adjacent defect modes is less than 0.3 nm. It is difficult to distinguish between those adjacent modes with a spectrometer resolution of 0.29 nm. Second, quasi-continuum defect modes are apt to decrease interferencing intensity and the specific defect mode is not easy to stand out of the emission background. Although we could fabricate DBR lasers with mirror separations smaller than 10 μm easily with dot-matrix holography in order to enlarge the spacing between adjacent defect modes, initial measurements were not successful. The extremely small excitation volume exhibits a challenge to the sensitivity of the spectrometer. When input pump energy is above 5 nJ, a narrow peak at 622.6 nm appears and dominates the emission as pump energy increases further. The mode at the reflection maximum of the Bragg mirror experiences the lowest optical loss when resonating between the two mirrors and the rest defect modes are well suppressed. As a result, the observed single lasing peak locates almost exactly at the dip. Its full width at half maximum (FWHM) was measured to be around 0.4 nm. Figure 4(b) shows the dependence of output lasing energy on input energy. The data slightly above the lasing threshold is not shown due to the sensitivity of the energy-meter, however the linear fit is fairly good and its x-intercept yields the threshold. The oscillation threshold is estimated to be 5.1 nJ (corresponds to a pump energy density of 13.5 μJ/cm2 and a pump power density of 1.68 kW/cm2). The demonstrated threshold is comparable with that in common high performance DFB polymer lasers [22,27,28] and is about to enter the regimes of LEDs pump (~1 kW/cm2) . It is also interesting to compare with previous results on DBR polymer lasers. The lowest threshold is 1.2 kW/cm2 demonstrated in an optimized DBR laser cavity which is fabricated via electron beam lithography [16,17]. Much higher thresholds (102 - 103 kW/cm2) have been reported with conventional interferometric lithography [19,20]. Thus dot-matrix holography not offers great flexibility in fabricating DBR laser cavities, but also provides devices with performance comparable with those by electron beam lithography. The device slope efficiency given by the slope of the fitted linear curve is 5.2%. The only report on the slope efficiency of DBR polymer lasers yields a value around 0.15% . Although a slightly higher slope efficiency of 7.8% has been demonstrated in a DFB polymer laser , the device here represents the current state of art for a DBR polymer laser.
Figure 5 shows device performance of DBR lasers with four different mirror separations. Photographs of operating laser devices are given in Fig. 5(a). The far-field emission from these DBR lasers is fan-shaped and resembles that in 1D DFB polymer lasers. Unlike the situation in DFB lasers where output coupling by diffraction takes place throughout the whole cavity, lasing is out-coupled only at the edges of the Bragg mirrors, thus the two lasing lines are spaced in accordance with the mirror separation. Figure 5(b) shows spectra of lasing from these devices. Output lasing between 621 nm and 623 nm can be observed and the corresponding mode effective refractive index is around 1.555. This value is reasonable, considering the refractive index of the silica substrate, the active film and the cured UV adhesive being 1.46, 1.76 and 1.51, respectively. The small variation in lasing wavelengths for different cavities should be attributed to thickness non-uniformity of the spin-coated conjugated polymer. It is also worth to note that there appear two peaks of D and E when the mirror separation is 2.0 mm. The multi-mode operation may be due to the competition between defect modes supported by the DBR cavity or simply due to the non-uniformity of the active film. The bandwidth of these lasing peaks is between 0.4 nm and 0.8 nm, typical values for single-mode lasing in DFB polymer lasers . In Fig. 5(c), the dependence of the working threshold on the mirror separation and thus the active area length is shown. The threshold lies in the range between 13.5 μJ/cm2 and 30 μJ/cm2. The variation in the threshold is not significant and no clear dependence on the mirror separation can be found. The small variation in the working threshold could thus be attributed to the quality difference in Bragg mirrors. The threshold dependence on the mirror separation has been previously analyzed in DBR lasers fabricated using electron beam lithography. Akiko Seki et al. found the threshold increases significantly when the active area length is below 3 mm (0.6 μJ/cm2 at 3 mm and 50 μJ/cm2 at 0.5 mm) . While A. E. Vasdekis et al. found the threshold changes little with a mirror separation between 20 μm and 110 μm . In DFB polymer lasers, solid evidences are reported for a critical pump length below which the working threshold increases significantly [30, 31]. This critical length may not exist for DBR lasers. In microcavity polymer lasers where the feedback mechanism is similar, they can in principle exhibit thresholdless lasing with a very thin gain medium . This is due to the modification of the spatial distribution of spontaneous emission by interference effects. As a result, most energy can be transferred into the lasing mode.
Feedback mechanisms for both the DBR laser and the DFB laser are shown schematically in Fig. 6(a) and (b). The DBR laser relies on the standing wave between the two Bragg mirrors while the DFB laser relies on coherent backward scattering at each grating corrugation. However, they all based on the sub-wavelength periodic structure and overall device performance is quite similar. In fact, the dot grating fabricated using dot-matrix holography can serve not only as the Bragg mirror but also as the DFB cavity. Such measurements have been carried out for the pair of Bragg mirrors with a separation of 0.2 mm. The pump position is varied to obtain DBR lasing and DFB lasing as shown in Fig. 2(a). Laser emission was observed when input energy was above corresponding thresholds and the spectra are shown in Fig. 6(c) (d) and (e). Each set of data is fitted with a Gaussian curve to obtain more precise values of lasing wavelengths and FWHMs. The lasing wavelength is 622.52 nm for the DBR laser while it is 622.55 nm and 622.48 nm for the left and right DFB laser, respectively. If measurement error is taken into consideration, they almost resonate at the same lasing wavelength. However, the underlying reasons can be different. In the DBR cavity, the mode at the dip experiences the lowest reflection loss. While in the case of the DFB cavity, gain modulation caused by thickness modulation in the active film may be the dominating coupling mechanism . The left and right DFB laser shows a threshold seven times and eight times higher than the DBR laser, respectively. This is also indicated in the lasing spectrum where there is some background spontaneous emission from DFB lasers. Two reasons are responsible for this. First, optical loss caused by out-coupling and scattering is less in the DBR laser . Second, the length of the grating dot in this study is 160 μm which is below the critical pump length for a DFB laser . As dot-matrix holography could provide grating dots with a size up to 500 μm, it may fabricate DFB lasers with performance comparable with those by electron beam lithography or conventional interferometric lithography. It is also worth to note that there is a trend in the field of OSLs to fabricate grating pixels for the realization of the DFB laser array [8,33]. They could be easily integrated into a micro-chip and function as a lab-on-a-chip system. Dot-matrix holography demonstrated in this paper is also feasible to achieve this goal and enjoys clear advantages over electron beam lithography.
3.2 Flexible DBR laser
In the next, a flexible DBR laser is demonstrated and the purpose is twofold. First, we want to show that do-matrix holography is compatible with the nanoimprint technique for mass production. Second, flexible lasers are of significance and flexible DBR lasers have not been reported yet. As mentioned in the experimental part, the silica mold fabricated using dot-matrix holography was used as the imprinting mold and four pairs of Bragg gratings are fabricated on the flexible PET substrate via the one-step UV imprinting process. Performance of the flexible DBR laser is summarized in Fig. 7. Figure 7(a) shows the photograph of the operating sample at high pump intensity. The lasing pattern consists of two tangent arc lines with most energy concentrated in the center. The curvature of the flexible PET substrate has little influence on device performance. Figure 7(b) shows the output lasing spectrum and its inset shows the dependence of peak intensity and FWHM on pump fluence. The emission spectrum quickly narrows down once pump energy exceeds the threshold. The lasing peak locates at 629.1 nm corresponding to an effective refractive index of 1.572 which is slightly higher than that in the last section. This is because the refractive index of the polymer grating is 1.51 which is slightly higher than that of the silica. The data in the inset were fitted using two linear curves with one for spontaneous emission below the threshold and the other for lasing above the threshold. The deduced working threshold is 16.8 μJ/cm2 and no performance deterioration is found compared with that on silica substrate.
In summary, high performance DBR polymer lasers are demonstrated using dot-matrix holography. Four pairs of Bragg mirrors with different mirror separations are defined in the photo-resist and transferred onto the silica substrate. A gain material blend consisting of PFO and MEH-PPV was spin-coated over the substrate. Output lasing locates around 622 nm with a bandwidth between 0.4 nm and 0.8 nm and the measured sloped efficiency reaches 5.2%. The distance between the two arc lasing lines in the far field is dependent on the mirror separation. No critical pump length for the working threshold was found due to the peculiar feedback mechanism in the DBR cavity. DFB lasing was also demonstrated by moving the pump strip onto the Bragg grating. Its lasing wavelength is the same with that of the DBR laser, while the working threshold is much higher. Finally, flexible DBR lasers on a PET substrate was demonstrated by UV-imprinting the Bragg mirrors into the curable adhesive. Dot-matrix holography demonstrated here represents a versatile approach for fabricating DBR polymer lasers. Future investigations would focus on realizing high performance micro-cavity laser arrays with wide wavelength coverage. They could serve as the light sources for spectroscopic analyzing or biomolecule sensing and be easily integrated into lab-on-a-chip systems.
We thank financial support from the National Natural Science Foundation of China (NSFC) (Grant No. 91323303, 61401292, 61505131), Jiangsu Provincial Natural Science Foundation of China (Grant No. BK20140350 and BK20150309), the China Postdoctoral Science Foundation (Grant No. 2015M571816), and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions.
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