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Polymer lasers are fabricated by an assembly method. A polymer membrane is directly attached on the one- or two- dimensional grating. The suspended membrane acts as an active waveguide, which is supported by the grating ridge, leaving air gaps in the grating valley. Most of the radiation is effectively confined within the active waveguide due to the strong reflection at the membrane/air interfaces. So, low threshold lasing can be achieved when the sample is optically pumped. This fabrication method provides an alternative to investigate low-threshold polymer lasers.
© 2016 Optical Society of America
1. Introduction
Over the past two decades, polymer lasers have attracted much interest and are potential candidates for sensing and display applications [1–4]. Distributed feedback (DFB) cavities have been considered as the most promising solution for polymer lasers [5–8]. A variety of fabrication schemes have been proposed to introduce the active materials in the DFB polymer lasers, such as spin coating [9], nanoimprint [10, 11] dipping process [12], ink-jet printing [13], and thermal evaporation [14]. In all these methods, the active layer clings to the DFB cavity. So, the film quality and the thickness uniformity of the active layer are limited to the topology and quality of the DFB cavity. Generally, the periodic height modulation can be observed on at least one surface of the active layer due to the influence of the DFB structure [15]. Thus, it is difficult to guarantee the thickness uniformity of the active layer. From this perspective, more flexible and robust method is required for the easy realization of polymer lasers based on the arbitrary DFB configuration.
In this paper, a simple assembly method is proposed to achieve DFB polymer lasers. The active layer and the DFB grating were prepared respectively. Then the wet active layer was directly attached on the DFB grating, forming a suspended membrane structure. Air gaps exist in the grating valley covered by the polymer membrane. When the sample was optically pumped, low-threshold laser emission was observed. It is attributed to both the flatness and uniform thickness of the active layer and the high refractive index contrast of the suspended membrane structure. Furthermore, the dependence of the laser performance on the polarization of the pump is investigated for the two-dimensional (2D) rectangular lattice cavity. The polarization direction of the laser output is parallel to the grating lines. This technique provides the flexibility to fabricate DFB polymer lasers.
2. Fabrication
Figure 1 presents the fabrication procedure of the polymer laser based on a suspended membrane structure. In the experiment, two thin films are constructed on the glass substrate by spin-coating, as shown in Fig. 1(a). A water-soluble polyvinyl alcohol (PVA 107, Celanese Chemicals, Germany) solution with a concentration of 0.04 g/mL is spin-coated firstly on the glass substrate at 3000 rpm, forming a 350-nm-thick film. A typical light-emitting polymer, poly [(9, 9-dioctylfluorenyl-2, 7-diyl)-alt-co-(1, 4-benzo-(2, 1', 3) -thiadiazole)] (F8BT), is employed as the active material. The solution of F8BT in xylene with a concentration of 23 mg/mL is spin-coated on the PVA film at 1500 rpm. The thickness of the F8BT film is about 150 nm. The different solvents of PVA and F8BT prevent the interpenetration of the different layers, guaranteeing the smoothness of the films. Then the bilayer structure is immersed in the deionized water for 30 mins to dissolve the PVA layer completely. A free-standing polymer membrane is obtained when the PVA layer disappears, as shown in Fig. 1(b). The film thickness is measured with a NanoMap-500LS contact surface profilometer. The absorption and photoluminescence (PL) spectra of F8BT can be found in our previous work [16].
Fig. 1 Fabrication procedures of polymer laser based on a suspended membrane structure. (a) The polymer/PVA bilayer structure is immersed in deionized water for 30 mins to dissolve the PVA layer. (b) A free-standing polymer membrane is obtained after complete dissolution of the PVA layer. (c) A PR film coated on the glass substrate is exposed to an interference pattern of two UV laser beams. α is the included angle between the two laser beams, which determines the period of the pattern. (d) The PR grating fabricated by interference lithography. (e) The polymer laser fabricated by directly attaching the wet polymer film in (b) to the surface of the grating in (d).
The grating structure is achieved by interference lithography in Figs. 1(c) and 1(d) [8]. The photoresist (PR, AR 300-47 developer, Allresist) solution is spin-coated on the glass substrate of area 15 mm × 15 mm and thickness 1 mm, forming a thin film about 180 nm. The spin-coating speed is 2000 rpm. The PR film is exposed to a two-beam interference pattern, generating from a continuous wave laser (He-Cd laser, Kimmon) with a wavelength of 325 nm and a power of 30 mW. Then a grating structure is obtained after developing in AR 300-47 developer for 3 seconds, which is employed as the DFB cavity. The period Λ of the grating is determined by the included angle α between two laser beams, i.e., Λ = λ/2/sin(α/2). The 2D grating structure can be fabricated by multiple exposure interference lithography [17]. Finally, the wet polymer membrane in Fig. 1(b) is attached on the grating structure in Fig. 1(d), forming a polymer laser based on a suspended membrane structure. The typical feature of the laser device is the air gap between the polymer membrane and the DFB cavity, as shown in Fig. 1(e). Note that no adhesive is needed because the membrane can stick tightly to the grating structure after drying naturally at room temperature due to the surface intension. The long-term reliability of the PR grating is acceptable for polymer lasers after developing, which can be kept for years after coating with polymers.
3. Assembled polymer lasers and spectra analysis
The morphologies of the DFB cavities are measured by using the scanning electron microscopy (SEM, Hitachi S-4800), as shown in Figs. 2(a) and 2(b). The period of the one-dimensional (1D) grating is 360 nm. The periods of the 2D rectangular lattice are 360 nm (Λ2) and 370 nm (Λ1) for the two orthogonal directions, respectively. The cross-sectional SEM image of the sample shows that air gaps exist in the grating valley in Fig. 2(c), increasing the strength of the diffraction effect of the grating. Also, it exhibits good adhesive strength between the polymer membrane and the grating structure.
Fig. 2 SEM images of the (a) 1D and (b) 2D DFB grating. The period of the 1D DFB grating is 360 nm. For the 2D rectangular grating, Λ1 = 370 nm; Λ2 = 360 nm. The inset in (a) is the AFM image of the 1D grating. The scale bar is 500 nm. (c) Cross-sectional SEM image and (d) photograph of the polymer laser based on the suspended membrane structure. The inset in (c) is the AFM image of the 1D grating covered by the membrane. The scale bar is 600 nm.
The insets in Figs. 2(a) and 2(c) demonstrate the atomic force microscopic (AFM, WITec alpha300-S) images of the 1D DFB cavity before and after covering by the polymer membrane, respectively. The height modulation of the grating in Fig. 2(a) is about 90 nm. After covering by the membrane, the height modulation of the sample is negligible, which is less than 5 nm. It implies that the morphology of the DFB cavity has little effect on the smoothness of the polymer membrane. Of course, the larger the polymer membrane, the more difficult the procedure. It is difficult to paste a large-area polymer membrane on the substrate without wrinkles. In the experiment, the area of the polymer membrane is about 100 mm2, which is adequate for polymer lasers. Figure 2(d) presents the photograph of the polymer laser based on a suspended membrane structure. The homogeneous diffraction colors imply that the free-standing polymer membrane is attached on the grating structure smoothly and successfully. The effective area of the DFB cavity is about 70 mm2 in Fig. 2(d).
For the emission measurements, the sample is excited by a 200-fs laser, which has a wavelength of 400 nm and a repetition frequency of 1 kHz. The pump light is directly incident on the sample without focusing. The diameter of the pump beam is about 3 mm. A neutral density filter is used to adjust the pump power. And a quarter-wave plate is employed to switch the polarization of the pump light from linear to circular, which will be discussed in detail later. The emission spectra of the polymer laser are measured by a spectrometer (Maya 2000 Pro, Ocean Optics) as shown in Fig. 3.
Fig. 3 Measured emission spectra of the polymer laser with (a) 1D and (c) 2D distributed feedback. The output intensity of the polymer laser based on the (b) 1D and (d) 2D DFB cavity. The threshold of the 1D polymer laser is 23.4 μJ/cm2. The thresholds of the 2D polymer laser are 14.2 μJ/cm2 and 15.2 μJ/cm2 for the two orthogonal cavities, respectively.
Figures 3(a) and 3(c) present the emission spectra of the polymer laser with different pump fluences. One laser emission peak can be found in the 1D device, which locates at 566 nm. For the 2D device, two laser emission peaks are observed at 568 nm and 579 nm, respectively. For the DFB polymer lasers, the laser oscillation satisfies the Bragg condition 2Λneff = mλ, where neff is the effective index of the laser mode, m is the Bragg order. For the surface-emitting DFB laser in this work, m equals 2. So, the effective index of the laser mode in the 1D device is 1.57. For the 2D rectangular cavity, the effective indices of the 568 nm mode and the 579 nm mode are 1.58 and 1.56, respectively. It can be seen that the effective index of the laser shows little change, which implies the relationship between the period of the cavity and the emission wavelength is more predictable.
Figures 3(b) and 3(d) show the output intensity as a function of the pump fluences. The threshold of the 1D DFB polymer laser is 23.4 μJ/cm2. The thresholds of the 2D DFB polymer laser are 14.2 μJ/cm2 and 15.2 μJ/cm2 for the two orthogonal cavities, respectively. The threshold of the 2D device is lower than that of the 1D device, which is attributed to the more efficient feedback of the 2D cavity [6]. The threshold of the proposed device is much lower than that of similar polymer lasers in our previous work [5, 17]. In the experiment, the pump slope efficiency of the laser device is in the range of 3% to 6%. The output intensity is slightly less than 1 μJ/cm2, which is enough to test the device.
All laser emissions show similar slope efficiency due to the same active material. When the pump power exceeds the laser threshold, the full width at half maximum of the laser emission peak is less than 1 nm. Besides, only pulsed output can be obtained due to the absorption of excited triplet states, which is characterized by a long lifetime [3].
The polarization dependency of the DFB polymer laser is studied systematically. The results show that the polarization of the pump light has significant influence on the performance of the DFB polymer laser. In the experiment, a rotating half-wave plate is inserted between the pump source and the sample, as illustrated in Fig. 4(a). When the half-wave plate is rotated from 0 to β, the polarization direction of the pump beam changes from 0 to 2β. To facilitate the measurement process, the detector and the pump source are separated by the surface-emitting DFB laser. Figure 4(b) shows that the output intensity of the laser device changes with the rotation angle β. For a pump power above threshold, a maximum output intensity is obtained when the polarization direction of the pump beam is parallel to the grating lines. Thus, a circularly polarized light is proposed to excite the polymer laser with a rectangular DFB cavity. An intriguing phenomenon is that the laser output beam consists of two linear polarization components, horizontal and vertical, which correspond to the 360 nm cavity and the 370 nm cavity of the rectangular cavity, respectively. Figure 4(c) and 4(d) demonstrate the characterization of the polarization of the output of the DFB polymer laser. The output beam is directed though a rotatable polarizer, which is analyzed by the Maya 2000 Pro spectrometer. A strongly polarized emission parallel to the grating lines is observed as shown in Fig. 4(d), implying that the rectangular cavity can be considered as a linear combination of two 1D cavities.
Fig. 4 (a) Schematic of the optical layout for measuring the polarization dependency of the polymer laser. The red dotted line indicates the optical axis of the half-wave plate. The double-headed red arrow denotes the polarization direction of the pump beam. β is the included angle between the optical axis and the polarization direction of the pump. (b) The output intensity as a function of β. (c) Enlarged view of the laser spot.① and ② identify the emission wavelengths of the 360 nm and 370 nm cavity, respectively. The red dotted line indicates the polarization direction of the polarizer. γ is the angle between the polarization direction of the polarizer and the lasing line. ② (d) The output intensity of the 2D DFB polymer laser with different angle γ.
The polarization dependency of the laser device can be explained by a slab waveguide theory. Waveguide modes exist in the polymer layers. The coupling of the waveguide modes to the free space depends on the Bragg scattering from the grating structure. The thickness of the polymer layer is such that, within the PL spectrum of F8BT, only the first transverse electric waveguide mode (TE0) is supported [18]. The polarization direction of TE0 mode is parallel to the grating lines, which is easier to be excited by light with same polarization state, as shown in Fig. 4(b). Similarly, the TE0 mode is Bragg-scattered out by the grating structure, forming the laser output in Fig. 4(d).
4. Conclusion
An assembly method is proposed to fabricate 1D and 2D DFB polymer lasers. The polymer laser consists of a grating structure and a polymer membrane. Low threshold laser emission can be observed when the device is optically pumped, which is attributed to both the smoothness of the polymer membrane and the air gaps in the DFB cavity. The polarization sensitivity of the polymer laser is discussed for the case of a rectangular cavity. The best performance of the polymer laser is achieved by adjusting the polarization direction of the pump along the grating lines. And the laser output consists of two linear polarization lights, which depend on the two orthogonal cavities, respectively.
Funding
National Natural Science Foundation of China (NSFC) (11474014, 11274031).
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