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Abstract
Continuously tunable polymer lasing was achieved in one-dimensional, two-dimensional, and compound chirped cavities. The chirped cavity was simply fabricated by using interference lithography and spin coating. Two-dimensional and compound chirped cavities were obtained by employing oblique exposure and double exposure, respectively. The tunability range of two-dimensional chirped cavities was much wider than that of one-dimensional chirped cavities, which varied from 557 nm to 582 nm. The interaction between lasing modes was studied in the compound cavity by introducing an additional nanostructure into the two-dimensional chirped cavities. The threshold of the compound chirped cavities changed with the coupling strength between lasing modes. These results may be helpful for designing compact polymer laser sources.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Semiconducting polymers are attracting more and more attention due to the broad photoluminescence (PL) spectra [1,2], large tunable emission wavelengths [3] and large stimulated emission cross-sections [4]. To date, optically pumped polymer lasers are demonstrated using various resonators [5–7], such as microcavities [8], microrings [9,10], and distributed feedback (DFB) geometries [11–17]. Due to the excellent performance, DFB cavities have been considered as the most promising solution for polymer lasers [18–21]. Furthermore, tunable lasers have been desirable for the laser source of multichannel optical communication system [22–24]. Therefore, a variety of approaches have been applied to achieve tunable polymer laser, such as flexible substrates [25–30], thermal sensitive materials [31], liquid crystals cavities [31–33], multi-period integrated devices [3,34–37], and chirped grating [22,38–43]. These successful devices open a door for multi-wavelength organic semiconducting DFB lasers to be applied in compact laser source. These advances make it timely to develop simple-fabricated, low-cost and high-performance DFB lasers. In the experiment, we take advantage of a simple and cheap method to fabricate one-dimensional (1D), two-dimensional (2D), and compound chirped cavities. The laser performance was investigated systematically based on different chirped cavities, including tunability and mode interactions.
In this paper, we studied the tunability of DFB polymer lasers based on 1D, 2D and compound chirped cavities. The chirped cavities were fabricated by a modified interference lithography method. An ultraviolet (UV) laser beam was split into two beams, which were converged by the cylindrical lens and overlapped to form the interference pattern. The 2D chirped cavities were obtained by an oblique exposure method. And, the compound chirped cavities were achieved by a double exposure method, which consisted of a regular grating and a 2D chirped cavity. Tunable polymer lasing was observed in all chirped cavities. The tuning range of the 1D, 2D chirped cavities were 8 nm, 25 nm, respectively. The lasing threshold of the compound chirped cavities increased with the decrease of the period difference between the chirped cavity and the regular grating. It can be attributed to the additional loss in the laser cavity caused by the coupling between different lasing modes.
2. Fabrication of chirped cavities
The photoresist (PR, AR-P3170, Strausberg, Germany) chirped cavities was fabricated on a glass substrate (15 × 15 × 1 mm) by a modified interference lithography method. The effective area of the PR structure was about 8 mm2. The PR was first spin-coated onto a glass substrate to form a thin film at a speed of 2500 rpm for 30 s. The sample was heated at 110 °C for 1 min on a hotplate. Then, the prepared sample was exposed to an interference pattern generated by a 343 nm pulsed laser (FLARE NX, Coherent, Santa Clara, CA, USA) for 15 s. After exposure, the sample was developed in a developer (AR-300-47, Allresist, Strausberg, Germany) for 5 s, forming a PR grating structure. A polymer poly [(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-(2,10,3)-thiadiazole)] (F8BT, Sigma-Aldrich, St. Louis, MO, USA) was dissolved in xylene at a concentration of 23.5 mg/mL. The F8BT solution was spin-coated onto the grating structure at a speed of 1700 rpm for 30 s. Later, the device was heated at 50 °C for 1 min on a hotplate forming a DFB polymer laser.
For the modified interference lithography method, two cylindrical lenses (f = 50 mm) were introduced into the interference lithography system, as shown in Fig. 1(a). The interference pattern was formed by two cylindrical beams in Fig. 1(b). As a result, a 1D chirped pattern was obtained as shown in Fig. 2(b).
Fig. 1. (a) Schematic optical layout of the modified interference lithography method. BS, R, and CL indicate the beam-splitter, the mirror and the cylindrical lens, respectively. Beam configurations for fabricating (b) 1D and (c) 2D chirped gratings. ${{\alpha }_\textrm{i}}\, ({{i = 1, 2, 3}} )$ denotes the angle between the two interference beams converged by the cylindrical lens. φ and ϕ are the angle between the z axis and the wave vector of the two interference beams, respectively. β presents the oblique angle between the angular bisector direction of two interference beams and the direction of the surface normal of the sample. ${\beta = 54}$° in our experiment.
Fig. 2. 1D and 2D chirped grating structures. (a) and (d) Photographs of the sample. The dash outline in (d) indicates profile of the UV laser spot. The scale bar is 5 mm. (b) and (e) Schematic diagram of 1D and 2D chirped gratings. The blue area denotes the UV laser spot. The red lines indicate the grating structures. (c) and (f) SEM images of the nanostructure at different positions of the chirped grating.
The period ${\Lambda }$ of the 1D chirped grating is determined by $\Lambda = \frac{\lambda }{{\sin \varphi (z) + \sin \phi (z)}}$, where λ is the wavelength of the UV laser, and $\varphi (\textrm{z})$ and $\phi (\textrm{z})$ are the wave vector direction of the two cylindrical waves as shown in Fig. 1(b). For some special cases in the upper panel in Fig. 1(b), the grating period can be estimated by $\Lambda = \frac{\lambda }{{2\sin {\alpha _i}}}$, where ${\alpha _i} = \varphi (z) = \phi (z)$ (i = 1, 2, 3), where ${{\alpha }_\textrm{i}}$ is the angle between two interference beams. The range of the chirped grating period can be estimated by ${R_{1D}} = [{\Lambda _3},{\Lambda _1}] = [\lambda /(2\sin {\alpha _3}),\lambda /(2\sin {\alpha _1})]$.
An oblique exposure method was employed to fabricate 2D chirped gratings. Figure 1(c) shows the beam configuration. A 2D chirped pattern was obtained as shown in Fig. 2(e). The period of the 2D chirped grating is determined by ${R_{2D}} = \left[ \begin{array}{l} {\Lambda _x}\\ {\Lambda _y} \end{array} \right] = \left[ {\left|\begin{array}{l} \frac{\lambda }{{\sin \varphi (z) + \sin \phi (z)}}\\ \frac{\lambda }{{\cos \varphi (z) - \cos \phi (z)}} \end{array} \right|} \right] z \in [{0,l \cdot \sin \beta } ]$. l is the length of the interference pattern as denoted in Fig. 2(b). The oblique angle ${\beta }$ is determined by the angular bisector direction of two interference beams and the direction of the surface normal of the sample in Fig. 1(c).
Figure 2 presented the 1D and 2D chirped grating. The photograph and the schematic of the 1D chirped grating were shown in Figs. 2(a) and 2(b), respectively. The period of the chirped gratings changed slowly from the middle to both sides due to the cylindrical symmetry. The scanning electron microscopy (SEM) images of three typical positions. Obviously, the tuning range of the period of the 2D chirped gratings is much larger than that of the 1D chirped gratings, as shown in Figs. 2(d)–2(f). The diffraction pattern in Fig. 2(d) indicated the overlapping area of two interference beams denoted by the white circles. The period of the 2D chirped grating changed from 290 nm to 350 nm with a shift of 3.5 mm along the y direction as shown in Figs. 2(e) and 2(f).
3. Spectroscopic characteristics of chirped polymer lasers
As shown in Fig. 3(a), the period of the 1D chirped grating increased from 330 to 340 nm with a shift of 1.8 mm along the x direction as shown in Figs. 2(b) and 2(c), implying a tuning rate of about 6 nm/mm. The distribution of the grating was symmetric as shown in Fig. 2(b). The tuning rate of the 2D chirped grating is about 17 nm/mm. Obviously, the tuning range of the chirped cavity was significantly improved by introducing an oblique angle β.
Fig. 3. (a) Variation of the period of 1D/2D chirped gratings at different positions of the chirped gratings. (b) Relationship between the output wavelength and the period of the 1D (red line and dots) and 2D (black line and dots) chirped gratings. (c) Scheme of optical layout for spectroscopic measurement. OA denotes the optical attenuator. θ is about 50°. Typical emission spectra of the different output wavelengths based on the 1D (d) and 2D (e) chirped gratings, as indicated in (b). Normalized PL and amplified spontaneous emission spectra of F8BT were denoted by the yellow dotted curve and the purple dotted curve, respectively.
For a DFB laser, the laser wavelength λe satisfies the Bragg condition $2{n_{eff}}\Lambda = m{\lambda _e}$. Λ is the grating period. neff is the effective refractive index of the laser mode. For the surface-emitting DFB laser in this work, m equals 2. In the experiment, the period of 1D and 2D chirped cavity were changed with changing the pump position. Thus, λe was tuned by changing the grating period. Figure 3(b) showed relationship between the output wavelength and the period of the 1D and 2D chirped gratings. It can be seen that the relationship can be expressed as ${\lambda _e} = 1.7\Lambda - 24.4$ and ${\lambda _e} = 1.7\Lambda - 33.4$ for the 1D and 2D chirped cavities, respectively. The yellow zone indicated the range of the lasing wavelength by using the gain material F8BT.
Figure 3(c) showed the optical layout of the optical pumping. A 400 nm femtosecond laser with a pulse duration of 200 fs and a repetition frequency of 1 kHz was employed as the pump source. An optical attenuator was inserted between the pump source and the sample to tune the pump energy. A lens (f = 150 mm) was used to focus the pump beam on a specific position of the sample surface with an incident angle of θ (∼50°). The radius of the pump beam was approximately 0.5 mm. The sample was mounted on an x-y translation stage. The emission spectra of the laser device were measured by an optical spectrometer (Maya 2000 Pro, Ocean Optics) with a resolution of 0.2 nm. The tunabilities of 1D and 2D chirped cavities were demonstrated in Figs. 3(d) and 3(e), respectively. The tuning range of the emission wavelength of the 1D and 2D chirped cavities were 8 nm (559 nm ∼ 567 nm) and 25 nm (557 nm ∼ 582 nm), respectively. So, the tuning range of the laser device can be greatly extended by employing 2D chirped cavities. The normalized PL and amplified spontaneous emission (ASE) spectra of F8BT were given in Figs. 3(d) and 3(e).
The tunability of the lasing wavelength in chirped cavities provides an opportunity to investigate the interaction between different lasing modes. The interaction between two laser modes has been carefully discussed in the lead halide perovskite microrods [7]. Here, we introduced an additional nanostructure into the 2D chirped cavities to form a compound cavity. In the experiment, the sample was exposed for 15 s in the light path in Fig. 1(c), forming a 2D chirped gratings. After rotating 90 degree, the sample was exposed for 12 s in a two-beam interference pattern (${\Lambda }$=334 nm). As shown in Fig. 4(b), the compound cavity consisted of a 2D chirped grating in Fig. 2(e) and a 1D grating. The compound chirped cavities were fabricated by a double exposure method. The photograph of the sample was presented in Fig. 4(a). The direction of the grating lines of the 1D grating is perpendicular to the y direction of the 2D chirped grating, as shown in Fig. 4(b). Figure 4(c) presented the SEM images of the nanostructure at different positions of the compound chirped cavity. It can be seen that the difference of the grating period changed at different positions of the sample. The wavelength of the lasing mode was decided by the grating period. The wavelength of the lasing mode changed in the 2D chirped cavity. The wavelength of the lasing mode kept unchanged in the 1D grating. So, it is possible to study the coupling strength between lasing modes in different cavities in a single laser device.
Fig. 4. (a) Photograph of the laser device based on the compound chirped cavity. The dash outline indicates the profile of the UV laser spot. The scale bar is 5 mm. (b) Schematic diagram of the fabrication of the compound chirped cavity. (c) SEM images of the nanostructure at different positions of the compound chirped cavity.
The emission spectra of the laser device based on the compound chirped cavity were measured by changing the pump position on the sample. The pump position was changed carefully along the y direction in Fig. 4(b) by a using a translation stage. Figures 5(a)–5(d) presented the emission spectra at the different pump positions of the compound chirped cavity. Two laser emission peaks were observed which were supported by the 1D grating and the chirped grating, respectively. The separation between two lasing peaks decreased with reducing the period difference between the two gratings in the compound cavity. Figures 5(e)–5(h) presented the output intensity of the laser devices as a function of the pump power intensity, indicating that there were two lasing thresholds for the compound chirped cavity. One lasing threshold corresponded to the 561-nm mode in the 1D grating structure, and the other corresponded to the mode in the 2D chirped grating structure. The interaction between two lasing modes became stronger when the separation between two lasing peaks decreased. When the two lasing peaks are nearly coincident as shown in Fig. 5(a), both thresholds of the laser device increase significantly. It can be attributed to the energy coupling between two lasing modes, which acts as a loss channel of the laser device.
Fig. 5. (a)-(d) Measured emission spectra at the different pump positions of the compound chirped cavity. The position was denoted by x and y. (e)-(h) Output intensity of the laser devices as a function of the pump power intensity. (i)-(l) Electric field distributions of the lasing modes in the grating structure with different periods.
The electric field distributions of the two lasing modes were studied by using the COMSOL software package based on a finite element method, as shown in Figs. 5(i)–5(l). In the simulations, the thickness of the polymer layer (npolymer=1.9) was 310 nm. The period of the PR grating (nPR=1.67) changed from 334 nm to 341 nm. The modulation depth of the grating structure was 160 nm. The refractive index of the silica substrate was 1.5. All refractive indices of the materials were measured using a spectroscopic ellipsometer (ESNano, Ellitop). Obviously, the difference between the mode distributions reduces with decreasing the period difference of two gratings. The match between two laser modes enhances the coupling effect.
The coupling effect can be described by the coupled-mode theory [44]. For compound chirped cavities, the coupling effect is decided by the difference of the grating periods and the angle between two gratings.
The coupling strength between the modes is determined by the coupling coefficient ${\kappa}$ [45].
Fig. 6. (a) Coupling strength as a function of the difference of the two lasing wavelengths (Δλ). (b) Coupling strength as a function of the angle ${\gamma}$ between two gratings. The white circle rings denoted the experimental values in Fig. 5(a).
4. Conclusion
1D and 2D chirped cavities were proposed to achieve continuously tunable polymer lasing. All cavities were fabricated by interference lithography. The tuning range of the output wavelength of 2D chirped cavities was approximately three times larger than that of 1D chirped cavities. For a compound chirped cavity, the coupling strength between different lasing modes was studied by measuring the threshold of the laser device. The coupling strength were decided by the period difference and the angle between the two gratings. The lasing threshold increased with increasing the coupling strength. It can be attributed to the additional cavity loss caused by the coupling process. These results can be utilized for realization of compact polymer laser devices.
Funding
National Natural Science Foundation of China (61822501); Natural Science Foundation of Beijing Municipality (Z180015).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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