1. Introduction

The broad photoluminescence (PL) spectra of polymers render them ideal for tunable spectroscopy in the visible region, which raise the possibility of continuous tunability of polymer devices [1]. In recent years, the tunability of the distributed feedback (DFB) polymer lasers makes them attractive candidates for compact photonic and optoelectronic devices. A variety of approaches have been developed to achieve continuous tunability of DFB polymer lasers, such as flexible materials [2–8], chirped or segmented cavities [9–11], dynamic cavities [12, 13], wedge shaped layers [14–17], incorporating liquid crystals [18–22], electric-directed reconfiguration [23], and changing refractive indices [24, 25]. The typical size of most tunable laser devices is about several centimeters, which is not suitable for the integration of such laser devices. So, a simple and convenient method that enables wide tuning range is required for the easy integration and more profound investigation of DFB polymer lasers.

In this work, a compact tunable polymer laser is demonstrated based on a fanshaped cavity. The periods of the fanshaped cavity change from 370 nm to 340 nm, which is fabricated by electron beam lithography (EBL) technique. Then a typical light-emitting polymer is spin coated onto the cavity, forming a DFB polymer laser with gradual increasing periods. The pump beam is focused by using a cylindrical lens to a stripe spot on the sample surface at oblique incidence. When the pump stripe is moved along the lines of the fanshaped cavity, the wavelength of the DFB polymer laser is continuously tuned due to the variation of the period of the DFB cavity. These results can be utilized in the design of compact, low-cost, continuously tunable laser sources.

2. Fabrication of the polymer laser based on a fanshaped cavity

In the experiment, a polymethylmethacrylate (PMMA) resist was used for defining the fanshaped cavity. PMMA (molecular weight 200K) dissolved in chlorobenzene with a concentration of 4.0% was spin coated on a glass substrate (15 × 15 × 1 mm) with a speed of 4000 rpm for 30 seconds. The thickness of the PMMA film is 160 nm. The sample substrate was baked on a hot plate at 180°C for 2 minutes in order to flat the film and evaporate the solvent. The fanshaped cavity was designed with the Raith nanosuit software and fabricated through an EBL process with Raith e-LiNE plus system with the accelerating voltage of 10 kV and working distance of 10 mm. The average producing time is about 10 hours for each sample. The production efficiency can be improved by the nanoimprint technique [26]. Then the sample was developed with MIBK: IPA(1:3) and stopped with IPA for 30 seconds respectively. The morphologies of the fanshaped cavity was measured by using the scanning electron microscopy (SEM, Raith e-LiNE), as shown in Fig. 1(a). The area of the fanshaped cavity is 1.84 mm² (x = 1.60 mm; L = 1.15 mm), and the modulation depth of the fashaped cavity is 160 nm. The width of the grating line is about 200 nm, as shown in the upper inset of Fig. 1(a). A typical light-emitting conjugated polymer, poly [(9, 9-dioctylfluorenyl −2,7-diyl)-alt- co-(1,4-benzo-{2,1’,3}-thiadiazole)] (F8BT, American Dye Source), was employed as the active material, which was prepared in xylene solution with a concentration of 22.5 mg/mL. The F8BT solution was spin coated onto the fanshaped cavity with a speed of 2000 rpm, forming a 150 nm thick polymer film. So, the proposed polymer laser consists of a fanshaped cavity and a polymer film, which is a surface-emitting device as shown in the lower inset of Fig. 1(a).

 

Fig. 1 (a) SEM images of the fanshaped cavity. The upper inset is the enlarged view of the SEM image. The scale bar is 200 nm. The lower inset shows the schematic of the polymer laser. (b) Schematic diagram of the fanshaped cavity. The red dotted line denotes the position of the pump stripe. The red arrow indicates the movement direction of the pump stripe. The blue line indicates the bisector the fanshaped cavity. L = 1.15 mm.

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When the pump stripe focused by a cylindrical lens moved from long period (Λ₁) to short period (Λ_n) (and vice versa) indicated as the red arrow in Fig. 1(b), the wavelength of the polymer laser can be continuously tuned. In the experiment, Λ₁ = 370 nm, Λ_n = 340 nm. The duty cycle of the grating varies from 59% to 54% when the period changes from 340 nm to 370 nm. These parameters were determined through a series of experiments. For a surface-emitting DFB laser, the laser wavelength λ satisfies the Bragg condition λ = Λn_eff, where n_eff is the effective index of the laser mode.

3. Spectra characterization of the polymer laser based on a fanshaped cavity

During the spectroscopic characterization, the fabricated polymer laser was excited by a 200-fs laser, which has a repetition frequency of 1 kHz and a wavelength of 400 nm. The emission spectra of the laser device were measured by an optical spectrometer (Maya 2000 Pro, Ocean Optics). The sample was mounted on a linear translation stage (Newport ULTRAlign M-462-XYZ-M) with a minimum incremental motion of 1 μm. So, when the sample was moved, the position of the pump stripe on the fanshaped cavity can be changed as shown in Fig. 1(b). The pump beam was focused by a cylindrical lens with 50 mm focal length to a stripe on the sample surface. The width of the pump stripe was about 75 μm and the length of the pump stripe was 10 mm, which were measured by a knife edge method. When the position of the pump stripe on the fanshaped cavity was changed from long period (370 nm) to short period (340 nm), the output wavelength of the polymer laser was tuned from 584 nm to 552 nm. Figure 2(a) shows the dependence of the output wavelength λ on the displacement d (y in Fig. 1, the position of the focus point on the sample), which follows a linear relationship of the form λ = −0.05d + 583.62. The period of the fanshaped cavity is 370 nm when the displacement equal zero. The discontinuous change of the wavelength is caused by the width of the pump stripe. Note that the effective area of the fanshaped cavity is about 1.12 mm² (x = 1.60 mm; d_max = 0.70 mm). Figure 2(b) presents the laser spectra measured at different pump positions perpendicular to the direction of the period. It can be seen that the tuning range is about 32 nm, which is mainly determined by both the extinction (cavity losses) and the PL spectra (cavity gain). The color change of the laser spot was clearly observed when the laser wavelength was tuned, as shown in Figs. 2(c)-2(f).

 

Fig. 2 (a) Tunability of the output wavelength of the polymer laser, which is achieved by changing the pump position (y) as shown in Fig. 1. (b) Typical emission spectra of the different output wavelengths indicated in (a). The tuning range of the wavelength is from 552 nm to 584 nm. The black and orange dotted curves indicate the extinction and PL of F8BT, respectively. (c)-(f) Photographs of the laser spot corresponded to different wavelengths in (b).

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From Fig. 1(b), it can be seen that the period of the fanshaped cavity follows the equation Λ = Λ₁-d(Λ₁-Λ_n)/L. So, the relationship between the output wavelength and the period of the laser device can be expressed as λ = 1.92Λ-125.55. As shown in Fig. 3(a), the tuning rate of the laser wavelength can be expressed as dλ/dΛ = 1.92, i.e., the laser wavelength shifts 1.92-nm per 1-nm change in the cavity period. The effective refractive index of the laser mode satisfies equation n_eff = 1.92-125.55/Λ. The model results agree well with the experiments in Fig. 3(a). Figure 3(b) illustrates the evolution of the chromaticity of the tunable polymer laser, which covers a wide spectral range.

 

Fig. 3 (a)The output wavelength/effective index as a function of the period of the fanshaped cavity. (b) Chromaticity of the emission spectra of the tunable DFB polymer laser in Fig. 2(b), as indicated by the eight red dots. The black arrow indicates the evolution of the chromaticity with changing the position of the focus as shown in Fig. 1(b).

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The thresholds of the laser modes in Fig. 2(b) are essentially the same. Figure 4 demonstrates the measured emission spectra of the 567-nm mode with different pump power intensities. The full width at half maximum of the laser peak is less than less than 1 nm when the pump intensity exceeds the threshold of the laser device. The period of the cavity is 361 nm. So, the effective index of the 567-nm mode is 1.57. Figure 4(b) shows the output intensity as a function of the pump intensities. The threshold of the 567-nm mode is 47 μJ/cm², which is similar to that of their counterparts fabricated by conventional methods [3, 27].

 

Fig. 4 (a) Measured emission spectra of the polymer laser at a specific wavelength (567 nm). (b) The output intensity of the polymer laser as a function of the pump power intensity, indicating that the threshold of the polymer laser is about 47 μJ/cm². The blue and red lines represent the linear fit of the output intensity below and above the threshold of the laser, respectively. The upper panel denotes the electric field distribution (TE₀) of the 567-nm mode.

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The upper panel of Fig. 4 presents the normalized electric field distribution (TE₀) of the 567-nm mode in the DFB cavity. Most of the energy concentrates within the F8BT layer due to its high refractive index, which implies the high efficiency of the laser device. The simulation was performed using the software COMSOL based on the finite element method. For the 567-nm mode, the refractive indices of glass, PMMA, F8BT, and air are 1.5, 1.4, 1.9, and 1.0, respectively. All refractive indices were measured by a spectroscopic ellipsometer (ESNano, Ellitop).

To investigate the polarization dependency of the polymer laser based on the fanshaped cavity, a half-wave plate is inserted before the laser sample to change the polarization of the pump beam as shown in the inset in Fig. 5(a). α is the angle between the polarization direction of the pump beam and the direction of bisector the fanshaped cavity as shown in Fig. 1(b).

 

Fig. 5 (a) Schematic diagram of the optical layout for measuring the polarization of the laser output. The blue and yellow arrows denote the pump beam and the output beam, respectively. The double-headed purple arrows imply the polarization direction of the pump. The black arrow indicates the optic axis of the half-wave plate. α is the angle between the polarization direction of the pump beam and the direction of bisector the fanshaped cavity. (b) The angle (β) between the lasing line and the polarization direction of the polarizer indicated by the white dotted line. (c) The output intensity of the laser as a function of α. (d) The polarization of the laser output .

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It can be seen from Fig. 5(c) that a maximum output intensity is achieved when the polarization direction of the pump beam is parallel to the direction of bisector the fanshaped cavity. The polarization of the output of the tunable polymer laser is measured as in Fig. 5(d). Clearly, it is a linearly polarized laser. The polarization properties of the proposed polymer laser is attributed to that only the first transverse electric waveguide mode (TE₀) can be excited due to the limited thickness of the polymer.

4. Conclusion

In conclusion, a continuous DFB polymer laser was achieved based on a fanshaped cavity, which was fabricated by combining EBL and spin-coating methods. When the pump stripe moved from the long period to short period of the fanshaped cavity or vice versa, the output wavelength was continuously tuned. The tuning range is 32 nm and the tuning rate is 1.92 nm for 1-nm change in the cavity period. The output wavelength, the effective index, and the period of the fanshaped cavity follow a linear relationship. Moreover, the proposed laser is a polarization sensitive device. This technique can be useful and could find potential applications as compact tunable sources for spectroscopy.