1. Introduction

Since the first theoretical prediction by Letokhov in 1960s [1] and the experimental demonstration by Lawandy in 1994 [2], the investigation of Random Lasers (RLs) has become an extremely attractive field [3–8]. RLs result from the interaction of light with disordered amplifying media, which are a consequence of multiple light scattering events by scatterers and the formation of closed loop paths from the sattered light [9–11]. RLs have been realized in variety of dielectric and metallic media with micro and nanometric dimensions, such as liquid laser dye with scatterers [12–14], semiconductors powders [15–18], fibers [19–21], polymer films [22–24] and dye-infiltrated opals [25]. Two types of feedback regimes in RLs are identified [24]: incoherent feedback, in which light propagation is diffusive and the probabilistic nature of diffusion means that interference contributes negligibly to the feedback process; coherent feedback, which occurs when the photon mean-free-path and the emission wavelength have the same order of magnitude making it possible for localization of the radiation field to occur within the structure.

The formation of an optical waveguide is a powerful approach to confine and control the propagation of light [26]. A waveguide structure based on dye-doped polymeric films is a typical random laser system. Several waveguide random laser structures have been reported. For instance, Zhai et al studied a waveguide-plasmonic scheme which is constructed by coating the matrix of randomly distributed gold nanoisland structures with a layer of dye-doped polymer [27]. Yuen et al room-temperature ultraviolet lasing is demonstrated in mirrorless zinc oxide thin-film waveguides on silicon substrate [28, 29]. Generally, nanoislands or nanoparticles are commonly utilized as scatterers, some special nanostructures are not exhaustively researched. Dominguez et al described a RL system with coherent feedback system in PVA films containing titanium dioxide nanomembranes [24], which demonstrates the possibility that special nanostructures can equally be used as scatterers in the waveguide RL system.

In the present paper, we demonstrate a new scheme for achieving coherent random lasing based on a waveguide structure consisting of a PMMA polymer film doped with Rhodamine 6G as gain media and a layer of ZnSe-nanosheets (ZnSe-NS) arrangement as scatterers. The cubic sphalerite structure ZnSe nanosheets is distributed disorderly exhibiting a flat and triangular morphology with typical length of 0.5 to 1 μm and average thickness of 30 nm, which is fabricated by a two-steps method (femtosecond pulsed laser ablation and hydrothermal technique). The random laser scheme reveals an obvious two-threshold behavior, which is corresponding to the thresholds of TM and TE modes. Moreover, the feedback mechanisms for laser action are investigated by power Fourier transforming of the spectra, implying that “scattering-total reflection- scattering” process and microcavities between nanosheets are attributed to the random laser action. In addition, as nanosheets in a large area are foreseen as new MEMS/NEMS building blocks, a new generation of combined active/passive photonic devices can be envisaged.

2. Experimental details

In the experiments, the ZnSe nanosheets are fabricated by a two-steps method. The first step in the samples preparation is a ZnSe nanoparticle film fabrication, which were grown on silica substrates by femtosecond pulsed laser ablation (FPLD) from a 99.999% pure ZnSe target. A regenerative Ti:sapphire mode-lock pulsed laser system with a central wavelength of 800nm (Coherent Inc, Lengend Elite, 40 fs pulse duration, a fluency of ~1 J/cm² and 100 Hz repetition rate) is employed. Silica substrates with a thickness of 1 mm are placed on a heater at 45 mm distance from the target, which is raster-scanned in a circular area of 15 mm in diameter with the laser beam fixed. During deposition, the substrates are kept at 400 °C, the chamber base pressure is 1 × 10⁻⁵ Pa. Similar to the fabrication technology in the previous studies [30, 31], the second step for preparation ZnSe nanosheets is hydrothermal technique. The ZnSe nanoparticle films obtained by FPLD was placed in a autoclave, heated to 80 °C for 36 hours and then cooled to room temperature. Films were then washed with distilled water and absolute alcohol three times before being dried in a vacuum at 40 °C for 30 minutes.

The Silica/ZnSe-NS/PMMA-Rh6G/Silica waveguide samples were prepared by spin coating of a PMMA film (thickness of ≈2 μm, n₁ = 1.51) containing Rh6G dye (Concentration of 10⁻³ M) on the surface of a quartz substrate (10 × 10 mm²) that supports the ZnSe-NS. In order to protect the PMMA-Rh6G film and obtain a flat waveguide structure, another silica slice (n₂ = 1.45) with a thickness of 0.5 mm was covered above the PMMA-Rh6G film before it dried, hereby a Silica/ZnSe-NS/PMMA-Rh6G/Silica waveguide structure has been achieved. For comparison, additional samples with the same characteristics were prepared depositing the PMMA film with Rh6G directly on the middle of two quartz slices without ZnSe-NS. The purity and morphology of the as-prepared ZnSe-NS was characterized by XRD (X’Pert Pro MPD, Philips Research) and SEM (HITAC SU8220).

The waveguide samples were excited by a second harmonic of Nd: YAG laser (532 nm, 7 ns, 10 Hz, Innolas, GWU-Lasertechnik). Through a pinhole filter, a slit and a cylindrical lens, the excitation laser beam was focused onto the waveguide perpendicularly in a strip with the size of 15 μm × 12 mm. The edge fluorescence and waveguide laser emission was collected by a quartz fiber. In this experiment, we used an Ocean Optics USB4000 spectrometer (spectral resolution ~1.5 nm, five points collected per nanometer) for spectra measuring. To measure the pump energy, we used a beam-splitter in our experimental setup, and the reflected light was recorded by an energy meter (Ophir PE9F-SH), as shown in Fig. 1. Furthermore, the pump energy was kept below the damage threshold to prevent the destruction of the films and nanostructures.

 

Fig. 1 Schematic illustration of Silica/ZnSe-NS/PMMA-Rh6G/Silica waveguide random laser and diagram of optical experimental setup, the inset is a photograph of emission when the pump energy is above the threshold.

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3. Results and discussion

A layer of 1-μm-thick randomly assembled ZnSe nanosheets were grown on a silica substrate by hydrothermal method. Figure 2 illustrates the SEM morphologies at different magnifications of ZnSe-NS on the silica substrate. As shown in Fig. 2(a), a typical dense disordered distribution sheet array morphology is revealed intuitively in the large area at the low magnification SEM. From the high magnification SEM, it is shown that most of the as-growth ZnSe-NS exhibits a flat and triangular morphology with typical length of 0.5 to 1 μm and average thickness of 30 nm. The inset of Fig. 2(a) describes the XRD patterns of ZnSe nanosheets film compared with those of ZnSe nanoparticles film and ZnSe bulk. The XRD researches were performed by angle X-diffraction with Cu Kα anode. The XRD peaks can be indexed as ZnSe cubic sphalerite structure.

 

Fig. 2 SEM images at different magnifications of ZnSe-NS grown on the surface of a quartz substrate using hydrothermal technique. The inset shows the XRD patterns of ZnSe bulk (Bottom blue line), ZnSe nanoparticle film fabricated by FPLD (Middle red line) and ZnSe-NS film (Top black line).

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Figure 3(a) shows the emission spectra of Silica/ZnSe-NS/PMMA-Rh6G/Silica waveguide structure as a function of pump energy. Typical random lasing emission evolution spectra have been observed. At low pump intensity (15 μJ/pulse), only broad and featureless spontaneous emission spectra of Rh6G can be measured. In this case, the total gain of a certain random cavity cannot compensate the total loss, so there is no lasing action. As the pump energy is around the threshold (21 μJ/pulse), a ultra-narrow spike (FWHM ≈0.65nm) suddenly emerges on the broad emission background at the wavelength of 563 nm (as shown in the inset of Fig. 3(a)). This ultra-narrow peak represents a certain laser mode, and it is a direct evidence of the generation of coherent random lasing action [6]. Sharp lasing peaks start to emerge and the number of lasing peaks increases with the increase of pump power until saturation is reached. The appearance of laser spikes in the emission spectra is stable for the integration over 50 excitation pulses, indicating that after each excitation pulse, the system lases in the same modes and therefore at the same wavelengths. As for laser action, the pump energy corresponding to the appearance of the marrow peak, which is accompanied by rapid dramatic peak intensity, is regarded as the threshold. Figure 3(b) shows the peak intensity as a function of pump energy, which clearly demonstrate a threshold behavior with two thresholds at ~20 μJ/pulse and ~53 μJ/pulse. This phenomenon is similar to the previous studies [28, 31–33], and the reasons will be discussed later.

 

Fig. 3 (a) Emission spectra measured along the pump stripe for different pump energy, inset in (a): emission spectra just below and above the pump threshold. (b) Evolution of emission intensity as a function of pump energy, inset in (b): three photographs of emission at different pump energy above threshold. Three lines are guides for the eyes, and circles represent two thresholds at ~20 μJ/pulse and ~53 μJ/pulse.

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Figure 4 shows the normalized emission spectra of Silica/ZnSe-NS/PMMA-Rh6G/Silica waveguide, Silica/PMMA-Rh6G/Silica and PMMA-Rh6G film, pumped at 532 nm with a fixed pumping energy. The spectrum FWHM of Silica/PMMA-Rh6G/Silica structure is about 10 nm, longer than that of Silica/ZnSe-NS/PMMA-Rh6G/Silica waveguide (~2 nm) but shorter than that of PMMA-Rh6G film (~40 nm), which may be attributed to the amplified spontaneous emission efficiency (ASE) caused by light diffusive amplification in gain-induced waveguide channels . The dashed line shown in the inset of Fig. 4 corresponds to the function I_ASE∝I₀(e^(g-α)L-1), which fits well the experimental data. g and α are the gain and loss coefficients, respectively, and L is the pumping stripe length. Compared to the ASE, the FWHM of waveguide random lasing is decreased by about five times. The spectral characteristic is an evidence for the random laser engendered in the Silica/ZnSe-NS /PMMA-Rh6G/Silica waveguide.

 

Fig. 4 The normalized emission spectra of Silica/ZnSe-NS/PMMA-Rh6G/Silica waveguide (I), Silica/PMMA-Rh6G/Silica (II), and PMMA-Rh6G film (III), pumped at 532 nm with a fixed pumping energy. The inset is emission peak intensity of Silica/PMMA-Rh6G/Silica as a function of the pumping pulse energy.

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A polarizer in the direction perpendicular (TM) and parallel (TE) to the waveguide film is ultilized to analyze the polarization properties of the lasing light from the lateral facets of the waveguide. Figure 5 shows the TM (Fig. 5(a)) and TE (Fig. 5(b)) spectra, and their threshold curves. It is obviously to observe that only two peaks appear with the pump energy increasing, implying that there are only two optical modes for the TM polarized light. However, the peaks of TE spectra are not immovable, the position and intensity of the peaks vary with the pump energy, indicating that there are several optical cavities formed the random laser. Moreover, the thresholds of TE and TM are different, ~18 μJ/pulse for TM and ~50μJ/pulse for TE respectively. This is the reason why we find two thresholds in Fig. 3(b). Due to the spectra measured in Fig. 3 consist of TM and TE polarization, the TM polarized random laser will firstly emerge at ~20 μJ/pulse, by increasing the pump energy to ~50 μJ/pulse, the TE random laser can be obtained. Consequently, the thresholds of TM and TE are corresponding to the two thresholds in Fig. 3(b).

 

Fig. 5 The TM (a), TE (b) spectra and threshold curves (c) of Silica/ZnSe-NS/PMMA-Rh6G/ Silica waveguide, pumped at 532 nm.

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If the lasing mechanism of the waveguide random laser is related to coherent random laser action, it is possible to deduce the closed-loop cavity length, l, of the corresponding random modes by power Fourier transform (PFT) [7, 25, 26]. Figure 6 shows the calculated PFT spectra of the TM and TE spectra. It is appreciated that the closed-loop cavity length of TM (l_TM) and TE (l_TE) is 8 and 2.6 μm respectively. Obviously, the l_TM is much longer than l_TE, which predicates that the closed-loop cavity types of TM and TE are different. The random lasing results from multiple events of light scattering taking place sequentially by the disordered ZnSe nanosheets at their interfaces with the PMMA doped with Rh6G, which is enhanced or multiplied by the PMMA layer through the confinement of the scattered light into the waveguide. This can be equalized to a kind of microcavity effect, where the “round-trip” forms between the total reflection at the PMMA-Silica interface and the strong scattering by the ZnSe nanosheets. Thus, each “round-trip” corresponds to a gain process through a “scattering-total reflection-scattering” process and the lasing action depends on the minimum mean free path length, which is defined as [27]: l_min = 2n²d/(n²-1)^1/2, where n and d are the refractive index and the thickness of the gain medium, respectively. This path length is multiplied by the total reflection process and extended within the waveguide. In our experiments, n ≈1.51, d ≈2 μm, and we obtain l_min = 8.1 μm. It can be found that the l_TM almost equals to l_min, demonstrating that the TM polarized laser is formed in “scattering-total reflection-scattering” process as shown in the inset (a) of Fig. 6. As for TE polarized laser, the l_TE (2.6 μm) is much shorter than l_min (8.1 μm) meaning it do not form in the waveguide structure. Meanwhile, due to the special morphologies of ZnSe nanosheet which can be regarded as small mirrors (as shown in Fig. 2), it is reasonable to conceive that some ring cavities or F-P cavities may form between the nanosheets (as shown in the insets (b) and (c) of Fig. 6). The separation between nanosheets is about 0.2 μm to 1 μm, implying that closed-loop cavity lengths may be analogous to l_TE. Hence, we can conclude that the TE polarized laser is formed in the microcavities between nanosheets, then guided by the waveguide structure to the edge facet.

 

Fig. 6 The calculated PFT spectra of the TM and TE spectra. The inset diagrams explain the closed-loop cavity formed in the waveguide (a), F-P cavity (b) and ring cavity formed between the nanosheets.

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4. Conclusion

We demonstrated a new RL scheme with coherent feedback in a sample consisting of a gain medium (PMMA film doped with Rhodamine 6G) adjacent to the scattering medium comprised of ZnSe nanosheets. The strong scattering by the nanostructures and the strong confinement by the active waveguide extend free path length significantly, enabling low pump threshold and high conversion efficiency of the random laser. The herein reported results open up new avenues to exploit nanomembranes arrangements as active integrated photonic devices. The incorporation of active random media, as demonstrated here, will certainly enhance these applications.