Coupled Modes Enhance Random Lasing in Plasmonic Thin Etalons

In this work, we experimentally and theoretically proved a exible ne random laser from a two-face etalon plasmonic structure based on PDMS. Accordingly, PDMS was fabricated using the nanoimprint lithography method and coated by a thin gold layer with a thickness of 35 nm using a PVD device and light-emitting polymer (F8PT) to enhance the scattering and eciency of the random laser. Using a plasmonic gold grating as a substrate, the simulation results compared the upside and downside of the plasmonic etalon structure. Moreover, an enhancement was observed in light transmission, and it is common to predict high eciency in random lasing properties. The experimental results showed a comparison between normal plasmonic etalon samples and symmetric and asymmetric etalon-based nanostructures with thicknesses of 200, 400 and 600 μm and reported that random lasing properties had better results for samples with thinner spaces based on coupled mode effects. Correspondingly, this was done by increasing the intensity and decreasing the lasing threshold from 22 𝜇 J in normal etalon to 16 μJ in the thinnest etalon structure. etalon etalon etalon etalon a.u.), peak intensities etalon symmetric etalon intensity


Ii. Experimental Part:
In this research, double 2D grating (or Etalon) nanostructures based on polydimethylsiloxane (PDMS) were fabricated using the nanoimprint lithography method, and the random lasing properties of the proposed samples were investigated.
In this regard, a schematic layout of the fabrication process of the etalon-based nanostructure is shown in Fig. 1(a). For this purpose, two charge-coupled devices (CCDs) have been used with two-dimensional grating patterns. First, the rst CCD was placed on a glass substrate, and a plexiglass frame was then placed around it. After that, the second CCD was placed on the top of the frame in front of the rst CCD and then xed by thermal adhesive. Additionally, a mixture of polydimethylsiloxane (PDMS) and curing agent (at a weight ratio of 10:1) was prepared and injected into the frame. The cell was placed inside a metal clamp to achieve good xation of the CCDs on both sides of the frame.
Subsequently, for drying and xing the 2D pattern onto the PDMS, the cell was placed on the heater at temperatures ranging from 50 to 100°C for 1 hour. Finally, the CCDs were removed from the sample after good drying of the PDMS composite, which took place for approximately one day at room temperature.
In this way, a double 2D grating nanostructure with a thickness of approximately 1.4 mm (normal etalon) was prepared to be coated on both sides by a thin gold layer with a thickness of 35 nm using a PVD device. Afterward, the sample was left for 24 hours before being coated with a layer of light-emitting polymer poly [(9,9-dioctyl uorenyl-2,7-diyl)-alt-co-(1,4-benzo-(2,1', 3)-thiadiazole)] (F8BT, American Dye Source) at a concentration of 8 mg/mL in xylene solutions. Next, these layers were coated onto one and two sides of the etalon sample using the drop weight method, and the actual image is shown in Fig. 1 The random lasing properties of the etalon sample were studied by a second-harmonic generation Nd:YAG laser with a repetition rate of 10 Hz and pulse width of 5 ns. The required data were then collected by ocean spectrometry at 45° to facilitate the actual test. The pumped energy of the laser beam was tuned continuously using an optical attenuator, as shown in Fig. 1(e).
Iii. Results And Discussions: Figure. 2 shows the light emitting polymer absorption spectrum, PL spectrum (F8PT) and electric eld distribution of our normal etalon structure in three parts. As indicated, the broad absorption peak is at 456 nm with a Au layer (blue line) and without a Au layer (green line), and the absorption peak of Au NPs is observed at 526 nm (black line), corresponding to a broad localized surface plasmon resonance (LSPR) spectrum. Of note, the absorption spectrum of light-emitting polymer (F8PT) with a Au NP layer appears higher than that without Au NPs, which may be due to the (LSPR) Au NPs. In addition, the red line in Fig.   2(c) shows the PL spectra of the light-emitting polymer (F8PT), which was observed at 542 nm and measured by the spectrometer.
The proposed structure was simulated using the mode solutions module of the Lumerical software, and the optical electric eld distribution was also investigated for our proposed structure. The simulated structure consisted of arrays of unit cells with lattice constants of a 1 = 3040 nm and a 2 = 5150 nm for both up-and downgrading, respectively, which were con rmed by scanning electron microscopy (SEM) images (Figs. 1(c)-(d)). The thicknesses of the PDMS, Au, and F8PT layers were estimated as t PDMS =8000, t Au = 35 nm, and t F8PT = 250 nm, respectively. In addition, a mesh size of 3. Correspondingly, this decreased the transmission of the central SLR wavelength, common in any plasmonic media. In addition, for the sample with a strong plasmonic substrate on the right edge of the etalon structure, we enhanced the transmission of input light. These two enhancements in the transmission of light from the sample with the plasmonic substrate make it common to predict high e ciency in random lasing by the sample with the plasmonic gold grating used as a substrate. Furthermore, we used a gold grating with a light-emitting polymer medium to enhance the scattering and e ciency, which are reported in the rest of this work.
Furthermore, re ection spectra of normal etalon and symmetric and asymmetric plasmonic etalons with thicknesses of 200, 400 and 600 μm were measured at incidence angles of 58 degrees, as shown in Figs.  3(a), (b) and (c), respectively. The excitation of the surface lattice resonance (SLR) of these 2D grating structures typically occurs at incidence angles ranging between 50 and 60 degrees [18,19]. In this research, sweeping of the incidence angle showed that a sharp and strong SLR response occurs at an incidence angle of 58 degrees. As shown, SLR dips were achieved for the fabricated plasmonic etalon-based nanostructure, corresponding to the coupling between localized surface plasmon resonances (LSPRs) of nanorods at the corner of each unit cell and diffracted order waves in the 2D periodic structure. As seen, the depth and width of the SLR mode increase with increasing thickness of the plasmonic etalon nanostructure. The plasmonic SLR mode is sharper for etalon with a thickness of 400 μm, and the broader SLR mode was measured for etalon with a thickness of 600 μm. In addition, several extra modes were measured for asymmetric plasmonic etalons (Fig. 3(b)), which come from breaking the symmetry of the structure and the coupling between plasmonic and cavity (etalon) modes.
To investigate the effects of these modes on the random lasing e ciency, we recorded random lasing of normal, symmetric, and asymmetric etalon-based nanostructures with different thicknesses of 1.4 mm, 200, 400, and 600 µm.
At rst glance, Fig. 4(a) shows the emission spectrum of the normal etalon plasmonic sample random laser at different pump energies. The results were analyzed based on the changes in the slope e ciency and threshold lasing of the etalon plasmonic random laser by comparison with the face-coating samples.
In normal etalon samples, spontaneous emission was observed with low pumping energy below the lasing threshold. With an increase in pumping energy above the threshold power, the emission of the gain medium was strongly scattered, which resulted in a rapid enhancement in the spontaneous emission due to an increase in the optical path and con nement of emitted light by multiple scattering of these samples. Thus, a very narrow emission peak emerged at 541 nm, with a full width at half-maximum (FWHM) less than 1 nm, which implies the occurrence of coherent plasmonic random lasing (Fig. 4(b)).
In addition, the threshold power of the random lasing appearance reaches 22 μJ, and the maximum intensity reaches 30874 a.u. at a high pumping energy of 46 J due to strong con nement provided by the two faces of the etalon sample (Fig. 4 (c)).
At second glance, as mentioned before, we changed the etalons into three different thicknesses to obtain more useful modes and additional e cient random lasing. Thus, the emission spectra of the symmetric plasmonic etalon samples at 200, 400, and 600 µm were recorded (Fig. 5). It is observed that the emission spectrum of the symmetric etalon sample with a thickness of 200 µm is higher than that for the symmetric etalon samples 400 and 600 µm due to the effect of backscattering, which is featured here and plays a prominent role in improving the emission from the thinner etalon sample in comparison with the standard etalon sample, in agreement with the simulation results in Fig. (2).
To more evaluate the performance of the random plasmonic laser under the in uence of the thickness of the plasmonic etalon samples, asymmetric etalon samples were used while maintaining the same thickness for the symmetric etalon samples used as shown in Fig. 5, by these samples, it is noticed that it offers better results than the rst sample through the apparent increase in the intensity of the emission at the same pumping power, due to this style, the con rming losses between the asymmetric etalon samples were overcome, the possibility of forming a con ned sharp peak with high intensity has attitudinized easier because the trapping of photon and forcing it to change its path has become more likely than in the symmetric etalon samples.
The emission spectrum of the thin symmetric etalon sample is higher than that of the thicker symmetric etalon samples (60009, 44038, and 35678 a.u.), respectively.
In addition, as shown in Figure 6(a), we can compare the laser emission of thin samples in the higher pumping environment with normal samples to con rm the higher laser emission intensity and ne lasing modes in asymmetric structures in comparison with symmetric ones. Likewise, modes overlapping in symmetric etalons separate into two main regions exactly in the same position of the normal etalon sample's lasing wavelength according to Fig. 6(b).