Impact of Hybrid Plasmonic and Temperature in Random Laser Tuning

This research explores the interaction between silver films and dispersed silver nanowires (Ag NWs) in the context of generating random laser emission. To achieve random lasing, we use a mixture of Rhoda mine B (RhB) dye and a polyvinyl alcohol (PVA) matrix as the gain medium. The combination of silver components plays a crucial role in trapping and controlling light. The surface characteristics of the film, including its roughness and the interplay between localized and extended surface plasmons significantly affect the performance of the random laser (RL). The laser’s threshold is closely linked to the thickness of the film, which is influenced by its surface roughness. Additionally, variations in film thickness lead to wavelength modulation, ranging from 597 nm to 606 nm, primarily due to the reabsorption of RhB. Moreover, this research demonstrates the intriguing capability to tune emission wavelengths in response to temperature changes, promising precise wavelength control for plasmonic devices and potential applications.


Introduction
In the context of random lasers, feedback is a central element, and it arises from the complex interactions of light scattering, which can be coherent (resulting in gradual spectral narrowing) or incoherent (yielding sharp, distinct emission peaks).Plasmonic random lasers hold promise due to their substantial scattering cross-section and effective light confinement [1][2][3].Evaluation center relies on two key parameters including the threshold enhancement and spectral modulation, which offer avenues for further exploration [4].Surface plasmons that occur on metal surfaces give rise to a phenomenon known as surface plasmon polaritons (SPPs) at the interface between the metal and the adjacent dielectric material.These SPPs have broad dispersion relations and prove versatile in various applications.Interaction of light with small metallic nanoparticles results in localized surface plasmon resonance (LSPR), which shapes the scattering cross-section and allows control of light at the nanoscale [5].Metal nanoparticles have shown potential in enhancing light-matter interactions and plasmonic devices.Fiber-based technologies utilizing surface plasmon resonances (SPRs) and LSPRs find applications, particularly in biosensors [6].It is worth noting that the complex interplay between LSPR and SPP, occurring within nanoscale distances less than 100 nm, unveils a rich tapestry of optical phenomena.These advancements result in high-sensitivity wavelength shifts, promising new opportunities for advanced optical sensing and manipulation [7].The inclusion of a metallic film enhances gain and scattering, enriching the field of plasmonics.
In this study, we investigate the influence of varying thickness and surface roughness of an Ag film and Ag nanowires (NWs) on random laser performance.We employ a Rhodamine B (RhB) dye within a polyvinyl alcohol (PVA) matrix as the gain medium [8].Ag NWs, acting as effective scatterers with robust light-scattering properties, play a pivotal role in our research.Changes in film thickness significantly impact the random laser's threshold and emission wavelength due to variations in surface roughness and electric field modes.In addition, our study delves into the fascinating phenomenon of temperature-induced wavelength tuning within the random laser system.Our cost-effective approach aims to enhance laser performance, creating new practical possibilities.

Experiment details
In the device fabrication process, Ag nanowires (Ag NWs) and Rhodamine B (RhB) were individually mixed with ethanol solutions at concentrations of 5 mg/ml and 6 mg/ml, respectively.Simultaneously, a 5% (w/v) polyvinyl alcohol (PVA) solution was prepared by dissolving it in deionized water with constant stirring for 12 hours.To ensure even distribution, the RhB ethanol solution, Ag NWs, and the PVA solution were ultrasonically blended for 10 minutes in a certain ratio.In this composition, RhB in the matrix serves as the gain medium, while Ag NWs facilitate a feedback mechanism through light scattering.The mixture of RhB-PVA-Ag NWs was evenly coated onto two types of substrates: simple glass and glass with Ag film of varying thicknesses (10 nm, 20 nm, 30 nm, and 50 nm).Uniform film deposition (~370 nm thickness) was achieved by spin-coating on 10 mm×10 mm glass substrates at the speed of 1500 rpm for 30 seconds.The Ag NWs have a diameter of 120 nm and a length of 45 μm.The silver film with four thicknesses values were chosen, specifically S1 = glass with t = 0 nm, S2 = glass with t = 10, S3 = glass with t = 20, S4= glass with t = 30 and S5 = glass with t = 50, where symbol t denotes the Ag film thickness.All samples were heated at 80°C for 30 minutes and transformed into plasmonic random laser devices after cooling.For efficient gain, a Q-switched Nd: YAG laser (532 nm, 5-7 ns, 10 Hz) was used as the pump source, and emission spectra were recorded using a spectrometer with a 0.4 nm resolution [9].

Spectral characterization of random lasers
In Fig. 2(a), we can observe the extinction spectra of Ag nanowires distributed on a glass substrate, with and without a silver film.Notably, across the entire visible spectral range, all the extinction spectra exhibit broadband characteristics without distinct resonance peaks [10].In Fig. 2(b), the photoluminescence spectra are presented, where solid lines represent Rhodamine B (RhB) mixed with Ag NWs and dotted lines denote RhB without Ag NWs.The symbol (S11，S21, S3，S4，S51) with Ag NWs and (S12, S22 ， S32 ， S42 ， S52) without Ag NWs correspond to glass with Ag films thicknesses, encapsulates the influence of different silver film thicknesses on the photoluminescence behaviour.This graph enables visual comparison of the photoluminescence enhancement with Ag NWs under various silver film thicknesses, providing insights into the impact of Ag NWs and film thickness on RhB's light emission characteristics.Importantly, the incorporation of Ag NWs enhances the photoluminescence processes, actively contributing to emissions within the visible spectrum [11].The electric field distributions around silver nanowires (Ag NWs) were determined using COMSOL software, employing a finite element approach.The results are shown in Fig. 2(c-e), using a 602 nm excitation wavelength and a 20 nm film thickness.Notably, a significant local field enhancement was observed near the NW edges, especially in regions between adjacent NWs and NWs and the film.This enhancement was primarily due to the presence of a silver film, which dramatically increased field confinement through reflection at the film interface.The Ag NWs in the simulation were 45 µm long with a diameter of 120 nm, and Johnson and Christy's experimental data for silver's permittivity were utilized [12].Additionally, it was found that Ag NWs not only enhanced plasmonics but also facilitated light extraction from the waveguide through intense scattering [13].
The emission spectra analysis of devices S1 to S5 revealed intriguing insights when exposed to varying pump power densities in Fig. 3(a-e).In Fig. 3a, which serves as a reference and features a glass substrate without a silver film (t= 0 nm), a visually striking pattern is observed.This pattern showcases distinct, sharp spikes, and their full width at half maximum (FWHM) is remarkably narrow, measuring just 0.5 nm.These features pointed towards coherent random lasing due to a strong scattering cross-section.While, the results given in Fig. 3(b-e) explored the influence of various silver film thicknesses (t = 10 nm, 20 nm, 30 nm, and 50 nm) on random lasing characteristics.A notable portion of scattered light was redirected at the gain medium-air interface and underwent further scattering due to Ag NWs, enhancing plasmonic coupling [14].The elongated Ag NWs improved electric field distribution, creating efficient gain channels, leading to higher optical confinement, lower lasing threshold, and improved laser performance [15].Increasing pump power resulted in the superposition of distinct spikes in the emission spectra, with a dramatic improvement in FWHM upon reaching the threshold.Similar observations were made for S2, S3, S4, and S5, with emission wavelengths of nearly 597 nm, 599 nm, 601 nm, 603 nm, and 605 nm, respectively.The shifts in emission wavelength were a consequence of alterations in the effective refractive index.In terms of lasing thresholds, the inset in Figs.3(a-e) displays lasing thresholds for different Ag film thicknesses.The minimum threshold for glass without a silver film is 0.22 mJ/cm².However, with varying Ag film thicknesses like t = 10 nm, t = 20 nm, t = 30 nm, and t = 50 nm, thresholds varied as, 0.13 mJ/cm², 0.09 mJ/cm², 0.06 mJ/cm², and 0.12 mJ/cm², respectively.
The presence of a silver film significantly influenced device scattering efficiency.Thinner films with higher surface-to-volume ratios allowed for greater interaction with gain material and silver nanowires, promoting energy transfer and lowering the lasing threshold.Notably, device S4 with a t = 30 nm Ag film achieved the lowest threshold at 0.06 mJ/cm 2 , approximately 3.6 times lower than S1 (t = 0 nm).These findings underscore the critical role of optimizing Ag film thickness for efficient laser operation, offering great potential for applications in sensing, imaging, and integrated photonic devices.
Subsequently, we evaluated random lasing performance while maintaining a constant pump power density of 0.25 mJ/cm², as shown in Fig. 3(f).The results demonstrate a noticeable red shift in the central wavelength as the silver film thickness increases.Specifically, the central wavelength shifts from 597nm to 606 nm, marking a total shift of 9 nm.This shift in the central wavelength is attributed to reabsorption, resulting in distinct spectral modulation [16].In the inset Fig, we highlighted the increasing trend in peak wavelengths with varying thicknesses which provides a clear illustration of how thickness impacts the material's peak properties.
We used Atomic Force Microscopy (AFM) to analyse 1 × 1 µm² surface areas of silver film-based glass substrates, viewed from the top, to investigate the relationship between film thickness and threshold variation as shown in Fig. 4.Among the examined silver film thicknesses including (10 nm, 20 nm, 30 nm, and 50 nm), the 30 nm film displayed the highest surface irregularities, with a range from -27.7 to 32.8, indicating enhanced light scattering potential.These surface irregularities, particularly in the 30 nm silver film, are expected to boost light scattering, affecting light propagation within the laser cavity and influencing optical properties such as mode confinement and coherence [17].Next, the statistical lasing threshold fluence data was methodically analyzed for a range of devices, as depicted in Figure 4(b).These devices included glass substrates with varying thicknesses of Ag films: 0 nm (no film), 10 nm, 20 nm, 30 nm, and 50 nm.A total of fifty samples were created for this statistical evaluation, comprising ten samples for each category of device.
For each sample, a minimum of two unique emission points were selected for thorough lasing threshold analysis.This approach was consistently applied across all fifty samples, encompassing each device category.The fluctuation in the lasing threshold for each device type was illustrated in the bar graph in Figure 4(b).To compute the average threshold for each device, the thresholds recorded at all points for a given category were summed and then divided by the total number of points.For instance, in the case of device S1, with ten samples and twenty lasing points, the sum of the thresholds for these twenty points was divided by twenty.Following this method, the average thresholds for devices S1, S2, S3, S4, and S5 were determined to be 0.22 mJ/cm², 0.13 mJ/cm², 0.09 mJ/cm², 0.06 mJ/cm², and 0.12 mJ/cm², respectively.These figures represent the average minimal energy (fluence) needed to initiate lasing in the nanowires situated on each specific Ag film.Plasmonic enhancement, a phenomenon where the interaction of light with metallic nanostructures (such as thin Ag films or nanoparticles) leads to intensified electromagnetic field localization and confinement, plays a crucial role in this context.This intensification can markedly boost the interaction between light and the active lasing medium, in this case, RhB-doped Ag nanowires (NWs), facilitating lasing at reduced energy thresholds.Specifically, the lesser variability in thresholds observed with the 30 nm Ag film indicates a more uniform and predictable plasmonic enhancement effect compared to films of other thicknesses.This implies a stronger interaction between the 30 nm Ag film and the RhB-doped Ag NWs, resulting in a decreased lasing threshold, underscoring the significant role of film thickness in plasmonic enhancement and lasing efficiency.
In the view of these, we believe that the spectral properties of random lasers (RLs) are fundamentally influenced by the scattering characteristics of the materials used.The scattering behaviour can be described through two critical dimensions: the scattering mean free path (Ls) and the transport mean free path (Lt).Ls is the average distance light travels between successive scattering events, which inversely correlates with the scattering cross-section.Conversely, Lt is defined as the average distance over which a wave travels before its direction of propagation becomes randomized.This can be estimated using Mie theory to be approximately twice Ls, as noted in reference [18].Therefore, the scattering mean free path plays a pivotal role in determining the spectral features of a random laser, with shorter Ls values indicating stronger scattering strength for RLs.Further insights on this are available in reference [19].Additionally, the thickness of the material can significantly impact the spectral characteristics of RLs.This effect can be attributed to changes in the effective refractive index, denoted as neff.The relationship between neff, the grating period Λ, the propagating mode m, and the laser wavelength λe, can be expressed as 2neff Λ = m λe, as elaborated in reference [20].It is hypothesized that on smoother surfaces, neff tends to be higher, whereas on rougher surfaces, it is lower due to the presence of more free-space paths.This variation in neff can potentially enhance the spectral properties of RLs, including the tuning of emission wavelength and reducing the lasing threshold.
In Fig. 5, we observe temperature-induced wavelength tuning in the random laser system.Focusing on glass with a 30 nm thick Ag film and a constant pump power of 0.23 mJ/cm², we note a gradual shift in emission peaks towards longer wavelengths as the temperature increases from 20°C to 60°C.This phenomenon is attributed to factors like the expansion of the lattice structure of silver nanowires (Ag NWs) with temperature, leading to alterations in interatomic distances and affecting photon emission conditions [21].Additionally, the decrease in the bandgap energy of silver NWs and quantum confinement effects contribute to this wavelength shift.The inset in Fig. 5 systematically illustrates the wavelength shift with rising temperatures, each data point corresponding to a specific temperature, reinforcing our empirical evidence of temperature-induced wavelength tuning, and offering a compelling visual insight into this intriguing behaviour.

Conclusion
This study successfully developed a plasmonic random laser device by combining Ag NWs and Ag film with varying thicknesses.Thinner Ag film, with a higher surface-to-volume ratio, showed enhanced interactions with the gain material and Ag NWs, resulting in a lower lasing threshold.The 30 nm Ag film was the most effective, enhancing emissions for highly efficient random lasing.The collaborative use of Ag NWs and Ag film improved lasing performance and allowed emission property customization.A spectral shift toward longer wavelengths was linked to changes in silver film thickness.Additionally, with a 30 nm Ag film on a glass substrate, our study revealed temperature-dependent spectral changes, presenting exciting opportunities for precise control over random laser emissions.

Fig. 1 .
Fig. 1.Schematic of RhB and PVA with Ag NWs.Micrographs display top view with red circles indicating Ag nanowires.Inset zooms on Ag NW.

Fig. 1 (
a) shows the concept of a random laser, where the gain is spin-coated on the Ag film, showcasing the confinement of light within nanowires, as depicted in the upper right inset.It also highlights the phenomenon of light trapping between a nanowire and a thin film, as visualized in the lower right inset.SEM images in Fig. 1b confirmed the random dispersion of Ag NWs in the dyedoped medium with a scale bar of 5 μm, ensuring sample homogeneity, where inset figure indicate the zoom view of silver NWs.

Fig. 2 .
Fig. 2. Presents spectral analyses of Ag NWs mixed with RhB under varied Ag film thicknesses, photolumnescence comparisons of RhB with and without Ag NWs across different Ag film situations, and electric field distributions for single and dual Ag NWs on glass substrates with Ag films.

Fig. 5 .
Fig. 5. Shows random laser wavelength tunability from 20°C to 60°C, with the inset indicating an increasing trend in emission peak wavelengths with temperature.