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Abstract
This research investigates the hybridized plasmonic response of silver film combined with dispersed silver (Ag) nanowires (NWs) to random laser emission. The mixture of Rhodamine B (RhB) dye and polyvinyl alcohol (PVA) matrix is taken as the gain medium for random lasing, and the silver combination provides feedback mechanisms for light trapping. Importantly, film roughness and the coupling between localized and extended (delocalized) surface plasmons play a vital role in RL performance evaluation. The laser threshold is strongly influenced by film thickness attributed to surface roughness. Furthermore, the variation in film thickness also supports the wavelength modulation of 9 nm (597 nm to 606 nm), which results from the reabsorption of RhB. Additionally, the intriguing capability of emission wavelength tuning under the variation of temperature facilitates exciting prospects for precise wavelength control in plasmonic devices.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In random laser, feedback is provided by multiple scattering, which can be either coherent or incoherent. In the former situation, scattering strength is poor, and the emission spectrum narrows progressively with linewidth lying in several nanometers. While in a later scenario, scattering strength is strong enough and provides sharp discrete peaks overlying the emission spectrum with a line width smaller than 1 nm. Importantly, plasmonic random lasers [1–4] suffer from drawbacks like absorption. However, these are still potential candidates owing to the substantially high scattering cross-section and immense light confinement features [5]. The threshold improvement and spectral modulations are key performance evaluation parameters in random laser fabrication, providing space for further exploration.
The conduction electrons in a metal surface oscillate collectively in a coherent way representing the surface plasmons capable of a couple with electromagnetic waves; as a result, the excitation of surface plasmons polariton (SPP) at the metal-dielectric planner interface occurs [6] . Due to continuous dispersion relations, these modes are supported by a broad frequency range. While, the interaction of light waves with metallic nanoparticles with dimensions much smaller than the incident wavelength leads to the excitation of localized surface plasmon resonance (LSPR). Scattering cross-section is strongly influenced by LSPR as it can be coupled directly with incident light [7]. Moreover, investigating metal nanoparticles (NPs) in optoelectronics and their interaction with light has attracted significant attention in recent decades. Promising outcomes in enhancing light-matter interactions and their enormous potential in plasmonic devices have been reported somewhere [8–12]. In this context, the fiber technology based on SPRs and LSPRs are available in the literature for the biosensor applications [13,14]. Notably, the complex interaction of LSPR with SPP, with spacing smaller than 100 nm, unveils rich, marvellous optical phenomena such as a wavelength shift with high sensitivity and pronounced consistency [15]. Moreover, the metallic film can strengthen the medium gain and scattering due to the strong electrical conductivity features [16,17].
In this study, we investigate the effect of thicknesses and respective roughness of Ag film combination with Ag NW on random lasing performance. We develop an optically active gain medium using Rhodamine B (RhB) dye solution with polyvinyl alcohol (PVA) as a matrix. [18]. Ag NWs are used as scatterers exhibiting light localization, and powerful light scattering characteristics [19]. The alternation of film thickness strongly influences the lasing threshold and emission wavelength, attributed to respective variations in surface roughness and electric field modes respectively. The laser performance can be enhanced using this simple and low-cost strategy.
2. Device fabrication and spectral characterization
The Ag NWs and RhB are mixed in an ethanol solution separately with concentrations of 5 mg/ml and 6 mg/ml, respectively. A separate solution of polyvinyl alcohol (PVA) (5% (w/v)) was prepared by dissolving it into deionized water by using magnetic stirrer for 12 hrs. Here, PVA acts as a polymer matrix to host the RhB molecules and provide structural stability to the device. The RhB ethanol solution, Ag NWs, and PVA solution were mixed with a certain ratio in an ultrasonic tank for 10 minutes to ensure uniform dispersion of the mixture (i.e., RhB:PVA:Ag NWs = 1:1:0.1). Here, RhB in the matrix provides medium gain, and Ag NWs facilitated with feedback mechanism through multiple light scattering, as shown by the schematic presented in Fig. 1(a). The inset top right Fig. indicates the light confinement between nanowire while bottom right corresponds to light confinement between nanowires and silver film. This hybrid confinement would be advantageous in improving the laser performance.
Fig. 1. (a) Schematic representation of random laser, light confinement between nanowires (as shown in top right inset), and light trapping between nanowire and thin film (as shown in bottom right inset). (b) The micrographs of the sample, viewed from the top, and the red circles represent the Ag nanowires. 5 µm is the scale bars.
The mixture solution of RhB-PVA-Ag NWs was spin-coated onto a simple glass substrate without silver film or glass substrate with silver film. The glass substrate with an area of 10 mm × 10 mm was chosen. Here, four different thicknesses of Ag film were prepared viz: t = 10 nm, t = 20 nm, t = 30 nm, and t = 50 nm by using the evaporation plate method. The Ag NWs have a diameter of 120 nm and a length of 45 µm as provided by the manufacturer. For mixture deposition during all the situations (i.e., either on a glass substrate or on silver film deposited glass substrate), spin-coated speed was set at 1500 rpm for 30s duration resulting in a uniform and controlled film thickness of ∼370 nm. The samples prepared on a glass substrate without an Ag film were used as a reference for comparison to assess the impact of introducing the Ag film on the random lasing properties. In simple words, five different types of samples were fabricated: a mixture deposited on a simple glass substrate and a mixture deposited on silver film with four thicknesses (t = 10 nm, t = 20 nm, t = 30 nm, and t = 50 nm) based glass substrate denoted as S1, S2, S3, S4 and S5 respectively. The samples were heated at 80°C on a hot plate for 30 minutes to promote the formation of cross-linked polymers and facilitate drying. Following this, the sample was effectively transformed into a plasmonic random laser device after cooling to room temperature.
Besides, to make the gain work efficiently, the device doubled Q-switched Nd: YAG laser (532 nm, 5-7 ns, 10 Hz) is chosen as the pump source. An emission spectrum was collected using a spectrometer with a resolution of 0.4 nm [20]. The SEM image given in Fig. 1(b) displays that the Ag NWs were dispersed randomly throughout the dye-doped medium with a scale bar of 5 µm. Indeed, the stirring method ensured the homogeneity of the sample.
3. Results and discussion
Figure 2(a) displays the extinction spectra of Ag nanowires dispersed over a glass substrate without and with a silver film of various thicknesses viz: t = 10 nm, t = 20 nm, t = 30 nm, and t = 50 nm. Over the entire visible spectral range, all extinction spectra have broadband characteristics without any prominent resonance peaks [21]. The photoluminescence spectra presented in Fig. 2(b) are observed to be centered around 592 nm, 598 nm, 602 nm, 606 nm, and 608 nm, corresponding to the respective five same samples chosen for considering extinction spectra (Fig. 2(a)). Indeed, the inclusion of Ag NWs enhances the optical processes of photoluminescence, as they contribute to emission within the visible spectrum [22].
Fig. 2. The extinction (a) and photoluminescence spectra (b) of Ag NWs mixed with RhB during various situations, including when deposited on glass without Ag film (black solid line) and glass with a silver film of variable thicknesses (i.e., red solid line (t = 10 nm), blue solid line (t = 20 nm), magenta solid line (t = 30 nm) and cyan solid line (t = 50 nm) respectively. Here magnifier of 5 (five) is used. The electric field distributions in a situation when (c) Ag nanowire on a glass substrate without silver film, (d) Ag nanowire on a glass substrate with silver film, (e) two Ag nanowires on a glass substrate with silver film. Here, length of Ag NW = 45 µm and diameter of Ag NW = 120 nm.
The electric field distributions near the Ag NWs were determined by using a commercially available COMSOL software tool with a finite element approach. The predicted results are given in Fig. 2(c-e). Here, for convenience, the excitation wavelength of 602 nm and a film thickness of only t = 20 nm were taken into account. A significant local field enhancement was observed near the edges of NWs, which is more pronounced when considering the situations like between the adjacent NWs and between NWs and film. It is inferred from the results that the inclusion of silver film dramatically enhanced the field confinement (i.e., as a result of reflection from the film). The length and diameter of the Ag NWs in the numerical simulation were chosen as 45 µm and 120 nm, respectively. Johnson and Christy's experimental data [23] for Ag's permittivity were used. In addition to plasmonic enhancement, it was found that the Ag NWs also extracted light from the waveguide through intense scattering [24].
Next, the emission spectra analysis of fabricated devices (S1, S2, S3, S4, and S5) is investigated, which revealed fascinating insights when subjected to varying pump power density levels. The results presented in Fig. 3(a) correspond to device S1 (where glass substrate without silver film acted as a reference for comparison), which showed a visually striking pattern characterized by distinct, sharp spikes with an impressively narrow full width at half maximum (FWHM) of 0.5 nm. These fascinating features pointed towards the occurrence of coherent random lasing due to a strong scattering cross-section. The inset given in Fig. 3(a) depicts the threshold pump fluence.
Fig. 3. The emission spectra for different pump intensities of the device based on (a) glass without silver film, glass with silver film of (b) t = 10 nm, (c) t = 20 nm, (d) t = 30 nm and (e) t = 50 nm. The insets show the corresponding lasing threshold denoted by Pth.
Next, Figs. 3(b-e) show the random laser action under the influence of various Ag film thicknesses viz: t = 10 nm, t = 20 nm, t = 30 nm, and t = 50 nm recorded to analyze the impact of these thicknesses on the random lasing characteristics. The symbole Pth given in ligends of Fig. 3 represents the threshold values which is depicted by arrows in Fig. 4. Importantly, it is worthnoting that a significant portion of scattered light is redirected back at the interface between the gain medium and air. This scattered light undergoes further scattering due to Ag NWs and their interaction with the Ag film, facilitating the coupling of plasmonic modes in such a hybrid structure [25]. Further, Ag NWs presence can modify the film's local electric field distribution. The elongated structure of Ag NWs assists in enhancing the electric field at the particular points along their length. The Plasmon resonance of the silver NWs along with metal film can be further enhanced by providing more effective light absorption and scattering. Thus, combining multiple scattering by Ag nanowires (NWs) and the waveguide confinement creates efficient gain channels, which in turn further enhances the overall optical performance [26,27]. Indeed, such hybridization leads to a higher optical confinement, lower threshold for lasing action, and improved overall laser performance. Additionally, the tunable nature of proposed hybrid plasmonic system allows for flexibility in tailoring the random laser emission characteristics, making it suitable for a wide range of applications, including sensing, imaging, and integrated photonic devices.
Fig. 4. The emission spectra and full width at half maximum (FWHM) versus pump power (a) glass without silver film, glass with silver film of thickness (b) t = 10 nm, (c) t = 20 nm, (d) t = 30 nm and (e) t = 50 nm. The threshold intensity is indicated individually in each case.
Again, from Fig. 3(b), it is investigated that the full width at half maximum (FWHM) of photoluminescence is approximately 38 nm. Increasing the pump power achieves the superimposition of various distinct spikes in the emission spectra. When the threshold is reached, the FWHM of the spikes is improved dramatically. Similar observations have been made for the emission spectra of S3, S4, and S5 devices, as shown in Figs. 3(c-e) respectively. Additionally, it is noticed that emission wavelengths for the devices S1, S2, S3, S4 and S5 lie around ∼597 nm, ∼599 nm, ∼601 nm, ∼603 nm and ∼605 nm respectively. This modulation wavelength shift is caused by variation in the effective refractive index. Silver film's presence can influence the device's overall scattering efficiency. Thinner films generally have a higher surface-to-volume ratio, which increases the available surface for interaction with the gain material and the silver nanowires. Such interaction enhancement can promote significant energy transfer and amplification of the incident light, resulting in a lower threshold.
Figure 4(a-e) represents the lasing thresholds associated with varying thicknesses of the Ag film. Like the minimum threshold in case of glass without silver film was measured to be 225 µJ/cm2. While, for the situation of glass with Ag film with various thicknesses as, t = 10 nm, t = 20 nm, t = 30 nm and t = 50 nm, the thresholds measured were 130 µJ/cm2, 91µJ/cm2, 63 µJ/cm2 and 125 µJ/cm2 respectively. In this context, it is found that as the Ag film thickness increases, the lasing threshold experiences an initial notable decrease, followed by a subsequent increase. The lowest lasing threshold was measured at value of 63 µJ/cm2 for device S4 (Ag film thickness of t = 30 nm). This value is approximately 3.6 times lower than the lasing threshold observed for device S1. These findings provide valuable insights into the dependence of lasing performance on the thickness of the Ag film, highlighting the importance of optimizing this parameter for achieving efficient laser operation.
Next,we analyzed the performance of random lasing based on the variation effects of silver film thickness while keeping the pump power density fixed at 234 µJ/cm2, as presented in Fig. 5(a). It is seen that with increase in silver film thickness, a clear red-shift in the central wavelength was observed. Here, the central wavelength is changed from 597 nm to 606 nm as the thickness increases with a total shift of 9 nm. The red shift in the central wavelength can be attributed to reabsorption [28–31], leading to respective spectral modulation.
Fig. 5. (a) The emission spectra at fixed pump power intensity of 234 µJ/cm2 for random laser with various film thicknesses. (b) The threshold statistics for glass without silver film and glass with silver film, lower statistics were obtained for glass with silver film of thickness 30 nm (c) Atomic Force Microscopy (AFM) images depicting Ag film of various thicknesses of (i) t = 10 nm, (ii) t = 20 nm, (iii) t = 30 nm and (iv) t = 50 nm.
Next, the statistical lasing threshold fluence data was determined for all prescribed devices and is presented in Fig. 5(b). The thicknesses of the Ag films are given as 0 nm, 10 nm, 20 nm, 30 nm, and 50 nm. For statistical measurements, we fabricated total of fifty samples including ten samples of each device category i.e., the glass substrates without Ag film (t = 0 nm) and glass substrates with Ag films of four different thicknesses (i.e., t = 10 nm, t = 20 nm, t = 30 nm and t = 50 nm). For a comprehensive analysis of lasing threshold measurements, we selected a minimum of two distinct points from each fabricated sample, representing different emission regions. This process was repeated for all fifty samples including ten samples from each device category. The deviations in threshold corresponding to each device are represented by bar graph of Fig. 5(b). The average thresholds were calculated by summing of all threshold values obtained in each individual category and divided with total number of values. For example, for device S1, total ten sample were prepared and total twenty lasing points were determined, then thresholds of all twenty points were summed up which is divided by number twenty. Same procedure was repeated for the rest of devices, where random lasing with respective thresholds were noted. In this context, the average thresholds of the devices S1, S2, S3, S4 and S5 were calculated as 237 µJ/cm2, 133 µJ/cm2, 94 µJ/cm2, 61 µJ/cm2, and 131 µJ/cm2, respectively. These values represent the average minimum energy (fluence) required to achieve lasing in the nanowires on each corresponding Ag film. Generally, Plasmonic enhancement refers to the phenomenon where the interaction of light with metallic nanoparticles or thin films, such as Ag in this case, leads to enhanced electromagnetic field confinement and localization. This localized electromagnetic field can significantly increase the interaction between the light and the active medium (Rhb-doped Ag NWs), thereby facilitating lasing at lower energy thresholds. In the context of the given experiment, the reduced variation in thresholds observed with the 30 nm Ag thin film suggests a more consistent and predictable plasmonic enhancement effect than the other films with different thicknesses. This enhanced plasmonic effect results in a more vital interaction between the Ag film and the Rhb-doped Ag NWs, leading to a lower lasing threshold.
Next, in view of investigating the physics behind the threshold variation with film thickness, Atomic Force Microscopy (AFM) measurement for silver film-based glass substrate with a surface area of 1 × 1µm2 (i.e.,viewed from the top) taken into account and respective images are given in Fig. 5(c). The surface roughnesses corresponding to 4 (four) film thicknesses are depicted adequately in subset Figs. viz: (i) t = 10 nm, (ii) t = 20 nm, (iii) t = 30 nm, and (iv) t = 50 nm. AFM is a valuable technique utilized for capturing detailed 3- dimensional images and evaluating the surface properties of various materials. This technique provided significant insights into the roughness characteristics of these films. Among the analyzed silver films, the 30 nm film exhibited the highest level of surface irregularities, suggesting a more significant potential for light scattering. Its roughness range of −27.7 to 32.8 indicated a broader distribution of surface variations compared to the other film thicknesses (t = 10 nm, t = 20 nm, and t = 50 nm). The positive values within this range indicated the presence of rougher regions on the film's surface leading to enhance the scattering which can disrupt the propagation of light within the laser cavity, affecting its optical properties, such as mode confinement and coherence [32]. In our situation, the 30 nm silver film is anticipated to exhibit higher light scattering as compared to other films. Thereby, the surface roughness can influence the laser performance by altering the scattering and propagation characteristics of light within the laser medium.
In view of these, we believe that the spectral response of RL strongly depends on scattering response of the materials involved. The scattering response can be illustrated by two closely related length scales including the scattering mean free path denoted by Ls and transport mean free path denoted by Lt. The lateral can be defined as the average light path between two scattering events and inversely related to scattering cross section while lateral can be defined as average distance of wave travels before the randomization of its propagation direction which can be approximated using Mie theory as ≈ 2Ls [33]. Thereby scattering mean free path is a key factor in describing the spectral characteristic of random laser. If scattering mean free path is smaller, then stronger will the scattering strength for RLs. More details can also be found from the Ref. [34]. Secondly, reason for thickness effect on spectral features of RL may be because of the variation in the effective refractive index i.e., 2neff Λ = m λe, where neff represents the effective refractive index, Λ denotes the grating period, m is propagating mode and λe is laser wavelength [35]. Through rough approximation, we believe that in a smooth surface, neff will be higher while in rough surfaces, it will be lower because of the availability of free-space paths. Consequently, it may facilitate the spectral characteristics (i.e., including emission wavelength tuning and lowering the threshold) of RL as well.
Another interesting phenomenon for the present system is that the emission wavelength for the random laser can be tuned through changing temperature. In Fig. 6(a), a phenomenon of emission wavelength tuning of random laser is observed under the variation of temperature. For simplicity, only glass with Ag film thickness of 30 nm is used. The increase in temperature from 20°C to 60°C leads to a gradual shift in emission peaks toward higher wavelength. This intriguing feature can be attributed to the material’s characteristics, as temperature rises, the lattice structure of the Ag NWs expands which results in to alterations in interatomic distances. These changes influence the resonance conditions for photons emission. Additionally, the bandgap energy in silver NWs decreases with increase in temperature, thereby contributing to the wavelength shift. Quantum confinement effects inherent in nanomaterials may also contribute to this observed phenomenon [36]. Our findings have significant implications for applications demanding precise wavelength control, such as plasmonic devices and optical filters, emphasizing the importance of accounting for temperature effects in material design and applications.
Fig. 6. Wavelength tunability of random laser as a function of temperature. (a) Illustrates the wavelength shift towards longer value in emission intensity with a variation of temperature ranging from 20°C to 60°C. (b) Shows the increasing trend in emission peak wavelengths with rise in temperature.
Figure 6(b) complements our exploration by illustrating the increasing trend in the emission peak points as a function of temperature. This graph provides a visual representation of the observed phenomenon, highlighting the gradual shift towards longer wavelengths with rising temperatures. Each data point on the graph represents the peak emission wavelength corresponding to a specific temperature point. The upward trajectory of these data points clearly demonstrates the systematic nature of the wavelength shift with increasing temperature. This graphical representation reinforces the empirical evidence of temperature-induced wavelength tuning observed in our study, providing a compelling visual insight into this fascinating behavior.
4. Conclusion
This study successfully realizes a plasmonic random laser device by synergistically incorporating Ag NWs and Ag film with varying thicknesses. Notably, thinner Ag film with higher surface-to-volume ratios exhibit improved interaction with the gain material and Ag NWs, resulting in a lower lasing threshold.The investigations reveal that the Ag film with a thickness of 30 nm offers the largest emission enhancement among the tested thicknesses, indicating its suitability for achieving highly efficient random lasing. Moreover, the combined utilization of Ag NWs and Ag film showed a remarkable mutual effect that can be harnessed for achieving enhanced lasing performance and tailoring emission properties. The observed red shift in the lasing emission is associated with variation in silver film thickness. Additionally, for the situation of glass substrate with Ag film of thickness, t = 30 nm, the temperature-dependent spectral characteristic of RL unveiled a remarkable redshift from 601 to 605 nm. This finding provides exciting opportunities for precise manipulation of the emission characteristics of random lasers.
Funding
Beijing Municipal Natural Science Foundation (Z180015).
Disclosures
The authors declare that there is no conflict of interest.
Authors contribution. A. G. fabricated samples, conducted experiment, writing-original draft preparation, Y. J. H. data collection, X. Z. performed numerical simulations, k. S. and H. A. performed formal analysis, N. I. involved in conceptualization, data analysis, review and editing of draft, X.S. writing-review and editing and T. Z. project supervision, review and editing of draft, and funding acquisition. All authors have reviewed and agreed to the final version of the manuscript.
Data availability
Data may be obtained from the authors upon reasonable request.
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