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Abstract
As a novel optical device, the plasmonic random laser has unique working principle and emission characteristics. However, the simultaneous enhancement of absorption and emission by plasmons is still a problem. In this paper, we propose a broad-band-enhanced plasmonic random laser. Two-dimensional silver (Ag) nanostar arrays were prepared using a bottom-up method with the assistance of self-assembled nanosphere templates. The plasmon resonance of Ag nanostars contributes to the pump light absorption and photoluminescence (PL) of RhB. Coherent random lasing was achieved in RhB@PVA film based on localized surface plasmon resonance (SPR) dual enhancement and scattering feedback of Ag nanostars. Ag nanostars prepared with different nanosphere diameters affect the laser emission wavelength. In addition, the random laser device achieves wavelength tunability on a flexible substrate under mechanical external force.
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1. Introduction
Random laser is a type of micro-nano laser source based on disordered induced multiple scattering effects and optical feedback, which is different from traditional lasers with specific cavities [1,2]. Random lasers can provide low spatial coherence due to radiation of several uncorrelated lasing modes [3,4]. The unique feedback mechanism endows random lasers with advantages such as small volume, flexible shape, and omnidirectional emission, making them widely applicable in fields such as information security, speckle free imaging, display, and sensing [255–9]. Multiple scattering increases the path length or dwell time of light in the active medium, and thus enhances light amplification by stimulated emission. The multiple scattering of random lasers can be divided into incoherent and coherent [10,11]. The scattering intensity of the former is relatively weak, and the emission spectrum gradually Narrows with the increase of pump energy, and the linewidth is about a few nanometers. The latter scattering intensity is strong enough, and the emission spectrum is characterized by sharp discrete peaks with linewidth less than 1 nm.
Random lasers do not require well-defined optics, and thus has considerably diversified the available types of laser gain materials including quantum dots [12,13], polymer films [14,15], dye solutions [16,17] and turbid ceramics [18]. In addition, the optical feedback in a random laser strongly depends on the scattering properties of the scattering medium. Up to now, many types of materials are used as scatterers in random lasers, such as metal nanostructures [19,20], dielectric nanoparticles [21,22], perovskite nanocrystals [23,24], biological tissues [25,26], nematic liquid crystals [27,28], etc. Among them, metal materials, especially metal nanostructures, have been extensively studied due to their surface plasmon resonance (SPR) properties [29,30]. Surface plasmons have strong and robust constraints on the light field, which is beneficial for light-matter interactions. The main preparation method of metal plasmon nanoparticles is chemical synthesis [19,31]. In most cases, however, the scattering medium is not controllable, resulting in aggregation of scattering particles that leads to unpredictable effective scatterer size and distribution. Most of studies on plasmonic random lasers are still focused on scattering amplification by disorder metal nanoparticles. The repeatability and unpredictability of laser emission remain key barriers to practical applications. Therefore, the study of metal scattering medium with two-dimensional distribution is of great significance. In recent years, researchers have prepared two-dimensional plasmonic nanostructures through various novel methods [32,33,34]. The focused ion beam etching method was used to etch nanoholes on the metal film and achieve localized SPR at the edge of the holes [35]. Large-area plasmonic lattices with high Q SLRs can be effectively prepared by combining AAO membranes with deposition technology [36]. A random laser with multiple scattering enhanced by disorder–order hybrid silver (Ag) nanorod arrays was prepared by electrodeposition [37]. Furthermore, the study of random laser with tunable wavelength has important theoretical significance and application value for the fields of spectroscopy, biology, information processing and communication. The emission wavelengths can be tuned by changing the nanostructure size [38], pump shaping [15], varying the humidity [39] and temperature [29,40]. By integrating random lasers on a flexible substrate, continuous tuning of laser wavelengths can be achieved while maintaining their laser performance [31,41].
In this paper, we propose a tunable plasmonic random laser with two-dimensional distributed scattering particles. Ag nanostars prepared by electron beam evaporation with the assistance of self-assembled microsphere arrays. The two-dimensional distribution of Ag nanostars prepared by this method is more conducive to the realization of a closed scattering loop. A flexible gain film was prepared by mixing rhodamine B (RhB) with polyvinyl alcohol (PVA). The laser emission properties of RhB@PVA polymer films combined with Ag nanostars have been investigated. Coherent random laser emission was realized with the assistance of nanospheres of different diameters. And experiments show that the size of Ag nanostars affects random laser wavelengths. A flexible device based on polyethylene terephthalate (PET) was fabricated, and the recoverable tuning of laser wavelength was realized by mechanical bending.
2. Design and fabrication
The preparation of Ag nanostars is based on a close-packed nanosphere structure, as shown in Fig. 1(a). Before preparing Ag nanostars, the close-packed monolayer nanospheres were prepared by the self-assembly method as a template. Polystyrene (PS) nanospheres after ultrasonic dispersion treatment are uniformly dispersed in deionized water with a mass fraction of 5%. The nanosphere dispersion was coated on the surface of the hydrophilic silicon wafer by spinning coating. Through experiments, it was found that after spin coating on the substrate at a speed of 1700 r/min for 60s, the nanospheres covered the silicon wafer in a relatively dense monolayer state. Slowly immerse the silicon wafer at a 30 ° angle into deionized water, and due to the effect of liquid surface tension, the nanospheres detach from the silicon wafer and float on the liquid surface. Drop a 5% mass fraction of sodium dodecyl sulfate (SDS) solution into deionized water to reduce liquid surface tension and make the microspheres more tightly aggregate. At this time, a close-packed monolayer of nanospheres was formed at the liquid surface. The microsphere film was then transferred to the plasma treated glass and heated (80°C, 5 min) to dry the water.
Fig. 1. Preparation and characterization of Ag nanostar arrays. (a) Preparation process diagram of Ag nanostars. (b) The scanning electron microscopy image of the close-packed nanospheres. (c) The scanning electron microscopy image of the Ag nanostar arrays.
The nanospheres were firmly attached to the glass surface to form a template. As shown in Fig. 1(b), the nanospheres present a monolayer close-packed structure, and the diameter of the nanosphere is 450 nm. A 100 nm Ag film was deposited on the surface of the microsphere template by electron beam evaporation. Noble metals such as Au, Ag, and Cu exhibit strong plasmon resonance effects [42]. At micro- or nano-scale, intense electromagnetic field enhancement (104-108) can be generated. Research has shown that Ag nanostructures have higher enhancement compared to Al [36]. The surface plasmon resonance absorption peaks of Au and Ag nanomaterials usually fall within the visible to near-infrared range of the electromagnetic spectrum. However, in comparison, Ag is cost-effective, and can be easily grown using coating techniques. Therefore, Ag was chosen in this study to prepare the nanostructures. Finally, the sample was subjected to ultrasonic treatment in an ethanol environment. Under the action of ultrasonic vibration, microspheres gradually detach from the glass surface. Only the Ag filled in the microsphere gap was left on the glass base, and the top view is a curved triangle, as shown in Fig. 1(c). Twenty Ag nanostars were randomly selected to calculate the average particle size, as shown in Fig. S1. A single Ag particle is 120 nm long and 100 nm high. The Ag nanostars prepared by this method have uniform size and two-dimensional distribution, which effectively avoids particle accumulation. In the case of in-plane size limitations, the probability of coherent random laser generation can be improved.
The structure of the designed random laser device is shown in Fig. 2(a). Monolayer Ag nanostars are overlaid on a glass substrate as scattered particles to provide coherent feedback and can generate localized SPR enhancement. Red fluorescent dye rhodamine B (RhB, Tianjin Fuchen Chemical Reagents Factory, Tianjin, China) was used as a gain material and mixed with polymer polyvinyl alcohol (PVA). In order to disperse RhB uniformly in solution, RhB and PVA were first dissolved in deionized water at the concentrations of 3 mg/ml and 0.1 g/ml, respectively. The PVA solution is heated at 80°C for 12 hours, and then mixed with the RhB solution at a 1:1 mass ratio. Finally, the RhB solution was spun on the surface of the Ag particles at 1400 r/min and heated at 60 °C for 5 min to cure. The addition of PVA improves the film-forming properties of RhB molecule and contributes to the mechanical stability of the device. The refractive index of the RhB@PVA film at 598 nm is 1.60 (Supplement 1, Fig. S2). The red lines in the Fig. 2(a) represent the scattering of light between Ag nanostars, and the scattering path forms a closed loop, which finally realizes the emission of coherent random laser.
Fig. 2. (a) Design of a random laser device. The red line represents the light scattering path. (b) Normalized absorbance (red solid line), PL (red dashed line) spectra of RhB@PVA film, and the extinction spectra (black dashed line) of Ag nanostar arrays. The green vertical line indicates the pump light wavelength (532 nm), and the red vertical line indicates the laser wavelength (598 nm).
The absorption spectrum and photoluminescence (PL) spectrum of RhB@PVA film are shown in Fig. 2(b). When the diameter of the nanosphere was 450 nm, the extinction peak of the silver nanostar array perfectly covers the absorption spectrum and the PL spectrum of RhB@PVA. This indicates that the scattering and plasmon resonance of Ag nanostars array not only enhance the emission of RhB@PVA but also enhance its absorption of the pump light. The scattering spectra of individual Ag nanostar and Ag nanostar arrays were calculated using the Finite difference time domain (FDTD) method, as shown in Fig. 3. Compared with a single Ag nanostar, the scattering spectrum of the nanostar array was significantly broadened. Therefore, Ag nanostars plays a double enhancement role.
Fig. 3. Calculated scattering spectra of individual silver nanostars and arrays of silver nanostars.
3. Results and discussion
The electric field intensity distribution of Ag nanostars was numerically simulated by COMSOL multi-physics 5.4. The cross-sectional electric field intensity distribution of individual Ag nanostar is shown in Fig. 4(a). The locally enhanced electric field distribution at the three tips of the Ag nanostar indicates the existence of the localized SPR mode at 598 nm. According to Fermi’s golden rule [37,43], the lifetime of an excited state is heavily influenced by the electromagnetic environment, specifically the local density of states. And the rate of spontaneous emission is directly proportional to the local density of states. The enhancement of spontaneous emission can be quantified by the Purcell factor, which is proportional to the ratio of the quality factor (Q factor) to the mode volume (V) [37]. Plasmonic nanocavities usually have an ultra-small mode volume and a high Q factor, which greatly increases the Purcell factor. Under the same periodic boundary conditions, as shown in Fig. 4(b), the localized SPR enhancement of Ag nanostar arrays is significantly better than that of individual Ag nanostar. Therefore, the Ag nanostar array is more beneficial to increase the fluorescence intensity and reduce the laser threshold. In addition, in Supplement 1, Fig. S3, there is a significant local electric field enhancement at the absorption wavelength λ=532 nm, which confirms that the Ag nanostar array enhances the absorption of RhB.
Fig. 4. Electric field distributions of (a) individual Ag nanostar and (b) Ag nanostar arrays at 598 nm.
Next, the laser output performance of the device was investigated. The intensity of the laser beam was tuned continuously by a variable optical attenuator. The emission spectrum was collected by a spectrometer (Maya 2000 Pro, Ocean Optics, Dunedin, FL, USA) as the integration time of 100 ms is used. The PL spectra of random laser device at different pump power densities are shown in Fig. 5(a). At low pump energy density, PL spectra appear as broad spontaneous emission peaks. When the pump energy density increases to 8.9 mJ/cm2 per pulse, a narrow laser peak with a linewidth of 0.14 nm appears on the PL spectra, as shown in the illustration. The appearance of such narrow modes is a signature of lasing resonance. And several discrete sharp peaks appear as the pump energy density increases. The appearance of discrete peaks with subnanometer line width indicates that the random laser is coherent. The full width at half maxima (FWHM) of coherent random laser modes can be represented by the discrete laser peak line width [44–46]. Line width is a pivotal parameter characterizing the temporal coherence of light sources, and the narrower the line width, the better the temporal coherence [46]. According to the calculation formula of quality factor Q = λ/Δλ, the Q value of this random laser mode is about ∼4300 [47]. Figure 5(b) shows the input-output characteristics of the device, indicating that the thresholds of the random laser is 8.96 µJ/cm2 per pulse. In addition, we also prepared silver nanostars with thicknesses of 80 nm, 100 nm, and 120 nm, and found that the device with a thickness of 100 nm exhibited the lowest threshold (Supplement 1, Fig. S4). Above the threshold, we observed a superlinear increase in output power as well as sharply narrowed laser peaks. It is worth noting that there is a sudden change in FWHM before reaching the threshold, which is due to the generation of amplified spontaneous emission (ASE) peaks. The discrete sharp peaks indicate that a closed scattering loop is formed in the Ag nanostars to achieve coherent random laser emission. As a contrast, device without Ag nanostars was prepared, and the spectra were shown in Fig. S5. As the energy density of the pump increases, only one amplified spontaneous emission (ASE) peak with a FWHM of a few nanometers from the waveguide mode appears. Thus, coherent random lasers are attributed to localized SPR enhancement and scattering feedback of Ag nanostar arrays.
Fig. 5. Spectra characterization of the coherent random laser. (a) The evolution of emission spectra of Ag nanostar array random laser with pump energy. (b) The emission intensity and FWHM dependent on the pump energy. (c) Emission spectra of random lasers with Ag nanostar arrays based on nanospheres of different diameters.
Then, the equivalent laser cavity size of the device is obtained by power Fourier transform (PFT) analysis. The PFT is widely used to obtain the size of the equivalent lasing path length from laser cavities, e.g., Fabry-Perot [48], whispering gallery mode [49] and random laser cavity [50,51]. According to the formula of the power Fourier transform:
where k is wave vector ($k = 2\pi /\lambda $) and r is position vector, the position vector r could be obtained by Fourier analysis of the emission spectra with a horizontal axis of $\lambda = 2\pi /k$. The peaks in the PFT result correspond to Fourier components, the path length ${p_m}$ given by ${p_m} = \; mn{L_c}/\pi $, where m is an integer denoting the Fourier harmonics, ${L_c}\; $ is the cavity path length, and n is the refraction index of the gain medium. Thus, the power Fourier transform plotted with a horizontal axis of “path length” is reasonable. The PFT of the random laser spectrum was calculated, and the results were shown in Supplement 1, Fig. S6, where the first Fourier component pm = 31.9 µm. The effective optical cavity length is Lc = 62.6 µm. This indicates that a closed scattering loop was formed in the silver nanostar array, which enables a coherent random laser. This scattering was not dependent on the periodic array, given that the metal loss limits the propagation range of the periodically driven light. In addition, both the angle-resolved transmission spectrum (Fig. S7) and the band structure diagram (Fig. S8) show that the array has no bandgap in the visible frequency. Therefore, there is no lattice resonance to produce feedback. The formation of photon localization comes from the multiple scattering of photons in the medium, which is actually the interaction between a large number of photons and scattering particles. The size of the particles will have an impact on the scattering process and ultimately affect the degree of localization of light waves. According to Rayleigh scattering theory, the scattering effect of scattering particles with the same size on light waves of different wavelengths is not the same. This suggests that the size of the scattered particles in the device may change the intensity of the radiated light at different wavelengths. Figure 5(c) shows the emission spectra of devices prepared based on nanosphere templates of different diameters. The size of Ag nanostars and arrays’ period size increases with the diameter of the nanospheres, resulting in coherent peaks at longer wavelengths.In addition, the dynamic tunability of random lasers is a crucial function that determines the applications of laser devices. A flexible random laser device was realized by growing Ag nanostars on a flexible PET substrate. The wavelength tunability of the flexible device was demonstrated by bending the sample. As shown in Fig. 6(a), the device was fixed to two fixtures, and the fixture spacing is changed by operating the translation stage, thus bending the sample. The initial length of the sample was 20 mm, and the compression amount ΔL in Fig. 6(b) was the translation distance of the fixture. As the ΔL increases, the emission wavelength of the flexible random laser device gradually shifts blue, and the results are shown in Fig. 6(c). When ΔL reaches 6 mm, the blue-shift of the laser wavelength exceeded 3.7 nm. The blue-shift of emission wavelength was attributed to the increase of inter-particle distances between Ag nanostars, which altered the mutual plasmon interaction and the scattering ability of different emitted light [31]. PFT analysis was performed on the spectra of the initial sample and the sample at ΔL = 6 mm, as shown in Fig. 6(d). The calculated cavity lengths are 67.7 µm and 34.48 µm, respectively. During the bending process, the transverse length (L) is compressed, but the longitudinal length (H) does not change. The transverse deformation disrupts the symmetry of the array and changes both the longitudinal and transverse scattering paths of light, which is a closed loop. The experimental results show that the scattering path in both directions decreases as a whole. After the stress was removed gradually, the shape of the device was restored, and the laser mode was also restored to the initial wavelength, which indicates that the flexible random laser device has excellent recoverability. In addition, a comparison of several different tunable random lasers mentioned in the introduction was conducted, and the results are displayed in Table 1 (Supplement 1). It can be observed from the table that there is not a significant difference in wavelength variations generated by different tuning methods.
Fig. 6. The optical spectrum characteristics were under the bending strain. (a) The device for bending the sample by adjusting the translation stage. (b) The schematic diagram of the principle of bending strain. The original length (L) of the sample was 20 mm, ΔL is the deformation length of the sample under bending strain. (c) Spectral evolution of the random laser under bending and recovering processes. (d) The power Fourier transform of the random laser spectra in (c).s
4. Conclusions
In summary, we propose a wavelength-tunable broad-band-enhanced plasmonic random laser device. Ag nanostar arrays were prepared with the assistance of microspheres, which exhibited SPR effect and simultaneously enhanced the absorption and emission of RhB. The two-dimensional distribution of Ag nanostars is more conducive to achieving a closed scattering path with high feedback efficiency. The emission of a coherent random laser with narrow line widths was achieved under the optical pump condition of a 532 nm nanosecond laser, and the laser threshold is 8.5 mJ/cm2 per pulse. The Ag nanostars prepared with the assistance of different diameter nanospheres achieved coherent random laser emission. And the wavelength was redshifted with the increase of the diameter of the nanospheres. The emission wavelength can be tuned by bending the device on a flexible substrate and was well recovered by releasing stress. The flexible devices fabricated by Ag nanostars arrays increase the possibility of random laser applications, such as multicolor random lasers, on-chip sensing, and microscale structural alteration detectors.
Funding
National Natural Science Foundation of China (62375007); Natural Science Foundation of Beijing Municipality (4222066).
Disclosures
The authors declare no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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