Dual-Wavelength Phase Transition Random Lasers with Switchable Modes

Abstract

Multi-wavelength random lasers with switchable modes have advantages in the fields of novel light source and information security. Here, we propose a dual-wavelength phase transition random laser, which can modulate lasing modes arbitrarily assisted by the phase transition hydrogel. Once the phase transition occurs in hydrogel, the scattering properties of light in the random system changes, affecting the optical feedback mechanism and enabling reversible switching of the dual-wavelength random laser mode between incoherent and coherent states. More appealing, random lasing mixed incoherent mode and coherent mode have been obtained for the first time by controlling the local phase transition of the sample. Based on these properties, an information encryption system is constructed by encoding spectral fingerprints at different modes. This work provides an effective way to precisely control the output modes at different wavelengths in the multi-wavelength random laser, further expanding the application of random lasers in multifunctional light sources, color imaging, and information safety.

1. Introduction

Random lasers (RLs), as a novel light source that breaks through the limitations of traditional resonators, operate by enabling feedback through multiple light scattering in disordered media to excite stimulated emissions and generate laser output [1,2]. Random lasers exhibit characteristics such as simple structure, low spatial coherence, unique spectral morphology, and novel physical phenomena, endowing them with potential application advantages in fields of light source integration [3], speckle-free imaging [4,5,6], sensing [7,8], information security [9,10,11], and basic interdisciplinary verification platforms [12,13]. For a long time, the unique spectral characteristics of random lasers have been widely discussed, specifically referring to the random laser spectra generated by two feedback mechanisms: incoherent feedback and coherent feedback [14,15]. Coherent feedback refers to the stimulated emission achieved when light forms closed loops after undergoing multiple scatterings in a random structure. Spectrally, this manifests as the formation of multiple discrete sharp peaks with a linewidth of less than 1 nm. Incoherent feedback refers to the stimulated emission achieved when light undergoes multiple scatterings in a random structure but fails to form a closed loop. The output spectrum has a single peak with a wide linewidth (up to several nanometers). Different feedback mechanisms lead to two typical lasing modes of random lasers, namely the coherent and incoherent modes. Incoherent random lasers excel in imaging [16,17], uniform illumination [18], and sensing [19], while coherent random lasers are suitable for information encryption [20,21] and quantum technology [22,23]. Consequently, the controllable mixed output of the two modes is highly valuable. Tang et al. established a physical model of “scattering intensity → photon path correlation → coherence”, which provides a design basis for multi-state photonic devices [24].

Scattering structures play a crucial role in random systems. Regulating scattering enables effective control of random lasing modes. Scattering structures, such as nanoparticles (silver–gold bimetallic [25], SiO₂ [26]), porous materials (alumina [27], PLGA spheres [28]), polymer hydrogels [29,30], liquid crystal [31], and photonic crystal [32] defects, can directly govern modes of optical feedback. Therefore, adding a scattering medium enables the overall modulation of the mode count, coherence, and threshold of the random laser. Recently, we first introduced a temperature-sensitive phase transition hydrogel into a random system as the auxiliary scatterer, realizing reversible switching between the coherent and incoherent modes of random lasers through phase transition modulation [33].

In this study, we further propose dual-wavelength phase transition random lasers with mixed modes, which is realized by introducing hydrogel as the scattering structure and irradiating the junction of two dye segments with pump light. Through independent control the phase transition at different segments, we obtain the mixed output of incoherent and coherent modes. Firstly, the reversibility of the spectrum of the dual-wavelength phase transition random lasers are tested, confirming that it can reversibly switch between coherent and incoherent random lasers. Secondly, by dynamically controlling the local phase transition of the sample, we conduct on the four states of the dual-wavelength phase transition random lasers: the changes in emission spectra when both segments undergo phase transition, both do not undergo phase transition, and only one end undergoes phase transition. Four spectral states (00, 01, 10, 11) are obtained, providing more “state dimensions” for information encoding. Finally, an information encryption transmission system is proposed by modulating laser modes to correlate binary encoding with phase transition states.

2. Design and Fabrication

2.1. Design Principles

Poly(N-isopropylacrylamide) (PNIPAM) (Innochem, Beijing, China) is a typical temperature-sensitive phase transition hydrogel characterized by a low critical solubility temperature (LCST) of approximately 32 °C. When temperature reaches LCST, the molecular structure of PNIPAM undergoes a reversible phase transition; from a hydrophilic and swollen disordered network structure at low temperatures (T < LCST), it rapidly transforms into a hydrophobic and contracted dense structure at high temperatures (T ≥ LCST). The transmittance spectra of PNIPAM hydrogel at different temperatures in visible range are shown in Figure S1. At temperatures below 32 °C, the hydrogel exhibits a high transmittance of approximately 80% in the visible spectrum. At the temperature above 32 °C, the hydrogel maintains a stable transmittance of less than 10%. Based on the result, we can deduce that scattering sharply enhances temperatures above the LCST. Here, PNIPAM is selected as the auxiliary scatterer. Rhodamine 6G (R6G) and Rhodamine B (RHB) serve as gain media, while titanium dioxide nanoparticles (TiO₂ NPs) act as the main scattering medium. The titanium dioxide nanoparticles used in this study are commercially available products (MΛCKLIN, anatase type, monodisperse) with a diameter of 100 nanometers, hydrophilic surfaces, and a metal oxide content of up to 99.8% (MΛCKLIN, Shanghai, China). As shown in Figure 1a, the sample is designed in two segments, each of which can be independently temperature controlled. The pump light irradiates the junction of the two dye segments to excite the sample, generating random laser emission. The state changes in the hydrogel before and after phase transition are illustrated in Figure 1b. When the temperature is below the LCST, the amide groups (-CONH-) in the hydrogel network form a large number of hydrogen bonds with water molecules, causing the hydrogel to swell with large and uniform internal pores. Light can transmit freely, resulting in extremely low scattering intensity (transparent state). When the temperature is above the LCST, the hydrogen bonds break, the hydrogel shrinks, and the polymer chains cluster and stack tightly, forming a large number of “scattering centers” with uneven sizes (such as polymer microdomains) inside. The scattering cross-section increases significantly, and the hydrogel changes from transparent to turbid. The reversible temperature-driven change in scattering properties enables reversible switching between coherent and incoherent random lasers. The arrows in the Figure 1b show the scattering path of photons. After phase transition, closed loops are formed, resulting in coherent random lasers. Figure 1c is a schematic diagram of the spectral changes in the dual-wavelength phase transition random lasers under independent phase transition control of the two dye segments, showing the corresponding relationship between the four phase transition state combinations (0 = non-phase transition/1 = phase transition) and the emission spectra.

2.2. Preparation Method

To fabricate the dual-wavelength phase transition random lasers, we prepared aqueous solutions of R6G (3 mg/mL) and RHB (12 mg/mL), respectively, then mixed them with 5% PNIPAM solution and 8% PVA solution in a 1:1:1 mass ratio, respectively. After mixing, the final concentration of R6G reached 1 mg/mL while that of RHB was 4 mg/mL. The solvents are all aqueous solutions. Figure S2 shows the emission spectrum of TiO₂ added at different concentrations. The results show that 0.2 mg/mL is the optimal concentration of TiO₂. In the experiment, a low-cost capillary tube was selected as the carrier. The sample solutions were sequentially dropped onto both ends of the same capillary tube, allowing gravity to form thin film structures. The two polymer films were distributed on both sides of the capillary tube without contacting each other. However, the polymer films should not be too far apart to ensure that the pump light can irradiate both dye segments simultaneously. We used a capillary with an inner diameter of 300 μm, an outer diameter of 500 μm, and a length of 60 mm. Each dye segment is 10 mm long.

2.3. Optical Measurement

A Q-switched Nd:YAG pulsed laser (model MINILITE II) (Continuum Lasers, Milpitas, CA, USA) was used as the pumping source, featuring a 10 Hz repetition rate, 5–7 ns pulse duration, and an operating wavelength of 532 nm. The emitted optical signal was captured by a fiber-optic spectrophotometer (Maya HR4000) (Ocean Optics, Orlando, FL, USA) with spectral resolution of 0.02 nm. A probe was used to detect the optical signal at the end face of the fiber. Given that the phase transition characteristics of the PNIPAM material are temperature-dependent, we employed a temperature control device to precisely regulate the operating temperature of the dual-wavelength phase transition random lasers. We maintained a fixed distance between the sections to ensure the heating lamp targeted only one end. The unheated end was cooled with a mini air conditioner. Handheld thermal imaging was used to monitor temperatures in real-time, ensuring the unheated end stayed below 32 °C. The intermittent pumping method was implemented during experiments to prevent thermal interference caused by prolonged continuous operation.

3. Results and Discussion

The optical images of the sample before and after phase transition are presented in Figure 2a. The appearance change directly reflects the phase transition state. The upper and lower sections show the color of samples with output wavelengths of λ₁ and λ₂. Red indicates samples containing R6G, while purple represents those containing RHB. Before phase transition, the sample is in a colloidal state, and the colors are dark pink and dark purple. After phase transition, the sample turns into a dense solid, showing light pink and light purple. In addition, a schematic diagram of the device used for collecting random laser spectra is shown in Figure S3. The pump light is amplified by the lens and irradiated to the junction of two dye segments to produce a random laser. Figure 2b shows the spectral changes during the process of the sample cooling from T ≥ LCST (33 °C) to T < LCST (31 °C). At 33 °C, the emission spectrum of both segments of the sample shows a coherent random laser. As temperature gradually decreases, the intensity of the random lasing gradually decreases. The spectra characteristics evolve from coherent feedback to incoherent feedback, indicating that the sample has good recovery. Figure S4 shows that the sample responds detectably to small temperature changes (0.3 °C).

As shown in Figure 2c,d, we collected the emission spectra of the dual-wavelength phase transition random lasers from the whole sample before and after it is phase-transformed. Before it is phase-transformed, the linewidth of the emission spectrum gradually narrows from a broad spectrum (about 16 nm) to a few nanometers (about 5 nm) as the pump energy density increases. The inset shows the corresponding variations in the peak intensity and linewidth. The threshold at 595 nm is 15.2 mJ/cm², and 17.1 mJ/cm² at 632 nm, respectively. These results indicate the emission spectrum undergoes a transition from a fluorescent spectrum to an incoherent random laser spectrum. Subsequently, the sample is subjected to overall heating treatment using a temperature controller. After both dyes undergo phase transition, the sample is excited with the same pump energy density. When the pump energy density is low, the spectrum is broad and smooth. As the pump energy density gradually increases, multiple discrete sharp peaks appear, indicating the onset of coherent random laser emission. The threshold at 595 nm is 6.4 mJ/cm², and 8.6 mJ/cm² at 638 nm, respectively. The threshold of the sample after phase transition is much lower than before phase transition. Figure S5 shows PFT of the random laser spectrum before and after phase transition. PFT further reveals the scattering characteristics of light. The variations in emission spectra when the sample is locally phase-transformed are shown in Figure 2e,f. With the gradual increase in pump energy density, the spectrum at the non-phase-transformed end gradually changes from fluorescence to incoherent random laser, and the spectrum at the phase-transformed end changes to coherent random laser. In addition, we show the spectral intensity and threshold variation for a single segment dye before and after phase transition (Figure S6). These results indicate random lasing that switches between incoherent mode and coherent mode can be obtained by controlling local phase transitions in the sample. Figure S7 shows the error bar of five repeated measurements, and the intensity difference in the error bar is small. This indicates that the stability of this dual-wavelength random laser is good.

As shown in Figure 3a, four spectral states can be obtained by independently controlling the phase transition of both segments under the same pump energy density. In state 1, samples with output wavelengths of λ₁ and λ₂ are not phase-transformed, and the emission spectra are both incoherent random lasers with low intensity. In state 2, samples with output wavelengths of λ₁ are not phase-transformed while samples with output wavelengths of λ₂ are phase-transformed. superposition of the two forms a combined spectrum: a non-coherent random laser with a central wavelength of approximately 595 nm (λ₁) and a coherent random laser at 632 nm (λ₂).

In state 3, samples with output wavelengths of λ₁ are phase-transformed, outputting a coherent random laser. Samples with output wavelengths of λ₂ are not phase-transformed, outputting an incoherent random laser. This state is symmetric to state 2 but with different peak intensity ratios. In state 4, samples with output wavelengths of λ₁ and λ₂ are phase-transformed, and the emission spectra are coherent random lasers with high intensity. In addition, we also investigated the chromaticity diagram of the CIE chromatic space in 1931, which corresponds to four states of the dual-wavelength phase transition random lasers (Figure S8). The results show that the wide range of color from orange to red can be achieved by controlling the phase transition. Figure 3b shows the measured random laser intensity variation for 12 cycles at a fixed pump energy density. The measurements show minimal spectral intensity fluctuations after repeated pumping. The wavelength of λ₁ is between 595 nm and 598 nm, while the wavelength of λ₂ ranges from 632 nm to 638 nm. These results show that the random laser performance of the four states is stable. Therefore, the combined spectrum of different modes can be achieved by controlling the phase transition of the two samples.

The multi-state spectra of random lasers (where each state corresponds to a binary code) provides an ideal physical layer carrier for information encryption. Based on the “state-binary” mapping, we construct a spectral information encryption system (Figure 4). Figure 4a shows a schematic diagram of the control of random laser pulses in a time sequence. By heating and cooling simultaneously, or heating a small area of one end with a heating lamp while shielding the other end with black paper for heat insulation, pulse sequences corresponding to coherent and incoherent random laser modes are generated.

As shown in Figure 4b, the two-segment states of the sample are mapped to binary codes, corresponding to four different binary encodings. Taking the character “BUCT” as an example, their ASCII codes are shown in the second column of the table in Figure 4c. Each ASCII code is divided into groups of two bits (for instance, “B” with ASCII code 01,000,010 is divided into 01, 00, 00, 10), corresponding to four state combinations (Figure 4b). As illustrated in Figure 4d, by sequentially adjusting the sample states through the temperature control system and emitting the corresponding spectra, characters can be encoded into spectral time sequences. The encoding sequence for “B” is as follows: 01 → 00 → 00 → 10, which corresponds to the sample being in the state of the only sample with an output wavelength of λ₂ which undergoes phase transition, samples with output wavelengths of λ₁ and λ₂ show no phase transition, samples with output wavelengths of λ₁ and λ₂ show no phase transition, and only samples with output wavelengths of λ₁ undergo phase transition in sequence. The samples in different phase transition states produce corresponding combined spectra. The receiving end collects the emission spectrum via a spectrometer, analyzes its state characteristics (coherent or incoherent random laser and wavelength). By analyzing the intensity and linewidth of random laser, the binary code output is determined. The binary sequence is then concatenated and decoded into characters.

4. Conclusions

In this study, a dual-wavelength random laser with switchable modes based on phase transition is proposed, which can achieve reversible switching between incoherent and coherent random lasers through temperature modulation. Meanwhile, through independent temperature control of two segments, dynamic regulation of spectra (coherent or incoherent random laser and wavelength), colors (orange, red, or transitional colors), and states (four types) are realized. In addition, by expanding the state space through the two-segment design, an information encryption system based on “state-binary” mapping is constructed. Taking the character “BUCT” as an example, encoding/decoding between spectral states and information is realized. The unforgeability of physical phase transition ensures high-security information transmission. This research provides a new scheme for the dynamic regulation and information application of random lasers. In the future, the device size can be further reduced and the state switching speed can be improved through methods such as microfluidic integration, promoting its practical application.