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Abstract
Introducing phase transition materials to random systems provides a promising route to create new optoelectronic functionalities of random lasers. Here, a phase transition random laser with switchable lasing modes is reported, which is designed with a thermoresponsive hydrogel as scattering medium. By manipulating the phase transition in hydrogel, random lasing modes can be switched reversibility between incoherent and coherent random lasing. The phenomenon derives from the changing of light scattering properties in different phase states, thus affecting the optical feedback path of random lasing. Besides, based on its controllable and easily detectable time-domain characteristics, the phase transition random laser is applied in information encoding and transmission. It is the first time that the transition from coherent to incoherent random lasing is observed by varying the sample phase states. This work will inspire the design and application of novel random lasers in photoelectric device.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
As a novel optical device, random lasers are unique in working principle and emission characteristics. Random lasing is achieved by the multiple scattering of light in the disordered gain medium [1]. Due to the resonance cavity-free, random lasers are characterized by simple structure, low cost, and rich physical phenomena [2–4]. Based on these characteristics, random lasers have been used for speckle-free imaging, information technology, sensing, and basic cross-disciplinary studies [5–11]. Even more appealingly, there are two typical lasing modes in random lasers, viz. incoherent mode and coherent mode [12]. In the former situation, the amplified light cannot return to the original scattering point through multiple scattering, resulting in the loss of space resonance and insensitive to phase. Thus, a significant broad spectrum is generally emanated in this case. While in the latter situation, the amplified light returns to the original scattering point through multiple scattering, forming a closed loop and occurring constructive interference at certain specific frequencies, which generates a spectrum that has discrete laser spikes with narrow linewidth.
Up to now, various random lasers with incoherent modes or coherent modes are reported [13–17]. Specifically, an incoherent mode based random laser can be used in illumination, imaging, and sensing [5,18,19]. While a coherent one exhibits potential applications in bio-monitoring, imaging, and secure communication [6,7,20–24]. Recent researches have focused on integrating the two lasing modes in one random system, which is valuable for studying the evolution mechanisms of random lasing and could promote the multifunctional applications of random lasers [25–27]. However, stable and recoverable coexisting or switching between incoherent mode and coherent mode in one random laser is still challenging.
Scattering plays a crucial role for the characteristics of random lasing. The optical feedback in random lasers is strongly dependent on the scattering properties of the scattering material. Various types of materials have been used as scatters in random lasers, including, viz. dielectrics/metal nanoparticles [28–30], liquid crystals [31,32] and biological tissues [33,34]. Among these, phase transition materials exhibit plenty of exotic features such as easy fabrication, highly stable, reliable, and quick switching functionality including the good optical characteristics. Lately, Xiao et al reported a work on phase transition microcavity lasers as high-sensitive sensor [35]. Nevertheless, phase transition materials have been rarely applied in engineering the controllable random lasers [36]. It is strong believed that the utilization of phase transition materials as scatters can bring new vigor to random lasers.
In the work, by varying the phase states of scattering material, the switching from incoherent to coherent random lasing is first observed. The proposed random laser is achieved by introducing a hydrogel as scattering structures. The hydrogel is a type of phase transition material controlled by temperature. The random lasing in this scenario can be switched reversibility between incoherent mode and coherent mode. The switching mechanism of random lasing mode can be well understood by analyzing the appearance of changes in hydrogel molecular structure before and after phase transition. Using power Fourier transform analysis, we calculate the closed cavity lengths and analyze the scattering properties in the random system. Besides, the time-domain characteristics of phase transition random laser are used for information encryption application.
2. Design and methods
Design principles. Scattering is a key factor for the optical feedback in random system. The selection of scatter is essential. Poly(N-isopropylacrylamide) (PNIPAM) is a typical phase transition hydrogel material, existing a lower critical solution temperature (LCST) [37]. Once reaching the point of phase transition, the hydrophobicity/hydrophilicity of PNIPAM hydrogel will change abruptly, leading to the change of transmission and scattering in visible wavelength range [38]. It is believed that the introduction of PNIPAM as scatter in a random system will influence the random lasing performances. Here, phase transition random lasers (PTRLs) are fabricated by selecting PNIPAM hydrogel and R6G as scatter and gain material respectively, as shown in Fig.1a. It is worth noting that the PNIPAM hydrogel behaves as a homogeneous system beneath its critical temperature (∼ 306 K) and exhibits a weak scattering feedback. The structure of PNIPAM monomer was optimized using the B3LYP/6-31 + G (d, p) base group in a drawing software for molecular structures (GaussView in Supplement 1). The optimized structure and Natural Bond Orbital (NBO) charge distribution are achieved (Fig. S1). The abundant negative charges are distributed on the oxygen atom in amide group, which can serve as receptor in hydrogen bond. Meanwhile, hydrogen atom in the N-H structure has a significantly higher number of positive charges compared with other hydrogen atoms, which can serve as donor in hydrogen bond. Therefore, hydrogen bonds can be formed within and between PNIPAM polymer chains, as well as between molecular chains and water molecules. In other words, the PNIPAM hydrogel is in a stretching and homogeneous state due to the hydrophilic property, which is not conducive to light scattering in random system. When temperature reaches the phase transition point of PNIPAM hydrogel, the considered system abruptly changes to heterogeneous state with strong scattering feedback. The strong scattering feature is evolved due to the increase of air-water interface portraying the hydrophobicity of PNIPAM hydrogel. Consequently, the hydrophobicity results in the changes of molecular chain and sample morphology, as shown in Fig.1b.
Preparation method. PTRLs can be fabricated using drip coating approach and packaging technology to ensure the stability and durability. A laser dye, Rhodamine 6 G (R6G), was used as the gain medium. The PNIPAM hydrogel was used as matrix for scattering. A mixed aqueous solution of R6G (3 mg/mL) and PNIPAM hydrogel (3 mg/mL) was prepared. Dispense the mixture onto a clean PET substrate. Then, use PDMS as package agent to prevent the evaporation of water. Finally, the PTRL was achieved after drying the PDMS in the air. With the same preparing method, the sample ‘BUCT’ PTRL was prepared using a ‘BUCT’ template. The ‘BUCT’ here is the abbreviation of Beijing University of Chemical Technology.
Phase transformation controlling. A temperature controller was used for controlling the phase transition in the random system. Connect a homemade hot plate to the temperature controller. Use a stepping motor to paste on or separate from sample.
Optical measurement. A doubled Q-switched Nd:YAG laser (532 nm, 5-7 ns, 10 Hz) was employed as the pump source. The pump lasing incident vertically on the sample surface and the reflected emission light was collected by an optical fiber spectrometer (Ocean Optics HR4000) with a spectral resolution of 0.02 nm. The pump power was controlled by pump voltage.
2. Results and discussion
The process of phase transition in PTRLs has been observed at macro and micro scales, as shown in the insets of Fig. 2(a) and 2(d). Before phase transition, the morphology is stretched colloid state with high light transmittance due to the hydrophilic property. Once the phase transition occurs, the morphology changes to compact solid state with low light transmittance. The corresponding transmission spectra of PNIPAM encapsulated by PDMS at different phase states have been measured, as shown in Fig.S2. Before phase transition, the hydrogel exhibited a high transmittance above 80% in the visible range, indicating a weak scattering in random system. After approaching the phase transition point, the transmission becomes weak sharply even in visible region.
The emission characteristics of PTRLs before/after transition have been analyzed. The experiment setup for collecting emission spectra is shown in Fig. 1(c). Before the phase transition of sample, the incoherent random lasing action is observed at the wavelength of 595 nm. With the increase of pump energy density, the emission spectrum is evolved from the typical fluorescent spectrum to the random lasing spectrum with a single peak, as shown in Fig. 2(a). The threshold behavior and narrowing linewidth have been presented in Fig. 2(b). The threshold value appears around 8.6 mJ/cm2. The FWHM decreases from 20 nm to around 9 nm. The results indicate the occurrence of incoherent random lasing mode. When temperature is raised to the phase transition point, the random lasing action is switched to coherent state (Fig. 2(d)), where discrete laser spikes with narrow linewidth (around 0.2 nm) and a lower threshold value around 4.7 mJ/cm2 were presented as shown in Fig. 2(e). Besides, the obvious changes observed through micrographs of respective PTRL, as shown in the insets of Fig. 2(a) and 2(d). To further demonstrate the emission properties of random lasing, we have collected the emission spectra when pumping different areas of the sample, as shown in Fig. S3. The spectrum profile changes randomly at different excited areas. However, the switching between incoherent and coherent random lasing can be observed near the phase transition point at different pumping areas. In addition, the influence of temperature on random lasing threshold and linewidth have been plotted in Fig. S4. Before the phase transition (301K∼305 K), the lasing mode keeps in incoherent mode when the lasing threshold slightly changes. The change of lasing threshold before phase transition is resulted from the increase of temperature. Near the phase transition point, the coherent mode generates, where the lasing threshold and FWHM decrease sharply. These results indicate the phase transition in hydrogel is determining factor for the switching of random lasing modes.
Fig. 1. Design and working principles. (a) Schematic of the phase transition random laser. The black dotted circle corresponds to the sample part in b. (b) Changes in molecular structure of PNIPAM hydrogel and sample characteristics before and after phase transition. (c) Experimental setup for collecting spectral characteristics of phase transition random lasers. A temperature controller is used for controlling the temperature.
Fig. 2. Normalized emission characterization of the random lasing from the sample before and after phase transition. Emission spectra under different pump energy densities (a) before and (d) after phase transition of random laser. The insets denote the micrographs and ‘BUCT’ PTRL optical photographs of random lasers at corresponding states. Emission intensity and linewidth of the random lasing as a function of the pumping fluence (b) before and (e) after phase transition of random laser. The black arrows indicate the lasing thresholds. (c) The PFT of the random lasing spectra at 8.6 mJ/cm2 (c) before and (f) after phase transition of random laser. The black arrows denote the optical path length of the first-order Fourier harmonic. The inset: schematic of the light path loops in different state of random lasers. The error bars in (b) and (e) are obtained by calculating the average value of 20 spectra.
Power Fourier Transform (PFT) is an effective channel to analyze the equivalent cavity of random lasers, which can indicate the strength of light scattering [39]. It is well known that the abscissa corresponding to the peaks of PFT spectra represents the Fourier component. The cavity length can be calculated by the expression ${L_c} = {{\pi {p_m}} / {mn}}$, where n is the effective refractive index of sample, m is the order of the Fourier harmonic. The fundamental Fourier component (m = 1) of PFT spectra is considered for the calculation of the path length of light in sample. Here, the PFT calculation is executed in the Origin software (Origin 2018 64Bit). Figures 2(c) and 2(f) show the PFT results of the random lasing spectra collected from the sample excited at 8.6 mJ/cm2 before and after phase transition, respectively. In Fig. 2(c), there is no peaks in the PFT spectra, indicating no multi-spikes generated in spectrum. The results further demonstrate the occurrence of incoherent feedback behavior in sample before phase transition. The inset of Fig. 2(c) shows a schematic of the light path corresponding to incoherent feedback mechanism. While in Fig.2f, the optical path length can be fetched from the PFT spectra with the value approximately 8 µm. For n = 1.42, the value of cavity length is about 18 µm. The calculated cavity length is far less than the size of pump spot, meaning that the random lasing modes are localized. The localized modes attribute to the strong scattering caused by phase transition. The inset of Fig. 2(f) shows a schematic of the optical loop corresponding to coherent feedback mechanism. In detail, we have studied the change of cavity length in the phase transition process, as shown in Fig. S6. When the phase transition occurred in sample, the equivalent cavity length becomes shorter, indicating scattering increases [20]. The increase of scattering leads to the random lasing evolution from incoherent mode to coherent mode. In other words, the switch between incoherent and coherent random lasing in our work is resulted from the change of scattering property in random system via phase transition. These results show that the phase transition materials provide an effective strategy for the optical field manipulation of random lasing.
In Fig. 3, the switchable modes and recoverability of random lasing have been achieved. All the spectra are collected when the sample in the stable states at phase 1 and phase 2, respectively. The modes switching are observed in many cycles. The lasing mode based on incoherent feedback (coherent feedback) mechanism is called state ‘OFF’ (‘ON’). The intensity of coherent random lasing is two orders of magnitude stronger than that of incoherent random lasing, as shown in Fig. 3(b). Besides, the FWHM of coherent random lasing is 0.2 nm, much smaller than the one of incoherent random lasing (9 nm), as shown in Fig. 3(c). The process of switching between states On and OFF is fast. Besides, the intensity and FWHM at different states are stable. Based on these characteristics, the PTRLs can be used as a binary system for information coding, which takes the state ‘ON’ as ‘1’, and the state ‘OFF’ as ‘0’.
Fig. 3. Dynamical control of random lasing modes in PTRLs. (a) The emission spectra before (OFF) and after (ON) phase transition of the sample of 8.6 mJ/cm2. (b) The peak intensity and (c) linewidth of random lasing at 595 nm before (grey area) and after (pink area) phase transition in PTRLs. The error bars in (b) and (c) are obtained by calculating the average of 20 spectra.
As a signal in time-domain is easily detectable, the photodetector and oscilloscope are used to record the pulses at different modes, as shown in Fig. 4. The photodetector used here is a commercial device (KG-PR-10M-B), which has the gain of 105 V/W. The oscilloscope with the bandwidth of 1 GHz and sample rate of 4GSa/s (Agilent Technologies: DSO7104B) is used to record the pulses. A filter is used for filtering out the pumping light. Before phase transition, the intensity of pulse sequence excited at 8.6 mJ/cm2 is at a low level about 0.03 V. When the phase transition occurs in sample, the signal intensity switches to a higher level about 0.12 V. Besides, single pulses before and after phase transition are recorded, which shows the raising time about 8.5 ns and 3.7 ns, respectively. The plotted pulses in (c) and (d) are the typical spectra that selected before and after phase transition when the sample is pumped at 8.6 mJ/cm2. Based on the previous publication [40], the faster the rising time in temporal profile is, the easier it is for random lasing generation. The signal features of random lasing in time-domain, such as the signal intensity and pulse width, are advantageous for dynamical information encryption and transmission.
Fig. 4. The characterizations of time-domain spectroscopy in PTRLs pumped at 8.6 mJ/cm2. The random lasing pulses series of (a) incoherent mode and (b) coherent mode obtained by a photodetector before and after phase transition, respectively. The single normalized pulse of random lasing (c) before and (d) after phase transition.
Based on the phase transition of PNIPAM hydrogel in PTRLs, information encoding and transmission is performed by controlling the random lasing pulse sequence. Figure 5(a) shows the schematic diagram of controlling the random lasing pulses in time series. The sample is attached or detached with hot plate through stepper motor in controlled manner giving rise the pulse sequence of respective coherent and incoherent random lasing mode. The temperature controller with a constant temperature at 306 K is mounted on a stepper motor, which can accelerate the sample cooling and avoid the time of heating the plate every cycle. According to the Fig. S5a, when attaching the hot plate on sample, the phase transition of sample begins within 3 s and complete in 4 s. When detaching the hot plate from the sample, the emission recovers within 2 s. The phase transition process is fast (∼6 s) and recoverable, as shown in Fig. S5. Based on the phase transition time, we can set the interval for obtaining adjacent pulses by program. Due to the distinct difference in voltage and raising time of pulse between incoherent and coherent random lasing, a barcode on time series is generated. In the experiment, the voltage lower (higher) than 0.04 V is defined as ‘0’ (‘1’). The process of information encryption and transmission is shown in Fig. 5(b). Based on the Feinam encode, a plaintext ‘lasing’ is given. The Feinam encoding is a binary based encryption method, which is to make each letter consist of seven binary digits in a specific order. The encryption principle is 0 + 0 = 0, 1 + 1 = 0, 0 + 1 = 1, 1 + 0 = 1 [20]. A photodetector is used to collect the electric signal series converting from the optical signals. A single-chip microcomputer is used for encrypting the plaintext with the key ‘random’, the corresponding signal series are shown in the inset of Fig. 5(b). Finally, the ciphertext can display on the receiving terminal. The proposed concept based on phase transition random lasers has unique scenario in information safety, such as encrypt and transmit the environmental information where the temperature changes near the phase transition point.
Fig. 5. The information encryption transmission system based on PTRLs. (a) The principle of controlling the random lasing pulses in time series. The phase transition of PTRLs is controlled by mechanical pasting or separating the hot plate with a constant temperature at 306 K. (b) Schematic diagram of information encryption transmission system based on the Feinam code. Inset: the key signal ‘random’ edited by the lasing pulse in time series based on the principle in (a).
3. Conclusions
Phase transition random lasers with switchable modes between incoherent mode and coherent mode are demonstrated by using the thermotropic phase transition property of PNIPAM hydrogel. Before the phase transition, the sample is homogeneous state with weak scattering. Once the temperature reaches the LCST point, the sample changes suddenly to heterogeneous state with strong scattering feedback. The strong scattering feedback is attributed to the increase of air-water interface portraying the hydrophobicity of PNIPAM hydrogel. The weak/high scattering feedback in random system is the key factor for the generation of incoherent/coherent random lasing mode. Besides, we calculated the cavity lengths in two typical lasing modes using PFT analysis, demonstrating the scattering paths have changed before and after the phase transition in sample. Finally, we applied the PTRL in information encoding based on its controllable and easy detectable time-domain characteristics.
Funding
Beijing Municipal Natural Science Foundation (Z180015).
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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