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Abstract
Several biological membranes have been served as scattering materials of random lasers, but few of them include natural photonic crystals. Here, we propose and demonstrate a facile approach to fabricating high-performance biological photonic crystal random lasers, which is cost-effective and reproducible for mass production. As a benchmark, optical and lasing properties of dye-coated Lepidoptera wings, including Papilio ulysses butterfly and Chrysiridia rhipheus moth, are characterized and show a stable laser emission with a superior threshold of 0.016 mJ/cm2, as compared to previous studies. To deploy the proposed devices in practical implementation, we have applied the as-fabricated biological devices to bright speckle-free imaging applications, which is a more sustainable and more accessible imaging strategy.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Random lasers (RLs) have attracted significant attention over several decades owing to its intriguing properties such as multiple emission spike spectrum, broad emission angle, and low spatial coherence. RLs have been reported from many different types of gain media such as disorder micro/nanostructures of semiconductors, polymers, organic matters, and liquid crystals [1–4]. Accompanying the demand for medical science raises, bio-compatible material becomes significant in the field of random lasers [5–8]. To date, RLs have possessed extensive applications like light illumination [9], diagnostic imaging [10,11], and security screening [12]. A conventional laser system, in which cavity mirrors are important elements that provide positive feedback to lower the lasing threshold and enhance spatial coherence. The spatial coherence becomes a negative character of conventional lasers that will produce speckle noise to damage imaging quality in image applications. Therefore, compared to standard lasers, random lasers have a low speckle noise owing to their low spatial coherence, which makes them become perfect brightening sources for speckle-free imaging systems [13–16].
To have a stable and high spectral purity of RLs, several designs, such as embedded plasmonic nanostructures [17], mirrors [18–20], or fluorescence resonance energy transfer [21] have been proposed. Bashar et al. used a SiO2/SiNx distributed Bragg reflector (DBR) to increase the output power of the electrically pumped ultraviolet random lasers [18]. Antonio et al. demonstrated a coherent random laser in which the scattering center are outside the active region by two walls [19]. Matsuhisa et al. fulfilled a low threshold laser action of dye-doped cholesteric liquid crystals (CLCs) in a multilayer cavity [20]. Liu et al. developed perovskite-based random lasers by sandwiching two TiO2/SiO2 DBR mirrors, which realized speckle-free imaging systems [14]. Meanwhile, with the demand for the need to engineer new advanced materials, natural photonic crystals have inspired diverse material designs. For example, the dielectric material mimicked photonic crystals in butterfly wings have been used for constructing structure color [22], vapor sensors [23], and infrared detection [24]. Another photonic crystal structure of ZrO2 inverse opal thin films has revealed an outstanding performance as catalysts [25]. A semiclassical multimode laser theory has introduced by Hakan et al. [26] and Oleg et al. [27]. Besides, Lepidoptera wings have fascinated scientists to make laser actions. Wang et al. coated ZnO nanoparticles as a gain medium on the surface of the butterfly wing to emit random lasing [28]. Zhang et al. made a liquid gain waveguide by biological scatter [29]. Xing et al. embedded SnF2-treated CsSnI3 perovskites in the butterfly skeleton for near-infrared lasing [30]. In these investigations, the ridges on the wings played the dominant role of lasing action based on Fabry-Perot or waveguide resonance.
In this work, the 3D photonic crystal in the Lepidoptera wings was first adopted to demonstrate the random lasing actions through a simple two-step fabrication approach, where the laser dye is served as the gain medium and the recurrent light scattering that utilizes the quasi-period structure on the wings of Papilio ulysses butterfly and Chrysiridia rhipheus moth. Experimental characterizations and finite-difference time-domain optical simulation of natural photonic crystals confirm the reflection range of different kinds of wings. We have recognized that the photonic crystal of Lepidoptera can be superior mirrors to enhance the emission signal and produce low threshold random lasers. We also investigated the relation of the optical path lengths and the direction of microstructures on the wings, the emission polarization, and the lifetime of the random laser. Moreover, we have applied these bio-material RLs to a bright speck-free imaging system, which provides a new route for sustainable and accessible imaging applications.
2. Materials and methods
2.1 Preparations of butterfly and moth wings
To investigate the characteristic of random lasing from wing textures, we use the laser dye Pyrromethene (PM597, Exciton Inc.) as a gain medium. In the combination of irregular nanostructure and periodic photonic crystal structure in scales, the brilliant wings of Papilio ulysses butterfly and Chrysiridia rhipheus moth are selected as scattering and reflecting device for our micro-cavity laser. The preparation process of a wing device is as follows: First, the samples were prepared by dissolving laser dye (PM597) in ethanol and then drop-coated on top of the wings of butterfly and moth. Because of the superhydrophobic properties of butterfly wings [31], it is hard to engineer the wing surface. Therefore, the whole device is sealed within two plastic PET films of 80µm, where the dye is physically attached to the wing surface through a laminator. Figs. 1(a), 1(b), and 1(c) show the photos of a lower dorsal wing from P. ulysses (S0), an upper dorsal wing from C. rhipheus (S1), and an upper ventral wing from C. rhipheus (S2). Due to the different alignment of photonic structures, the colorations of S0, S1, and S2 show blue, yellow-green, and cyan-yellow-green light. Figure 1(d) reveals the reflectance of the dorsal wing (S0) of P. Ulysses, the dorsal (S1), and ventral (S2) wings of C. rhipheus. It indicates different high reflection bands of S0, S1, and S2. The ranges of S0, S1, and S2 are from 425 nm to 550 nm, 550 nm to 625 nm, and 500 nm to 625 nm, respectively. Owing to melanin inside the scale [32], the reflectance of P. ulysses is smaller than C. rhipheus. In a comparison with the P. Ulysses, the reflection band of C. rhipheus exactly overlays at the photoluminescence peak of the laser dye (PM597) [red curve in Fig. 1(d)] around 580 nm. It means that the emission light from laser dye can be efficiently reflected by the wing of the butterfly to increase its conversion efficiency.
Fig. 1. The pictures of (a) a lower wing of P. Ulysses, (b) an upper dorsal wing of C. rhipheus, and (c) an upper ventral of C. rhipheus. (d) The reflection spectra of the colorful wings and the photoluminescence spectrum of P597. The blue, green, and cyan lines indicate the reflection from the wing of P. Ulysses (S0), the dorsal wing of C. rhipheus (S1), and the ventral wing of C. rhipheus (S2), respectively.
According to the iridescences from different stacking photonic crystals, the photonic wing of Lepidoptera classifies into three types [33]. A notable one is the color iridescence resulting from the multilayer of the ridges on scales shown by the Morpho butterfly [22]. Unlike Morpho butterfly, the primary iridescence in the wings of P. ulysses and C. rhipheus comes from the multilayer arrangements within the scale body. Both the scale bodies of P. Ulysses and C. rhipheus were characterized in previous works [32–35]. The blue scale of P. Ulysses consists of 11 cuticle-air multilayers, where the cuticle layer has a thickness of 60 nm and an air/cuticle bottom layer with a thickness of 110 nm, as shown in the scanning electron microscope (SEM) image of Fig. 2(a). The yellow-green scale body of dorsal C. rhipheus is composed of 9 cuticle-air multilayers, where the cuticle layer has a thickness of 90 nm and an air/cuticle bottom layer with a thickness of 120 nm as shown in SEM image of Fig. 2(b). The blue body of P. ulysses consists of bowl-shaped structures, which are different from the colorful scales of C. rhipheus having flat surfaces.
Fig. 2. The scanning electron microscope (SEM) images of the cross-section view of (a) a blue scale from P. ulysses and (b) a yellow-green scale from C. rhipheus. Both scale bars are 1 µm. The schematic plots to illustrate the dye covered (c) P. ulysses wing, and (d) C. rhipheus wing.
Figures 2(c) and 2(d) illustrate the structure of the prepared samples S0, S1, and S2 that comprise gain media (orange color) on top of bio-photonic crystals. These two species have at least two-layer different color scales on wings. The black ground scale, beneath the colorful cover scale, doesn’t have a multilayer stack in the scale body. It constitutes a layer of circular or porous rectangle structures that absorb visible light and make the wings exhibit dark black [35]. As shown in Fig. 2(b), only one side of P. ulysses (S0) exhibits plenty of color scales, while C. rhipheus has colorful scales on both sides [S1 and S2 in Fig. 2(d)]. Hence, both sides of C. rhipheus can provide light scattering for the laser systems. The distances between two ridges on the colorful wings of S0, S1, and S2 are around 5 µm, 3.6 µm, and 2.5 µm. The reflective index of a dry PM597 film at 580 nm (n = 2.5) was fitted from an ellipsometry measurement. The reflective index of PET is n = 1.6. The cuticle layers are chitin alike [34], which has a refractive index of 1.55. Because of the high refractive index of PM597, the structure forms a complex waveguide system. The stimulated emission light from laser dye forms several closed loops between PET and scales on the wings, which produce positive feedback for the random laser generation (yellow emission). In these laser devices, the photonic crystal in the wing acts as effective biological mirrors to obtain the high Q factor of microcavity lasers and reduce the pump threshold energy.
2.2 Simulation section
To investigate the role of the photonic crystal structure from the moth wing, we adopt the finite-difference time-domain (FDTD) method to obtain the relation between the incident light angle θ and the reflection spectra. The simulated structure of the photonic crystal of S1 is shown in Fig. 3(a), where the porous sheet-like stack has 9 cuticle layers. The diameter of the hole in the air/cuticle layer is 90 nm. E represents the polarized direction of the electric field, and k represents the propagation direction of the incident light. In our simulation, perfectly matched layers were set at both sides of the z-axis to completely absorb electromagnetic waves, and periodic boundary conditions were set at the x-axis and y-axis. The results showed that the estimated reflections are similar no matter with or without ridges in the simulated models. Figure 3(b) shows the simulated results of S1, the high reflection band blue shifts when the incident light angle θ increases from 0° to 35°. However, the reflectance at 580 nm remains high when the incident angle increases to 35°.
Fig. 3. (a) The structure of S1 in Fig. 2(b) for the simulation model. (b) The simulated reflection of the structure from S1 under different incident light angles. The color bar indicates the reflectance from 0 to 1.
2.3 Experimental section
Figure 4(a) shows the experimental setup and the measurement of the RL from the dye covered biological tissue. A linearly polarized frequency-doubling Q-switched Nd: YAG laser (NL200 series, EKSPLA Inc.) with a central wavelength of 532 nm, a 10 Hz repetition rate, and 2.2 ns pulse duration, was used as the pump source. A combination of a half-wave plate (λ/2) and a polarization beam splitter (PBS) was used to control the pulse energy. Because exciting random lasing needed sufficient area, a cylindrical lens of a 7 cm focal length was used to produce the extended line stripe pump area. The direction of the incident electric field is parallel to the long edge of the pump spot. The emission signals were collected through fiber and measured by a spectrometer (HR-4000, Ocean Optics Inc.) with a resolution of 0.06 nm. The angle between the pump beam and the lasing beam was around 15° ∼ 35°. To measure the beam spot size, we set up a blade on a micrometer stage of a 10 µm step. A power meter detected the laser beam through a blade on the stage, which moved step by step from the appearance of the laser to the extinction of the laser. The method is the so-called knife-edge method, and the beam area is 6.67 mm x 0.35 mm ≈ 0.023 cm2. Figure 4(b) shows the experimental setup and the RL for the images of the 1951 US Air Force (AF) resolution test chart, which project into a CCD camera.
Fig. 4. Schematic illustrations of the setup for (a) the generation and measurement of random lasing from prepared samples and (b) the imaging of an AF resolution test chart. (λ/2: the half-wave plate, PBS: the polarization beam splitter, CL: the cylindrical lens, PC: the personal computer, AF: AF test chart, Obj: microscope objective, CCD: CCD camera.)
3. Results and discussion
3.1 Random lasers of butterfly and moth
To confirm the lasing characteristics under different biology-based photonic reflectors, the emission intensities of a butterfly P. ulysses wing (S0), a moth C. rhipheus wing of dorsal (S1) and ventral (S2) sides as a function of different pump energies are shown in Figs. 5(a), 5(b), and 5(c). When the pump energies were below 0.2 µJ, the sample only reveals broad spontaneous emission signals. As exciting energy increased, the quasi-periodic arrangement of textures and the ridges on the Lepidoptera wings efficiently scatter light to form many closed loops. Thus, random emission spikes, with aperiodic period and large amplitude fluctuation, can be seen in Figs. 5(a), 5(b), and 5(c). The emission spikes from the dye covered wing located near the maximum emission peak of laser dye (PM597) around 580 nm. The output intensity as a function of pump intensity in Fig. 5(d) can be well fitted by two linear lines. The intersection of two fitting lines indicates the threshold of a laser device. The threshold energies of S0, S1, and S2 membranes are located at 0.38 µJ, 0.48 µJ, and 1.65 µJ, respectively. The threshold energies of S1 and S2 are lower than S0 because of the reflection band of the photonic crystal from moth (C. rhipheus) overlapping the emission wavelength of PM597, as shown in Fig. 1(d). To facilitate the signal collections, we slightly rotated the wing to make the angles between the pump beam and collected emission are 15°∼35°. Even the angle of incident light increases to 35°, the high reflection region still including 580 nm from the simulation results in Fig. 3(b). Owing to the reflection of random lasing signals near 580 nm from the wing of the moth, S1 and S2 have better performances than S0. The full width at half maxima (FWHM), Δλ, of the selected peaks are 0.4 nm, 0.3 nm, and 0.24 nm for S0, S1, and S2 as the insets in Figs. 5(a), 5(b), and 5(c). The FWHM (red lines) of S1 (△) and S2 (●) tie together in Fig. 5(d). We evaluate the quality of a resonant device by calculating the Q factor, which defined as Q =λ/Δλ, where λ is the central lasing peak near 580 nm, and Δλ is FWHM of the primary lasing signal. Accordingly, S0, S1, and S2 have slightly different Q factors of 1456, 1937, and 2416, respectively.
Fig. 5. Evolution of emission spectra with different exciting energies for the devices from (a) wing of P. ulysses, (b) dorsal wing of C. rhipheus, and (c) ventral wing of C. rhipheus. (d) Output intensity and the corresponding FWHM versus pump energy of the devices of S0(○), S1(Δ), and S2(●). Solid lines represent the fitting curves. Black arrows indicate the threshold power of S0, S1, and S2. Orange lines indicate FWHM. The FWHM curves of S1(Δ) and S2(●) are overlapping.
3.2 Comparison of RLs using PM597 as the gain medium
PM597 has been widely reported as a gain medium to combine many scattering materials to form RLs in Table 1. Many approaches in the list are to reduce the threshold of RLs. For example, RLs act in the solution of MnCl2 and PM597 due to energy transfer that dye PM597 absorb emitted photos of MnCl2 molecules to enhance emissions. After doping MnCl2, the threshold of the dye PM597-doped polymer-dispersed liquid crystal (PDLC) decreases from 14.25 µJ to 7.65 µJ [36]. Hsiao et al. have embedded silver (Ag) nanoparticles in the dye-covered polyvinyl alcohol (PVA) to tune the threshold of RL from 15.75 µJ to 1.12 µJ [37]. Some reports have demonstrated that metal oxide nanoparticles in dye-doped polymer also assist the reduction of pump threshold [38]. Optimized polymeric whispering gallery mode (WGM) lasers can be fulfilled by adjusting the cavity size. According to the cavity size, the threshold varies from 0.09 mJ/cm2 to 20 mJ/cm2 [39]. Besides, an optimized PM597 concentration is able to achieve low-threshold for the dye-doped polymer films with high surface roughness by surface rubbing [40]. In comparison to previous literature, the RLs based on the nanophotonic crystal of butterfly and moth, such as samples S0-S2, show a relatively low pump threshold in Table 1.
Table 1. The performances of different scattering materials with gain medium Pyrromethene (PM597)
3.3 Effect of the exciting polarization
To investigate the scattering mechanism from the ridge of the moth, the emission spectra are measured as the long edge of the exciting stripe line perpendicular and parallel to the ridge of scales respectively, as shown in Figs. 6(a) and 6(b). The insets of Figs. 6(a) and 6(b) show the images of sample S1 under an optical microscope (OM), in which the red arrow indicates the direction of the long edge of the incident beam stripe. The beam stripe size of 6.67 mm x 0.35 mm is larger than the photo size of the inset figure, which is 379 µm x 284 µm. The area inside the purple rectangle represents a scale of 78 µm x 165 µm. The cyan arrows indicate the distance between two ridges. Because of the polarization beam splitter, the electric field is parallel to the long edge of the incident beam. Every scale on the wing combines different photonic crystal structures exhibiting yellow and green colors. It is obvious to see that the generated random laser signal is more accessible when the exciting stipe line is perpendicular to the alignment direction of the ridges on scales (S1a) in Fig. 6(a). However, the laser actions are more challenging as the direction of the beam stripe is parallel to the alignment direction of ridges (S1b) in Fig. 6(b). The Fourier transform spectra in Fig. 6(c) show that S1a has a smaller position peak of 7.1 µm than S1b of 18.8 µm. Figure 6(d) summarizes the peak intensity and FWHM as a function of pump energies from the emission spectra of Figs. 6(a) and 6(b). It indicates that the slope efficiency of S1a is far higher than S1b, and the threshold is significantly declined from 1.48 µJ to 0.74 µJ. This implies that the ridges contribute main light scattering as polymer walls to reduce the threshold of random laser [41]. Since the pump stripe area is large enough to cover many ridges when the stripe line is parallel to the ridge. Thus, light scattering from the ridge can still be induced to form several emission spikes when the pump stripe line parallels to the ridges.
Fig. 6. Evolution of the emission spectra from the dorsal C. rhipheus as the long edge of the exciting beam (a) orthogonal and (b) parallel to the ridges. The inset photos are S1 under an OM. The red arrow indicates the direction of the incident beam position. The two cyan arrows indicate the distance between the two ridges. The purple rectangle indicates a scale on the wing. (c) Fourier transform spectra versus optical path length of S1a and S1b. (d) Output intensity and FWHM as a function of pump energy of S1a and S1b.
3.4 Properties of the random lasers
To understand the mechanism of the coherent feedback of our sample, the cross-section view of SEM image from the cut wing of the moth in Fig. 7(a) shows only the physical adhesion between wing and dye. In Fig. 7(b), the dried PM597 had attached the scale tight for the clear ditches made by the ridges. Owing to the hydrophobic property of the wing, the PM597 is difficult to penetrate the photonic crystals of scale. To confirm the polarization of RL, a linear polarizer (LP) is put in front of the detector to observe the emission spectra of S1 with different angles as shown in Fig. 7(c). In this measurement, the angle between the sample and the exciting pump is around 30 degrees. Figure 7(d) is the corresponding polar plot of normalized emission signals with a thirty-degree interval and it demonstrates no specific oscillation direction of the electric fields. The result confirms that the recurrent light scattering of the RL doesn’t have polarized property. Furthermore, we record emission signals every fifteen minutes to confirm the stability or the lifetime of random laser S1. The red and blue lines in Fig. 7(e) indicate the emission intensity with the pump energy of 4 µJ and10 µJ, respectively. After three-hour excitation, the normalized intensity of RL reduces to 0.073 times of the initial intensity with the pump energy of 10 µJ. On the contrary, it remains 0.75 times of the initial intensity with the pump energy of 4 µJ. Under the excitation with proper pump energy, the moth laser can function well for more than 3 hours. The micro/nano-structures on the wings will damage and break down the device if the exciting laser keeps irradiating on the surface with a high power pump laser.
Fig. 7. The SEM images of the cross-section view of wing and dye with the scale bar of (a) 10 µm and (b) 1 µm. (c) Emission spectra through a rotation of polarizer with different angles and (d) polar plot of the lasing of S1. (e) Emission spectra of S1 with 10 µJ (blue), and 4 µJ (red) pump energy under different exposure time. (f) the variation of normalized intensity from PL of S1 as a function of exposure time
4. Imaging applications
Random lasers have excellent performance in imaging applications. To realize the difference between the speckle formation of a conventional laser and the Lepidoptera random laser, we collect the images by a CCD camera without imaging the object on the object plane first. Figures 8(a) and 8(b) show the images of a conventional laser and the random laser of S1. The speckle contrast C estimated from every pixel of the extracted images by the formula $\textrm{C} = {\; }{\sigma _I}/I$, where ${\sigma _I}$ is the standard deviation, and $I$ is the average intensity. In comparison with the C = 0.116 from the speckle image of a conventional laser, the speckle formation by the RL from the moth’s wing is far less with C = 0.042. The quantitative information of the speckle contrast also clarifies the low spatial coherence of the RL from the moth’s wing. Following, we demonstrate that the RL of moth improves the image quality by imaging a 1951 US Air Force (AF) resolution test chart. Figures 8(c) and 8(d) are the images of an AF test chart with a conventional laser and the random laser of moth. The contrast-to-noise ratio (CNR) is a quantitative evaluation for image quality, and it is defined as $$\left( {{I_f} - {I_b}} \right)/\left( {\left( {{\sigma _f} + {\sigma _b}} \right)/2} \right).$$ Here, ${I_f}$ is the average intensities in the future of interest. We select some line pairs of the transmission bar in AF. ${I_b}$ are the average intensities in the background, the dark area in AF, and σ is the standard deviation of the intensity. Table 2 shows the CNR of a conventional laser and the RL of the moth with different spatial frequencies. For example, the CNRs of a conventional laser and the RL of the moth are 7.4 and 21.06 at 5.66 spatial frequency. The resolution of the RL of the moth is better than a conventional laser 2.85 times. The CNR of the RL of the moth decreases with the spatial frequency increase. From the CNR of three spatial frequencies, it convinces that the imaging resolution by the random laser of the moth is 2∼3 times of that by the conventional laser.
Fig. 8. Scattering images for evaluating the speckle contrast by (a) a Q-switched Nd: YAG laser of 532 nm wavelength, and (b) the random laser of S1. Images of 1951 US Air Force (AF) resolution test chart by (c) a Q-switched Nd: YAG laser of 532 nm wavelength, and (d) the random laser of S1.
Table 2. CNR of different spatial frequencies ((group2, element4), (group3, element4), and (group4, element4) in USAF resolving power test target 1951)
5. Conclusions
In summary, we have developed a facile method to improve laser performance by incorporating natural photonic crystal as functional scattering media. The photonic Lepidoptera effectively generates coherent emissions owing to its constructive interference and behaves as a reflector to enhance random lasers, which achieve a relevant, high Q factor of 2416 and a low threshold near the dye emission peak of 580 nm. The biology-based laser device has demonstrated its excellent performance for speckle-free imaging, which may facilitate modern random laser applications such as medical therapy or signal detecting. These sustainable materials also inspire us one step closer to the designs of the circular economy.
Funding
Ministry of Science and Technology, Taiwan (MOST-108-2112-M-027-001-, MOST-108-2221-E-027-072-).
Acknowledgments
The authors would like to acknowledge the center for precision analysis and materials research, National Taipei University of Technology, for providing instrument facilities.
Disclosures
The authors declare no conflicts of interest.
References
1. H. Gao, Y. G. Zhao, S. T. Ho, E. W. Seelig, Q. H. Wang, and R. P. H. Chang, “Random Laser Action in Semiconductor Powder,” Phys. Rev. Lett. 82(11), 2278–2281 (1999). [CrossRef]
2. M. Kuwata-Gonokami and K. Takeda, “Polymer whispering gallery mode lasers,” Opt. Mater. 9(1-4), 12–17 (1998). [CrossRef]
3. A. J. C. Kuehne and M. C. Gather, “Organic Lasers: Recent Developments on Materials, Device Geometries, and Fabrication Techniques,” Chem. Rev. 116(21), 12823–12864 (2016). [CrossRef]
4. A. D. Ford, S. C. Morris, and H. J. Coles, “Polymer whispering gallery mode lasers,” Opt. Mater. 9(1-4), 12–17 (1998). [CrossRef]
5. S. Caixeiro, M. Gaio, B. Marelli, F. G. Omenetto, and R. Sapienza, “Silk-Based Biocompatible Random Lasing,” Adv. Opt. Mater. 4(7), 998–1003 (2016). [CrossRef]
6. M. Humar and S. H. Yun, “Intracellular microlasers,” Nat. Photonics 9(9), 572–576 (2015). [CrossRef]
7. S. Nizamoglu, M. C. Gather, and S. H. Yun, “All-Biomaterial Laser Using Vitamin and Biopolymers,” Adv. Mater. 25(41), 5943–5947 (2013). [CrossRef]
8. S. Toffanin, S. Kim, S. Cavallini, M. Natali, V. Benfenati, J. J. Amsden, D. L. Kaplan, R. Zamboni, M. Muccini, and F. G. Omenetto, “Low-threshold blue lasing from silk fibroin thin films,” Appl. Phys. Lett. 101(9), 091110 (2012). [CrossRef]
9. D. O’Carroll, D. Iacopino, A. O’Riordan, P. Lovera, É. O’Connor, G. A. O’Brien, and G. Redmond, “Poly(9,9-dioctylfluorene) Nanowires with Pronounced β-Phase Morphology: Synthesis, Characterization, and Optical Properties,” Adv. Mater. 20(1), 42–48 (2008). [CrossRef]
10. R. C. Polson and Z. V. Vardeny, “Random lasing in human tissues,” Appl. Phys. Lett. 85(7), 1289–1291 (2004). [CrossRef]
11. Y.-J. Lee, T.-W. Yeh, Z.-P. Yang, Y.-C. Yao, C.-Y. Chang, M.-T. Tsai, and J. K. Sheu, “A curvature-tunable random laser,” Nanoscale 11(8), 3534–3545 (2019). [CrossRef]
12. M. Karl, J. M. E. Glackin, M. Schubert, N. M. Kronenberg, G. A. Turnbull, I. D. W. Samuel, and M. C. Gather, “Flexible and ultra-lightweight polymer membrane lasers,” Nat. Commun. 9(1), 1525 (2018). [CrossRef]
13. B. Redding, M. A. Choma, and H. Cao, “Speckle-free laser imaging using random laser illumination,” Nat. Photonics 6(6), 355–359 (2012). [CrossRef]
14. Y. Liu, W. Yang, S. Xiao, N. Zhang, Y. Fan, G. Qu, and Q. Song, “Surface-Emitting Perovskite Random Lasers for Speckle-Free Imaging,” ACS Nano 13(9), 10653–10661 (2019). [CrossRef]
15. M. Barredo-Zuriarrain, I. Iparraguirre, J. Fernández, J. Azkargorta, and R. Balda, “Speckle-free near-infrared imaging using a Nd3+ random laser,” Laser Phys. Lett. 14(10), 106201 (2017). [CrossRef]
16. R. Ma, Y. J. Rao, W. L. Zhang, and B. Hu, “Multimode Random Fiber Laser for Speckle-Free Imaging,” IEEE J. Select. Topics Quantum Electron. 25(1), 1–6 (2019). [CrossRef]
17. Z. Wang, X. Meng, A. V. Kildishev, A. Boltasseva, and V. M. Shalaev, “Nanolasers Enabled by Metallic Nanoparticles: From Spacers to Random Lasers,” Laser Photonics Rev. 11(6), 1700212 (2017). [CrossRef]
18. S. B. Bashar, M. Suja, W. Shi, and J. Liu, “Enhanced random lasing from distributed Bragg reflector assisted Au-ZnO nanowire Schottky diode,” Appl. Phys. Lett. 109(19), 192101 (2016). [CrossRef]
19. A. Consoli and C. López, “Decoupling gain and feedback in coherent random lasers: experiments and simulations,” Sci. Rep. 5(1), 16848 (2015). [CrossRef]
20. Y. Matsuhisa, Y. Huang, Y. Zhou, and S.-T. Wu, “Cholesteric liquid crystal laser in a dielectric mirror cavity upon band-edge excitation,” Opt. Express 15(2), 616–622 (2007). [CrossRef]
21. L. Sznitko, L. Romano, A. Camposeo, D. Wawrzynczyk, K. Cyprych, J. Mysliwiec, and D. Pisignano, “Interplay of Stimulated Emission and Fluorescence Resonance Energy Transfer in Electrospun Light-Emitting Fibers,” J. Phys. Chem. C 122(1), 762–769 (2018). [CrossRef]
22. A. Ruiz-Clavijo, Y. Tsurimaki, O. Caballero-Calero, G. Ni, G. Chen, S. V. Boriskina, and M. Martín-González, “Engineering a Full Gamut of Structural Colors in All-Dielectric Mesoporous Network Metamaterials,” ACS Photonics 5(6), 2120–2128 (2018). [CrossRef]
23. R. A. Potyrailo, T. A. Starkey, P. Vukusic, H. Ghiradell, M. Vasudev, T. Bunning, R. R. Naik, Z. Tang, M. Larsen, T. Deng, S. Zhonga, M. Palacios, J. C. Grande, G. Zorn, G. Goddard, and S. Zalubovsky, “Discovery of the surface polarity gradient on iridescent Morpho butterfly scales reveals a mechanism of their selective vapor response,” Proc. Natl. Acad. Sci. U. S. A. 110(39), 15567–15572 (2013). [CrossRef]
24. F. Zhang, Q. Shen, X. Shi, S. Li, W. Wang, Z. Luo, G. He, P. Zhang, P. Tao, C. Song, W. Zhang, D. Zhang, T. Deng, and W. Shang, “Infrared Detection Based on Localized Modification of Morpho Butterfly Wings,” Adv. Mater. 27(6), 1077–1082 (2015). [CrossRef]
25. G. I. N. Waterhouse, W.-T. Chen, A. Chan, and D. Sun-Waterhouse, “Achieving Color and Function with Structure: Optical and Catalytic Support Properties of ZrO2 Inverse Opal Thin Films,” ACS Omega 3(8), 9658–9674 (2018). [CrossRef]
26. H. E. Türeci, L. Ge, S. Rotter, and A. D. Stone, “Strong Interactions in multimode random lasers,” Science 320(5876), 643–646 (2008). [CrossRef]
27. O. Zaitsev and L. Deych, “Recent developments in the theory of multimode random lasers,” J. Opt. 12(2), 024001 (2010). [CrossRef]
28. C.-S. Wang, T.-Y. Chang, T.-Y. Lin, and Y.-F. Chen, “Biologically inspired flexible quasi-single-mode random laser: An integration of Pieris canidia butterfly wing and semiconductors,” Sci. Rep. 4(1), 6736 (2015). [CrossRef]
29. H. Zhang, G. Feng, S. Wang, C. Yang, J. Yin, and S. Zhou, “Coherent random lasing from liquid waveguide gain channels with biological scatters,” Appl. Phys. Lett. 105(25), 253702 (2014). [CrossRef]
30. G. Xing, M. H. Kumar, W. K. Chong, X. Liu, Y. Cai, H. Ding, M. Asta, M. Grätzel, S. Mhaisalkar, N. Mathews, and T. C. Sum, “Solution-Processed Tin-Based Perovskite for Near-Infrared Lasing,” Adv. Mater. 28(37), 8191–8196 (2016). [CrossRef]
31. Z. Han, J. Fu, Z. Wang, Y. Wang, B. Li, Z. Mu, J. Zhang, and S. Niu, “Long-term durability of superhydrophobic properties of butterfly wing scales after continuous contact with water,” Colloids. Surf. A: Physicochem. Eng. Aspects 518, 139–144 (2017). [CrossRef]
32. P. Vukusic, R. Sambles, C. Lawrence, and G. Wakely, “Sculpted-multilayer optical effects in two species of Papilio butterfly,” Appl. Opt. 40(7), 1116–1125 (2001). [CrossRef]
33. H. Ghiradella, Microscopic Anatomy of Invertebrates, Vol. 11c (F. W. Harrison and M. Locke, eds.), John Wiley & Sons, New York, USA1998.
34. S. Yoshioka, T. Nakano, Y. Nakano, and S. Kinoshita, “Coloration using higher order optical interference in the wing pattern of the Madagascan sunset moth,” J. R. Soc. Interface 5(21), 457–464 (2008). [CrossRef]
35. P. Vukusic, J. S. Sambles, and C. R. Lawrence, “Structurally Assisted Blackness in Butterfly Scales,” Proc. R. Soc. Lond. B 271(suppl_4), S237–S239 (2004). [CrossRef]
36. Z. Shang, M. Yang, and L. Deng, “Low-threshold and high intensity random lasing enhanced by MnCl2,” Materials 9(9), 725 (2016). [CrossRef]
37. J.-H. Hsiao, S.-W. Chen, B.-Y. Hung, K. Uma, W.-C. Chen, C. C. Kuo, and J.-H. Lin, “Resonant energy transfer and light scattering enhancement of plasmonic random lasers embedded with silver nanoplates,” RSC Adv. 10(13), 7551–7558 (2020). [CrossRef]
38. L. Ye, Y. Feng, Z. Cheng, C. Wang, C. Lu, Y. Lu, and Y. Cui, “Coherent Random Lasing from Dye Aggregates in Polydimethylsiloxane Thin Films,” ACS Appl. Mater. Interfaces 9(32), 27232–27238 (2017). [CrossRef]
39. S. Krämmer, S. Rastjoo, T. Siegle, S. F. Wondimu, C. Klusmann, C. Koos, J. Zhang, and H. Kalt, “Size-optimized polymeric whispering gallery mode lasers with enhanced sensing performance,” Opt. Express 25(7), 7884–7894 (2017). [CrossRef]
40. L. Ye, Y. Wang, Y. Feng, C. Zhao, G. Hu, and Y. Cui, “Random lasing based on rough dye-doped polymer thin film,” Opt. Quant. Electron. 48(4), 253 (2016). [CrossRef]
41. A. Consoli, E. Soria, N. Caselli, and C. López, “Random lasing emission tailored by femtosecond and picosecond pulsed polymer ablation,” Opt. Lett. 44(3), 518–521 (2019). [CrossRef]
