1. Introduction

In the past two decades, random lasers (RLs) have been developed extensively with promising applications in diverse areas, such as speckle-free imaging, biomedical diagnosis, and microscopy [1–3]. RLs are a special type of laser, in which the laser cavity is irregularly formed within the amplifying and scattering medium without substantially relying on external feedback apparatus, unlike conventional lasers. Consequently, RLs exhibit unique and distinctive characteristics, including low spatial coherence, multi-directionality, flexibility, and low fabrication cost. [4,5].

Recently, research on developing optoelectronic devices exploiting biomaterials that are eco-friendly with biodegradability and biocompatibility characteristics has received a lot of attention [6,7]. As well in the field of RLs, a variety of studies utilizing unique biomaterials, such as human tissues, bird feathers, butterfly wings, cicada wings, silk fibroin, and eggshell membrane, have previously been reported [8–14]. Most of the studies used biomaterials as scattering materials for controlling the lasing characteristics via their distinct structural properties. For example, these studies include the use of RLs for cancer cell diagnosis [8], single-mode laser emission [9], and a flexible light-source platform [11].

Among various biomaterials, scarab beetles, which are also called “jewel beetles”, have gained a lot of attention from the scientific community in that their elytron surface exhibits a considerable level of circular dichroism (CD) [15–20]. In particular, the optical properties of the scarab beetle Chrysina gloriosa (LeConte, 1854) have extensively been investigated due to its selective circularly-polarized-light (CPL)-reflecting properties depending on the area of elytron surface, which includes the Mueller matrix spectroscopic ellipsometry (MMSE) analysis of the elytron of them [21,22]. In fact, these distinct characteristics originate from the special elytral structure of C. gloriosa [21,22]. In our previous investigation reported in [23], the scarab beetle Chrysina adelaida (Hope, 1841) was used as a substrate of an RL, in which a right-handed circular polarization (RCP)-dominant light emission was observed at a specific illumination angle whilst the experimental characterization and analysis remained preliminary. In this study, the angular distribution and directional characteristics of the RL emission under various pump illumination conditions are comprehensively measured and analyzed. We observed, for the first time to the best of our knowledge, a special case in which the circular handedness of the RL emission could vary with the direction of the emission as well as with the pump beam’s illumination angle. RLs with such emission characteristics, capable of emitting different CPL handedness at different angles, have numerous potential applications. For example, they can be used as a light source for a 3-dimensional holographic display to make a different image visible in each direction [24] and for an encoded barcode system that makes the security level even higher by adding another variable, i.e., the degree of circular polarization, in addition to wavelength information [25]. Moreover, the importance of this study lies in developing an authentic RL, which fully possesses the advantages of simple fabrication, low cost, high luminescence, and biocompatibility.

This paper is thus organized as follows: Section 2 describes the materials used in the experiment, the experiment setup, and the laser measurement method. Section 3 presents the visible CPL-reflecting characteristics of C. gloriosa, including the surface structure analysis through a microscope and scanning electron microscope (SEM), the RL emission characteristics, the angle-dependent CPL emission characteristics of the RL, and the performance comparison with other devices. Section 4 provides the conclusion of this study.

2. Methods

2.1 Materials

The scarab beetle C. gloriosa was purchased through a commercial vendor (BicBugs, LLC). Elytra of the beetle were separated from the body and cleansed with deionized water to remove dust on the surface and completely dried in the air for several hours. A 1:1 mixture of 1 mg/mL of rhodamine 6G (252433, Sigma-Aldrich Co.) dispersed in ethylene glycol (99.8%, 252433, Sigma-Aldrich Co.) and 0.5 mg/mL of TiO₂ (637262, Sigma-Aldrich Co.) in ethyl alcohol (99.45%, E7023, Sigma-Aldrich Co.) was prepared. Then, 2 μL of the mixture was dropped onto the selected area of the beetle’s elytron. The surface of the beetle’s elytron is hydrophobic, so that the rhodamine 6G-TiO₂ solution was not adhered to the surface. However, it could remain on top of the surface with no noticeable change as long as it was untouched in the normal laboratory environment. In fact, the beetle’s elytron was used as a substrate to the gain medium.

2.2 Experimental setup

The schematic diagram of our experiment is shown in Fig. 1. The second harmonic of an Nd:YAG laser (532-nm wavelength, 10-ns pulse duration, 10-Hz repetition rate, ILTECH, Inc.) was used for laser excitation. The incident pulse energy was set between 0.15 and 1.92 mJ via neutral density filters (NE01A ∼ NE20A, Thorlabs, Inc.), and a polarizing beam splitter (CCM1-PBS251/M, Thorlabs, Inc.) was used to separate parallel- and perpendicular-polarized pump beams. A parallel polarized, with respect to the scarab beetle’s stripe direction, pump beam was directed onto the sample. Protected silver mirrors (PF10-03-P01, Thorlabs, Inc.) were aligned to direct the linearly polarized pump beam onto the sample. A focusing lens (f = 150 mm) in front of the sample was used to control the size and intensity of the pump beam. The beetle sample was placed on a mechanical stage. The emission from the sample was collected with a power meter (S120C, Thorlabs, Inc.). A 532-nm notch filter (NF533-17, Thorlabs, Inc.) and a left/right-handed circular polarizer (LCPR/RCPR, CP1L532/CP1R532, Thorlabs, Inc.) were placed in front of the power meter to filter out the pump beam and to measure the circular polarization properties of the output emission, respectively. Both LCPR and RCPR had a polarized transmission of ∼ 80% in the wavelength range of 500 ∼ 700 nm with a polarization extinction ratio (PER) of ∼ 1000:1 (∼ 30 dB). The FLAME-S spectrometer (0.32-nm resolution, Ocean Optics, Inc.) was used to record the emission wavelength spectra.

 

Fig. 1. Arrangements for experiments and measurements: (a) Schematic of the experimental setup. BD: Beam dump; ND: Neutral density filter; PBS: Polarizing beam splitter; M: Mirror; L: Focusing lens; NF: 532-nm notch filter; LCPR/RCPR: Left-/right-handed circular polarizer. The inset shows a schematic of the circularly polarized light-emitting random laser system. (b) The XYZ coordinate system showing the pump illumination angle θ_yz,p and the two emission measurement angles θ_yz,e and φ_xz,e.

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As depicted in the inset of Fig. 1(a), the feedback mechanisms of the RL sample comprised a variety of scatterings and reflections by the scattering nanoparticles and interfaces therein, which included TiO₂ nanoparticles, interfaces among the rhodamine 6G-TiO₂ solution, the ambient air, and the surface and internal Bouligand structure of the beetle’s elytron. Whilst TiO₂ nanoparticles could provide the main feedback mechanism to RL, the feedback by the surface and internal Bouligand structure of the beetle’s elytron was expected to play a crucial role in giving rise to angle-dependent CPL emission characteristics.

2.3 Measurement of the lasing characteristics

For measuring the intensity and full width at half maximum (FWHM) of the RL, the pump pulse energy was adjusted from 0.15 to 1.92 mJ while the pump beam diameter was fixed to 1 mm on the transverse plane to the pump beam axis at the location of the sample, which corresponds to the nominal incident pump pulse fluence of 19.1 ∼ 244 mJ/cm². The size of the RL sample was smaller than the pump beam diameter so that nearly uniform pump illumination was achieved. The plane of incidence for the pump beam was fixed to the yz-plane as illustrated in Fig. 1(b), in which the direction of the pump beam was recorded in terms of θ_yz,p. The emission of the RL was measured on two different planes: one was on the yz-plane and the other was on the xz-plane as illustrated in Fig. 1(b), in which the directions of the RL emission were determined in terms of θ_yz,e and θ_xz,e, respectively. The pump illumination or the RL emission for a specific angle was quantified by measuring the corresponding radiation within a solid angle of $9.5 \times {10^{ - 3}}$ steradian. To analyze the pump-angle-dependent polarization characteristics of the RL, the pump illumination angle, i.e., θ_yz,p, was varied from 20 to 65°, and the corresponding output laser emission was measured on the yz-plane and the xz-plane, depending on θ_yz,e and θ_xz,e, respectively, as shown in Fig. 1(b). Either LCPR or RCPR was placed in front of the power meter to quantify each CPL component out of the total RL emission. The luminescence dissymmetry factor, $g_{\textit {lum}}$, was calculated as $g_{\textit {lum }}=2\left(P_{L}-P_{R}\right) /\left(P_{L}+P_{R}\right)$ where ${\textit{P}_\textit{R}}$ and ${\textit{P}_\textit{L}}$ denote the power components of the RL output in the right- and left-handed circular polarizations (RCP/LCP), respectively, and the inclusion of a factor of 2 is for keeping consistency with Kuhn’s dissymmetry factor [26]. While measuring $g_{\textit {lum}}$ of the emission of the prepared sample, the nominal incident pump pulse fluence was fixed to 191.0 mJ/cm², which was, in fact, well above the lasing threshold although the effective pump pulse fluence projected onto the sample could vary with its incidence angle. The average power of the RL output was calculated for 100 consecutive pulses.

3. Results and discussion

The images of the scarab beetle C. gloriosa were taken while an LCPR, no polarizer, or an RCPR was placed in front of the camera, and its polarization-dependent reflection properties were then analyzed. The results are illustrated in Figs. 2(a-c). One can see that the scarab beetle appeared dark with an RCPR placed in front of the camera as shown in Fig. 2(c), while it appeared as a bright green color with an LCPR or no polarizer placed in front of the imaging camera as shown in Figs. 2(a-b). These consequences imply that the reflected light was mostly in the LCP. However, it is noteworthy that whilst the elytral surface of the beetle predominantly reflects light in the LCP, it can also reflect light in the RCP within a narrow window of the incident wavelength or the incident angle as reported in [17]. Thanks to these properties, one can expect to obtain angular-dependent LCP/RCP laser emissions if this extraordinary material is used as a chiral reflector or substrate.

 

Fig. 2. Images of the scarab beetle C. gloriosa taken with (a) an LCPR, (b) no polarizer, and (c) an RCPR placed in front of the camera. Optical microscope images and scanning electron microscope (SEM) images of (d-f) green and (g-i) gold-colored areas of the beetle C. gloriosa. A corresponding scale bar is shown in each figure.

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In Figs. 2(d-i) the optical microscope images and the scanning electron microscope (SEM, JSM-6700F, JEOL Ltd.) images of the beetle’s elytron surface are shown. The green area was composed of ∼ 10-μm width pentagonal-, hexagonal-, and heptagonal-shaped cells as shown in Fig. 2(d). The SEM images of the green area shown in Figs. 2(e, f), reveal that there were small cracks with a few micrometers in size, corresponding to the center of the hexagonal cells in the optical microscope image. On the other hand, as shown in Figs. 2(g-i), the gold-colored area appeared to have a nearly flat surface, although it has been reported that it consists of fine substructures underneath the surface (see [17] for detailed SEM images of such kinds).

Next, the lasing characteristics of the fabricated sample were analyzed. The normalized spectra of the emission from the sample are shown in Fig. 3(a) for different illumination conditions in terms of the nominal incident pump pulse fluence. The maximum intensity and the FWHM of the emission spectrum are shown in Fig. 3(b). As in a typical RL [4], the bandwidth narrowing was observed above the lasing threshold. It is noteworthy that 100 consecutive pulse outputs were measured to account for the fluctuation of the RL emission. Whilst the RL demonstrated a stable operation within the time scale of our experiment, the stability would be an issue in an even larger time scale due to the gradual degradation of the rhodamine 6G molecules, which is their inherent material property [27]. To verify that the lasing effect was not from the fluorescence of the cuticle material of the scarab beetle, the wavelength spectrum of the pump pulse reflected from the elytron of the scarab beetle without laser dye was recorded and shown in Fig. 3(c). It is clear that there was no peculiar emission from the scarab beetle alone, as only the reflected pump pulse at 532 nm was recorded in that case. In addition, it is noteworthy that when we first tested the same RL sample prepared on top of a glass substrate instead of the beetle’s elytron, we could not observe any significant angle-dependent CPL emission characteristics out of the RL sample although other characteristics, such as the peak wavelength and the FWHM linewidth, showed similar trends observed in Fig. 3.

 

Fig. 3. Typical characteristics of the fabricated RL sample with the green area of the elytron of the scarab beetle C. gloriosa: (a) Normalized emission spectra from the RL sample and (b) the maximum intensity and full-width-at-half-maximum (FWHM) of the RL sample under different nominal incident pump pulse fluence conditions. (c) Wavelength spectrum of the reflected light from the bare elytron of the scarab beetle C. gloriosa when illuminated by the pump pulse without laser dye upon it.

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In contrast, the polarization properties of the RL output from the RL sample fabricated on top of the beetle’s elytron indeed varied depending on the pump illumination angle θ_yz,p. The $g_{\textit {lum}}$ factor describes the relative difference between the LCP and RCP light emissions, which indicates the degree of circular polarization. Large positive values of $g_{\textit {lum}}$ imply a high degree of LCP, whereas large negative values imply a high degree of RCP. The circular polarization properties of the laser output formed upon the green and gold-colored areas exhibited distinct features as shown in Fig. 4. It is noteworthy that all the linewidths of the RL emissions recorded in this measurement remained below 5 nm as shown in Fig. 3(b), clearly indicating that the RL operated well above the lasing threshold.

 

Fig. 4. Contour plot of the average of the luminescence dissymmetry factor $g_{\textit {lum}}$ for the RL upon the gold-colored (a, b) and green area (c, d) of the elytron of the scarab beetle C. gloriosa excited at the nominal incident pump pulse fluence of 191.0 mJ/cm² while adjusting the pump illumination angle θ_yz,p and the measurement angle θ_yz,e in the YZ plane (a, c) and θ_xz,e in the XZ plane (b, d). Note that no measurement was carried out when the pump illumination angle overlapped with the measurement angle as shown in (a, c).

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For the gold-colored area, the $g_{\textit {lum}}$ values for some specific angles in terms of the pump illumination angle θ_yz,p are shown in Fig. 5. The most dominant LCP and RCP light emissions were observed when the pump illumination angle θ_yz,p was set to 20° and the measurement angle θ_yz,e was in the YZ plane as shown in Fig. 4(a) and the top row of Fig. 5(a). In this case the upper and lower bounds for the luminescence dissymmetry factor $g_{\textit {lum}}$ was measured to be 0.90 and −0.29 at θ_yz,e = 110° and θ_yz,e = 70°, respectively. At the pump illumination angle θ_yz,p of 35°, the $g_{\textit {lum}}$ value was not at a considerable level, but the dominant LCP and RCP light emissions were again observed at the pump illumination angle θ_yz,p of 50°, in which the upper and lower bounds for $g_{\textit {lum}}$ were measured to be 0.62 and −0.50 at θ_yz,e = 120° and θ_yz,e = 90°, respectively. For the measurement angle θ_xz,e in the XZ plane, as shown in Figs. 4(b) and 5(b), the significant levels of $g_{\textit {lum}}$ were only observed as the pump illumination angle θ_yz,p was increased to 50° and 65°. In particular, in the case of θ_yz,p = 65°, the upper and lower bounds for $g_{\textit {lum}}$ were measured to be 0.44 and −0.48 at θ_xz,e = 40° and θ_xz,e = 100°, respectively. In addition, at the measurement angles θ_yz,e’s that were too close to the pump illumination angle θ_yz,p under the given conditions, the RL emissions could not be measured properly, in which cases no data points were produced, as shown in Fig. 5(a).

 

Fig. 5. The average and standard deviation of the luminescence dissymmetry factor $g_{\textit {lum}}$ for the RL sample upon the gold-colored area of the elytron of the scarab beetle C. gloriosa excited at the nominal pump pulse fluence of 191.0 mJ/cm² while adjusting the pump illumination angle θ_yz,p and the measurement angle (a) θ_yz,e in the YZ plane and (b) θ_xz,e in the XZ plane. Note that the error bars represent the standard deviations of the corresponding $g_{\textit {lum}}$ values measured for 100 consecutive pulses.

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For the green area, the $g_{\textit {lum}}$ values for some specific angles in terms of the pump illumination angle θ_yz,p are shown in Fig. 6. The dominant LCP light emission was observed for most angles in the YZ plane as shown in Fig. 6(a) whereas the dominant RCP light emission was only observed in the case of θ_yz,p = 35°. For the measurement angle θ_yz,e in the YZ plane, the upper and lower bounds for $g_{\textit {lum}}$ were measured to be 0.78 and −0.37 for θ_yz,p = 35° at θ_yz,e = 140° and θ_yz,e = 70°, respectively. In contrast, for the measurement angle θ_xz,e in the XZ plane, the $g_{\textit {lum}}$ value was reduced close to zero for most angles, as shown in Figs. 4(d) and 6(b). The dominant LCP or RCP light emission was only observed in the case of θ_yz,p = 65° within a very narrow window, in which case the upper and lower bounds for $g_{\textit {lum}}$ were measured to be 0.52 and −0.17 at θ_xz,e = 80° and θ_xz,e = 70°, respectively. Again, at the measurement angles θ_yz,e’s that were too close to the pump illumination angle θ_yz,p under the given conditions, the RL emissions could not be measured properly, in which cases no data points were produced, as shown in Fig. 6(a). In addition, the situations of $g_{\textit{lum}} = 0$ can correspond to linearly polarized, un-polarized light, or both. We experimentally verified that in various situations of $g_{\textit {lum}} = 0$, the emitted light was partially linearly polarized; however, the situations did not have any clear correlation with the degree polarization or the degree of linear polarization as they varied significantly depending on the pumping and emission angles even with the nearly same level of $g_{\textit {lum}}$ close to zero.

 

Fig. 6. The average and standard deviation of the luminescence dissymmetry factor $g_{\textit {lum}}$ for the RL sample upon the green area of the elytron of the scarab beetle C. gloriosa excited at the nominal pump pulse fluence of 191.0 mJ/cm² while adjusting the pump illumination angle θ_yz,p and the measurement angle (a) θ_yz,e in the YZ plane and (b) θ_xz,e in the XZ plane. Note that the error bars represent the standard deviations of the corresponding $g_{\textit {lum}}$ values measured for 100 consecutive pulses.

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In fact, the circular polarization properties of the laser output originate from the chiral structure in the exocuticle of the scarab beetle C. gloriosa [17], as illustrated in the inset of Fig. 1. Inside the RL sample, the incident photons experience multiple scattering and amplification through the scattering particles and gain medium. At the same time, the elytron of the scarab beetle C. gloriosa functions as a chiral reflector and converts the polarization of the light into circular polarization. As a result, the light emitted from the RL sample tends to be circularly polarized. The elytron structure of the scarab beetle C. gloriosa has been investigated extensively in [21,22]. It has been found that there are nested arc microstructures in the outer exocuticle of the scarab beetle C. gloriosa, which is responsible for the iridescent properties of the beetle [22]. Moreover, the green area is made up of pentagonal, hexagonal, and heptagonal cells, which form a rough, scattering surface [17]. In contrast, the gold-colored area is made up of a relatively flat, reflecting surface with fine underneath substructures, resulting in a further difference in the polarization characteristics [17].

The MMSE study has found that the reflected light from the surface of the scarab beetle C. gloriosa is mostly LCP, but RCP light can also be observed in a narrow window depending on the wavelength, incident angle, and measurement setup [17]. In fact, this observation is similar to our experimental outcomes in that large positive $g_{\textit {lum}}$ values of the LCP light emission were observed in many angles whereas negative $g_{\textit {lum}}$ values of the RCP light emission were seldom observed and their magnitudes were relatively smaller than those of the LCP light emission. However, it should be noted that our results cannot be directly compared to the MMSE study due to the complex nature of the light interaction within the RL sample. On the other hand, light emission from the RL sample is multi-directional, and, therefore, the circular polarization properties could be measured at even wider angles compared with the MMSE measurement result previously reported [17]. Whilst the exact origin of the angle-dependent CD properties of the elytron of the scarab beetle C. gloriosa is still unknown, a further extensive investigation on the elytral structure of the scarab beetle C. gloriosa would help understand its functional characteristics and be exploited into developing a biomimetic device that emits asymmetric LCP/RCP light at different angles.

In comparison with the other CPL-emitting devices, our device displays a good CPL-emission property in terms of $g_{\textit {lum}}$. Typical $g_{\textit {lum}}$ values for the CPL-emitting devices with the different fabrication methods can be found in [28]: Most devices tend to have $g_{\textit {lum}}$ values of around 0.3, while the device with the maximum $g_{\textit {lum}}$ value of 1.6 was based on a three-layered-reflector configuration. As for our device, the largest $g_{\textit {lum}}$values were obtained from the RL based on the gold-colored area when the pump beam was set to 20°. In this case, the $g_{\textit {lum}}$ values of LCP/RCP-dominant emissions were measured to be 0.90 and −0.29, respectively. Overall, our device exhibited high degrees of circular polarization in comparison with the other conventional devices. even through a notably simple and easy fabrication method, also being capable of achieving simultaneous emissions of LCP and RCP light at different angles.

4. Conclusion

The CPL-emitting RL sample has been fabricated on the surface of the scarab beetle elytron and its characteristics have been investigated experimentally. The inherent chiral structure of the beetle’s elytron gave rise to the RL sample emitting both LCP and RCP light at different angles. This biocompatible, eco-friendly laser with a high degree of circular polarization will pave the way for developing a unique light source with many potential applications. For example, a light source with angle-dependent CPL-emission property can be used in the study of structured light or a 3D holography generation [24,29,30], through which different images can be produced via LCP and RCP light. Moreover, a biomimetic light source with dual LCP/RCP emission from a single device can also be developed through further exploiting the iridescent properties of the scarab beetle.