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Abstract
The surface of a gold-coated robust cicada wing, with nanometer-sized pillars as bristles, exhibits a broadband light absorber property (wavelengths of 400 – 800 nm), which cannot be observed in the case of a gold-coated brown cicada wing. This difference is owing to the different surface structures of the cicada wings. Scanning electron microscopy reveals that the pillars on the wing surface of the robust cicada are thin (diameter of 0.1 $\mu$m) and those of the brown cicada are thick (diameter of 0.5 $\mu$m). The spacing between the pillars of the robust cicada is $\sim$0.14 $\mu$m, whereas, that of the brown cicada is wide (1 $\mu$m). The calculated results, obtained using the finite-difference time-domain (FDTD) method, consider the surface structures, and are consistent with the experimentally obtained spectra. The FDTD calculation also reveals that compared with gold, platinum is a slightly more suitable metal for surface coating, which has a large imaginary permittivity in the visible wavelength range.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Broadband light absorbers (BLAs) over a wide spectral range have received considerable attention in nanophotonics. They can be used for a variety of applications, such as in light harvesting and emission devices. Several reports have been published on the fabrication of BLAs, [1–10] which are based on surface plasmons in metallic nanostructures or in carbon nanotubes. The mechanism of the BLA properties have also been reported. [11–13]
We have recently reported that gold-coated surfaces of Lotus (Nelumbo nucifera) and Taro (Colocasia esculenta) leaves exhibit BLA property, [14,15] despite their metallic surface. Although their surface structures have different shapes, both surfaces exhibit low reflectivity and scattering efficiency, over the visible spectral range. Such natural nanostructures provide us with large functional surfaces without requiring nanofabrication techniques. [16] Among them, silver-coated cicada wings, with their surfaces covered with arrays of hexagonally ordered nanometer-sized pillars, have widely been investigated. [17–28] The sharp tip of the pillar enables the fabrication of effective surface-enhanced Raman scattering (SERS) substrates. [29–35] Using the surface as a template, a large area SERS substrate can be conveniently fabricated of a size of $\sim$cm$^2$.
A few studies have described the BLA or light-trapping properties of the nanometer-sized pillar array coated with gold or high-index material. [5,6,22,36,37] In this paper, we report the BLA property of the cicada wing surface coated with gold and interpret the BLA mechanism using a finite-difference time-domain (FDTD) method calculation. The gold-coated wing of the robust cicada (Hyalessa maculaticollis) or black cicada (Cryptotympana facialis) behaves as a BLA, while that of brown cicada (Graptopsaltria nigrofuscata) is gold-colored and does not exhibit a BLA property. This difference is owing to the different sizes, shapes, and orderings of the surface pillars: the diameter of the pillars of the robust cicada is $\sim$0.1 $\mu$m and the distance between the cicada is $\sim$0.14 $\mu$m, while those of the brown cicada are $\sim$0.5 $\mu$m and 1 $\mu$m, respectively. The results of the numerical calculations, based on the FDTD method, are consistent with the spectra obtained experimentally.
2. Experimental
The brown cicada was collected at the Suzukakedai campus of the Tokyo Institute of Technology, Yokohama, and the robust cicada was collected near the Toyosu campus of Shibaura Institute of Technology, Tokyo. The black cicada was purchased from an institute providing insect specimens. The wing of the cicada was cut off and fixed by tape on a glass substrate. A 30-nm-thick metal layer (gold or platinum–palladium alloy) was sputtered on the wing in air at low pressure, using an E-1030 sputter coater (Hitachi). The reflection and scattering spectra were acquired by a MCPD-3000 spectrometer (Otsuka Electronics) and a halogen lamp was used as a light source. The light was guided to the sample by optical fiber to illuminate the sample surface at normal incidence. For the scattering measurements, the back-scattered light was collected at an angle of approximately 60$^\circ$ with respect to the surface normal. An SRS-99 diffuse reflectance standard (Labsphere) was used as a reference. The SEM observations were performed using an S-4500 scanning electron microscope (Hitachi).
The FDTD calculation was performed with using a simulation software by Lumerical FDTD Solutions. The refractive indexes for the metals were taken from literature [38]. The refractive index of the cicada wing was set as 1.4. Pulse light with wavelengths in the range of 300–900 nm was incident along the z-axis at normal incidence. The size of the Yee lattice was 1 nm $\times$ 1 nm $\times$ 1 nm. The boundary condition was periodic in the $x$- and $y$-directions and perfect absorption was achieved in the $z$-direction using perfectly matched layers. In this geometry, the calculated reflectivity and transmittance using the FDTD method include the back- and front-scattering light, respectively.
3. Results and discussion
3.1 Photographic and SEM images
The scanning electron microscopy (SEM) images of the brown, robust, and black cicada wings coated with sputtered gold are shown in Figs. 1(a)–1(c), respectively. The photographic images of the cicadas are also shown in the inset. Figures 1(d)–1(f) also show the higher magnification SEM images for the brown, robust, and black cicadas, respectively. While the pillars are observed at the wing surface of each cicada, their sizes, shapes, and orderings are different. Notice that the scale bar in Fig. 1(d) is 1 $\mu$m, whereas it is 0.2 $\mu$m in Figs. 1(e) and 1(f). This indicates that the pillars of the brown cicada are approximately 5 times thicker than that of the other cicadas. The mean spacing between the pillars is $\sim$1 $\mu$m in the brown cicada, whereas it is 0.14 $\mu$m in the robust or black cicadas. A SEM image of the brown cicada wing with fallen out pillars is shown in Fig. 1(g), to illustrate the length of the pillars. The pillars are nearly spindle-shaped with lengths in the range of 1–1.5 $\mu$m and their mean trunk diameter is $\sim$0.5 $\mu$m at the center. A SEM image of the robust cicada wing acquired with the sample stage tilted by 30$^{\circ }$ is shown in Fig. 1(h), to determine the size of the pillars. The diameters of the pillars are 0.06 $\mu$m at the top and 0.1 $\mu$m at the bottom. The pillars of the black cicada are shown in Fig. 1(i), which have similar size to that of the robust cicada. Therefore, the shape of the pillar can be approximated by circular truncated cones for both cicadas. It can also be seen that the pillars of the robust and black cicadas are weakly hexagonally ordered with a mean spacing of 0.14 $\mu$m, and there is no positional order in the brown cicada. Figures 1(j) and 1(k) show the Fourier-transformed profiles of the SEM images for the brown and robust cicadas. The Fourier-transform was performed for an area of 40 $\mu$m$\times$ 40 $\mu$m for the brown cicada and 8 $\mu$m$\times$ 8 $\mu$m for the robust cicada. Both Fourier-transformed profiles have the shape of a ring; however, a hexagonal-order can be observed in the SEM image of the black cicada. This indicates that there is no long-range hexagonal order. The ring of the robust cicadas is much sharper than that of the brown cicada, implying that the distribution of the inter-pillar distance is smaller for the robust cicada.
Fig. 1. SEM images of cicada wings coated with gold of thickness of 30 nm: (a) brown cicada, (b) robust cicada, and (c) black cicada. Photographic images of the corresponding cicadas are also shown in the inset. Higher magnification SEM images of cicada wings: (d) brown cicada, (e) robust cicada, and (f) black cicada. (g) SEM image of a brown cicada wing where the pillars are fallen out. SEM images of (h) robust cicada wing and (i) black cicada wing, acquired with the substrate tilted by 30$^{\circ }$. Fourier-transformed profiles of the high magnification SEM images of (j) brown cicada and (k) robust cicada.
Figure 2(a) shows a photographic image of the brown cicada wings coated with sputtered gold of 30 nm thickness. The cicada wing was fixed with adhesive tape. Both surfaces of the cicada wing and the tape are gold-colored because the thin gold films are deposited together. Figure 2(b) shows the robust cicada wing coated with gold sputtered in a thickness of 30 nm. The gold-coated wing surface of the robust cicada becomes black whereas the surface of the adhesive tape is gold-colored. Similarly, the gold-coated wing of the black cicada becomes black, as shown in Fig. 2(c). However, in both images, the wing vein of each cicada is gold-colored, because of there are no nanostructures.
Fig. 2. Photographic images of the 30-nm-thick gold-coated cicada wings: (a) brown, (b) robust, and (c) black cicadas. Measured spectra of (d) absorption, (e) transmission, and (f) $S/S_0$ of the gold-coated cicada wings of brown (blue line), robust (red line), and black (black line) cicadas.
3.2 Measured spectra
The reflectance and transmittance spectra of the gold-coated cicada wings are shown in Figs. 2(d) and 2(e), respectively. The wing surface of the brown cicada has the largest reflectivity, as expected from the photographic images. Those of the black and robust cicadas are below 1% at wavelengths shorter than 650 nm, and less than 3% at wavelengths longer than 650 nm. This is consistent with the photographic images shown in Figs. 2(a)–2(c). The transmittance of the gold-coated brown cicada wing is nearly negligible, while those of both the brown and black cicadas are relatively large. This is because the bare brown cicada wing is opaque and brown, indicating low transmittance, as shown in the inset of Fig. 1(a). In contrast, the wings of the robust and black cicadas are transparent with low reflectivity, as shown in Figs. 1(b) and 1(c).
The scattered light intensity $S$ in the 60$^\circ$ direction normal to the surface was measured. Since it was observed that the scattering is negligibly weak from the samples of the robust and black cicadas with the naked eye, the measurements was carried out only in the direction of 60$^\circ$. It was also observed that the surface of the gold-coated brown cicada wing is rather shiny. The scattered light intensity is normalized by the scattered light intensity from a SRS-99 diffuse reflectance standard used as a reference, $S_0$, and $S/S_0$ is plotted as a function of wavelength. The brown cicada has a large $S/S_0$ throughout the visible wavelength range, whereas $S/S_0$ remains low for both robust and black cicadas. These results are consistent with the photographic images and the BLA properties of the robust and black cicadas.
To interpret the BLA properties, we plotted the reflectivity $R$ and transmittance $T$ of the gold-coated wings of the robust cicada as a function of the thickness of gold, at a wavelength of 600 nm, as shown in Fig. 3. While the transmittance obviously decreases with the decrease in the thickness of the gold coating, the reflectivity increases with the increase in thickness gradually. This tendency is similar at other wavelengths. This suggests that the low reflectivity results from the surface morphology of the gold-coated wing, which barely changes with the thickness. There are two possible reasons for the increase in reflectivity: the decrease in the transmittance and change in the shape of the surface pillars, with the increase in the thickness of the gold coating.
Fig. 3. Measured reflectivity $R$ and transmittance $T$ of the robust cicada wing as a function of the thickness of the gold coating at a wavelength of 600 nm.
3.3 FDTD calculations
To understand the mechanism of the BLA properties observed in the gold-coated robust and black cicadas, we performed FDTD calculations with the models shown in Figs. 4(a)–4(c). The 30-nm-thick gold-coated flat substrate is denoted as model A. Based on the SEM images, the wing of the robust or black cicada, with circular truncated-cones coated with a 30-nm-thick layer of gold, is denoted as model B. The diameters of the circular top and bottom bases of the cone are 0.06 $\mu$m and 0.1 $\mu$m, respectively. The height of the gold-coated cone was set as 0.3 $\mu$m including the 30-nm-thick gold-coating. The spacing between the pillars was set as 0.14 $\mu$m. Model C describes the wing of the brown cicada, which has spindle-shaped pillars with a height of 1 $\mu$m and the diameter of its circular bottom base is 0.5 $\mu$m. The spacing between the pillars was set as 1 $\mu$m.
Fig. 4. Models for the FDTD calculation: (a) for a 30-nm-thick gold-coated flat substrate, (b) for the wing of the robust or black cicada, and (c) for the wing of brown cicada. Calculated reflectivity (d), transmittance (e), and absorptance (f) for each model.
Figure 4(d) shows the reflectivity spectra, obtained by FDTD calculation. The reflectivity of model A is high throughout the visible wavelength range, which is in agreement with that obtained by analytical calculation based on the transfer matrix method. [39] The reflectivity of model B is low at wavelengths shorter than 650 nm, and increases with the increase in the wavelength. In the measured spectra shown in Fig. 2(d), the reflectivity remains low over the visible wavelengths. The higher reflectivity in the calculated spectra in the long wavelength range is due to the back-scattering light intensity, which cannot be observed in the measured spectra. This is supported by the higher $S/S_0$ at longer wavelengths, shown in Fig. 2(f). The reflectivity of model C is higher than that of model B, which is in agreement with the measured spectra shown in Fig. 2(d). Figure 4(e) shows the transmission spectra obtained by FDTD calculation. The transmittance of model B remains low over the visible wavelengths, whereas that of model C is significantly higher. Figure 4(f) shows the absorptance spectra calculated from the reflectance and transmittance obtained by FDTD calculation. The absorptance of model B is more than 80% higher at wavelengths shorter than 650 nm, whereas model C has a significantly lower absorptance. The BLA properties of the gold-coated robust and black cicada wings result from the high absorptance of the wing surface.
To determine which coating metal provides the highest BLA properties, we performed the FDTD calculation for model B with different coating metals, including gold, silver, copper, and platinum, and their spectra are shown in Fig. 5(a). The reflectivity of model B with the silver coating is high owing to the small imaginary permittivity of silver. The reflectivity of model B with gold or copper coating is low at wavelengths shorter than 650 nm, while at wavelengths longer than 650 nm, the reflectivity is relatively high. In contrast, model B with platinum coating has low reflectivity in the wavelength range of 400 – 800 nm, which is due to the large imaginary permittivity of platinum in this wavelength range. Figure 5(b) shows the transmittance spectra of model B coated with different metals. The transmittance is low for all metal coatings, except for the silver coating at wavelengths of 400 – 500 nm, which probably stems from the fact that silver has imaginary permittivity much lower than that of gold or copper [38]. The lower absorption due to the small imaginary permittivity of silver results in the rather high transmittance at this wavelength range. Figure 5(c) shows the absorptance spectra. The absorptance of model B with silver coating is relatively low owing to the small imaginary permittivity, while the models of gold and copper have high absorptance at wavelengths shorter than 650 nm. This is because of the large imaginary permittivity of gold and copper at this wavelength range. The model with platinum coating has high absorptance over the visible wavelength range. Consequently, platinum is the most suitable metal for coating.
Fig. 5. Calculated spectra of (a) reflection, (b) transmission, and (c) absorptance for cicada wings coated with different metals, gold, silver, copper, and platinum.
We studied the mechanism of the high absorptance in the metal-coated pillars using the FDTD calculation. The analysis of the temporal electric-field distribution revealed that the electric field is localized between the pillars for a while, when pulse light is illuminated. The localized electric-field scarcely reradiates and is absorbed by the metal-coated pillars. Namely the metal-coated pillars effectively catch the illuminated light with little reflection. Details of the mechanism will be published elsewhere.
Finally, we measured the reflectivity, transmittance, and $S/S_0$ spectra of a robust cicada wing coated with a 30-nm-thick platinum–palladium alloy that we have as a target for sputtering. The imaginary permittivity of palladium is similar to that of platinum; however, its real part is slightly different. Thus, the spectrum of the platinum–palladium alloy coating is expected to be similar to that of the platinum coating. Figure 6 shows the reflectivity, transmittance, and $S/S_0$ spectra with a photographic image shown in the inset. Although the transmittance is 5–10%, the reflectivity and $S/S_0$ are low throughout the visible wavelength range. As a metal other than gold, platinum or platinum–palladium alloy can be used for the coating of the cicada wing to achieve BLA.
Fig. 6. Measured spectra of (a) reflection $R$, (b) transmission $T$, and $S/S_0$ for the robust cicada wing coated with sputtered platinum–palladium alloy. The inset shows a photographic image.
4. Conclusion
We have reported the BLA property of the cicada wing surfaces coated with gold. The gold-coated wing of the robust and black cicada exhibits BLA property, while that of the gold-colored brown cicada shows no BLA property. The different optical properties are due to the structural difference of the pillars in the surface of the cicada wing, which is supported by the FDTD calculations for the models considering the shape of the pillars. The pillars at the surface of the brown cicada wing are an order of magnitude larger than those of the robust cicada, and the shape of the pillars of the cicadas are different. Compared with gold, the most suitable metal for the surface coating is platinum, which has a large imaginary part in its permittivity in the visible wavelength range. These findings will be useful for the design of metamaterials as light absorber.
Funding
Japan Society for the Promotion of Science (JSPS) (25109707, 26286058, 26600023).
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