1 l. The refractive indices were set at the average values of 3.56 and 1.4 using the effective medium approximation. It is apparent from Figure 6d that as the size of an opaque square increases, the number of local scattering angle minima also increases. There is no local minimum at l = 100 nm because the size is sufficiently smaller than the wavelength. In the
size range above the wavelength, some local minima exist, and the angle was determined by Equation 3. This trend is similar to that of scattering by a sphere, i.e., Mie scattering [23]. The local minima ITF2357 manufacturer shown in Figure 5b for a wavelength of 1,050 nm are similar to the minima of the integrated phase function given in Figure 6d for l = 1,500 nm, which is also in good agreement with the size of the SiNW bundle illustrated in Figure 6b. This suggests that the strong light confinement observed in SiNW arrays is derived from Mie-related scattering, and it is important to adjust the apparent size of SiNWs to the wavelength of the incident light. Figure 5 ADF of transmittance of SiNWs with lengths of (a) 1 μm and (b)10 μm. Figure 6 Cross-sectional SEM images of SiNW arrays attached to silicon substrates. (a) 1-μm- and (b) 10-μm-long arrays.
(c) A diagram of the calculation model of an opaque rectangular obstacle illuminated by a plane wave. (d) Integrated phase function at a wavelength of 1,050 nm for various length opaque rectangular obstacles. Conclusions We succeeded in measuring the key optical properties see more of SiNW arrays that were prepared with metal-assisted chemical etching and separated from the substrates by peeling. The absorptance of a SiNW array composed of 10-μm-long nanowires Thiamet G is much higher than the theoretical absorptance of a 10-μm-thick flat Si wafer. Therefore, SiNW arrays demonstrate a strong optical confinement effect. To investigate the reason why SiNW arrays demonstrate such a strong optical confinement, their scattering properties were observed. For an array with 10-μm-long SiNWs, the range of high transmittance was expanded to high scattering angles for wavelengths
above 1,000 nm. Since high-angle scattering leads to the enhancement of photocurrent, the 10-μm-long SiNW array demonstrates strong light confinement for wavelengths above 1,000 nm. This enhancement of light scattering may be due to Mie-related light scattering because the ADF of this array is similar with the scattering patterns calculated by Mie-related theories. Acknowledgements This work was supported in part by JST, PRESTO, and the Nissan Foundation for Promotion of Science. References 1. Kurokawa Y, Kato S, Watanabe Y, Yamada A, Konagai M, Ohta Y, Niwa Y, Hirota M: Numerical approach to the investigation of performance of silicon nanowire solar cells embedded in a SiO2 matrix. Jpn J Appl Phys 2012, 51:11PE12. 11PE12–4CrossRef 2. Hu L, Chen G: Analysis of optical absorption in silicon nanowire arrays for photovoltaic applications. Nano Lett 2007, 7:3249–3252.CrossRef 3.
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