Three-dimensional photonic crystals fabricated by simultaneous multidirectional etching

Abstract

We discuss three-dimensional (3D) photonic crystals fabricated by simultaneous multidirectional plasma etching. First, we investigate a method for controlling the ion sheath used in reactive ion etching for obtaining multidirectional etching. We then discuss the fabrication tolerance from an analytical perspective. Based on our results, we demonstrate the fabrication of 3D photonic crystals with thicknesses of 1, 1.5, and 2 lattice periods in the surface-normal direction on single-crystalline silicon wafers, which show high reflectance $(\ensuremath{\sim}100%)$ and low transmittance $(\ensuremath{-}17\phantom{\rule{0.28em}{0ex}}\mathrm{dB})$ at optical communication wavelengths, suggesting the formation of a complete photonic band gap. We reveal that the shape of the etched holes limits the performance of 3D photonic crystals and suggest possible ways to improve the band-gap effect. Moreover, we show that 3D photonic crystals with short lattice constants show high reflectance $(\ensuremath{\sim}80%)$ at visible to near-infrared wavelengths. By investigating the influence of absorption on the characteristics of 3D photonic crystals, we reveal that the reflectance remains as high as 94% in the photonic band-gap range even when the absorption of silicon is taken into account. We find that a unique increase of absorption occurs at several discrete wavelengths below the photonic band gap, suggesting the possibility of manipulating light absorption. These results not only simplify the fabrication of 3D photonic crystals, but also provide a basis for realizing 3D photonic nanostructures that include other materials.