Metamirrors Based on Arrays of Silicon Nanowires with Height Gradients

Abstract

CommuniCation(1 of 5) 1600933 bersome due to the technical complexity required to achieve high spatial resolution.Herein we show that a careful design of the nanostructure height is indeed an efficient way to spatially tune the optical properties in high-index dielectric metamaterials. We demonstrate that arrays of silicon nanowires with tailored height gradients can generate phase gradients to develop metamirrors enabling light focusing in arbitrary shapes. In addition, the combination of height gradients and nanowires with anisotropic cross-section permits simultaneous light focusing and strong polarization conversion in the focused light. Such height-induced phase gradients can complement recently developed planar phase gradient metamaterials [21,22] to achieve even stronger lensing effects.We employ a recently developed fabrication method based on mechanically controlled metal-assisted chemical etching of silicon [23] to obtain ordered arrays of Si nanowires with height gradients. This fabrication method makes use of a thin gold layer comprising an array of nanoholes as catalyst to etch the silicon substrate. During the etching process the catalytic metal mesh is plastically deformed, thereby generating the mechanical stress that spatially modulates the etching rate and gives rise to the array of nanowires with height gradients. The height gradients can be tuned by varying the stiffness of the catalytic metal mesh, i.e., by modifying the size and shape of the nanoholes array, the edge-to-edge separation distance between nanoholes, and the metal thickness. All the metamirrors shown in this work are produced using 20 nm thick gold films and the same etching conditions of ref. [23] , with an etching time of 2 min. The height gradients could be larger by increasing etching time and by designing rupture points in the catalytic metal film. [23] The Si nanowire metamirror concept is shown in Figure 1a, which displays a circular array of cylindrical nanowires with height gradients in the radial direction, i.e., the height of the nanowires increases toward the center of the array. In the fabricated example of Figure 1b, the diameter of the array is 10 µm, the pitch is 300 nm, and the nanowires height increases from zero at the border of the array, up to 500 nm at its center. In all the arrays in this work, the 20 nm Au film used for etching is kept at the bottom of the array to increase the reflectivity of the metamirror. In these arrays, the radial height variation is converted into a radial phase variation in the light reflected by the array, which strongly depends on the nanowire diameter and pitch.As Figure 1d shows, such radial phase gradient enables tight focusing the reflected light within a spot with a focal length of 12 µm. Since the nanowires have circular cross-sections, the focusing effect in Figure 1 is polarization insensitive. Similar focusing behavior can be theoretically observed in finite difference time domain (FDTD) calculations of circular arrays of cylindrical nanowires with equivalent radial hei...

Development of a microstructured surface using the FIB

“…It can also concentrate the incident light through localized surface plasmon resonance. 4 Surfaces patterned with nanowires, 5 nanoholes, 6,7 metallic nanostructures 8 or metallic nanogratings 9 have been used to enhance the performance of solar cells. Additionally, patterned surfaces have important applications in plasmonics, [10][11][12] cell culture system, 13,14 , fluidics, 15 etc.…”