Tailored Height Gradients in Vertical Nanowire Arrays via Mechanical and Electronic Modulation of Metal‐Assisted Chemical Etching

Abstract

In current top-down nanofabrication methodologies the design freedom is generally constrained to the two lateral dimensions, and is only limited by the resolution of the employed nanolithographic technique. However, nanostructure height, which relies on certain mask-dependent material deposition or etching techniques, is usually uniform, and on-chip variation of this parameter is difficult and generally limited to very simple patterns. Herein, a novel nanofabrication methodology is presented, which enables the generation of high aspect-ratio nanostructure arrays with height gradients in arbitrary directions by a single and fast etching process. Based on metal-assisted chemical etching using a catalytic gold layer perforated with nanoholes, it is demonstrated how nanostructure arrays with directional height gradients can be accurately tailored by: (i) the control of the mass transport through the nanohole array, (ii) the mechanical properties of the perforated metal layer, and (iii) the conductive coupling to the surrounding gold film to accelerate the local electrochemical etching process. The proposed technique, enabling 20-fold on-chip variation of nanostructure height in a spatial range of a few micrometers, offers a new tool for the creation of novel types of nano-assemblies and metamaterials with interesting technological applications in fields such as nanophotonics, nanophononics, microfluidics or biomechanics.

Metamirrors Based on Arrays of Silicon Nanowires with Height Gradients

“…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.…”

“…This fabrication method makes use of a thin gold layer comprising an array of nanoholes as catalyst to etch the silicon substrate. [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. 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.…”

“…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. We demonstrate that arrays of silicon nanowires with tailored height gradients can generate phase gradients to develop metamirrors enabling light focusing in arbitrary shapes.…”

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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. [23] , with an etching time of 2 min. 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.

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“…We demonstrate that arrays of silicon nanowires with tailored height gradients can generate phase gradients to develop metamirrors enabling light focusing in arbitrary shapes. [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. Such height-induced phase gradients can complement recently developed planar phase gradient metamaterials [21,22] to achieve even stronger lensing effects.…”