3D fabrication for optical metamaterials and metasurfaces

Metamaterials enable the engineering of refractive index into the intriguing negative regime. Such negative index metamaterials (NIMs) can give rise to unique optical properties like negative refraction and backward waves. Similarly, zero index metamaterials (ZIMs) are widely studied recently for they open a new route to realize uniform phase distribution and coherent coupling even within a large volume. However, bulk NIMs and ZIMs of high performance necessitates sophisticated 3D nanofabrication technology. Here we show the fabrication of 3D NIMs and ZIMs using 20-layer electron beam evaporation without vacuum break on a thin 50nm silicon nitride membrane, followed by precise focused ion beam milling from the backside of membrane. The suspended multilayer-on-membrane configuration allows the remnant Gallium ions to penetrate through the structure, thereby significantly reducing the absorption of the fabricated samples. With the high optical transmission, we could then perform many interesting experiments on the bulk NIMs and ZIMs, including nonlinear four-wave mixing measurements to study the phase-matching condition, back-focal plane imaging to measure the topological transition between the elliptic and hyperbolic dispersions, heat experiments to examine the enhanced thermal emission due to hyperbolic dispersion, and a more recent single-photon experiment to test the fundamental principles of quantum mechanics. On the other hand, we also showed that metasurfaces, which are fundamentally 2D, can display exciting new properties by applying them on a 3D object. We first used the focused ion beam to carve out the 3D structures, followed by a specially-developed electron beam lithography technique with extremely precise alignment accuracy to fabricate metasurface on the curved 3D structures. An ultra-thin invisibility cloak capable of hiding 3D objects is thus realized in the visible wavelength.