Efficient Silicon Metasurfaces for Visible Light

Dielectric metasurfaces require high refractive index contrast materials for optimum performance. This requirement imposes a severe restraint; either devices have been demonstrated at wavelengths of 700 nm and above using high-index semiconductors such as silicon, or they use lower index dielectric materials such as TiO2 or Si3N4 and operate in the visible wavelength regime. Here, we show that the high refractive index of silicon can be exploited at wavelengths as short as 532 nm by demonstrating a crystalline silicon metasurface with a transmission efficiency of 71% at this wavelength and a diffraction efficiency of 95% into the desired diffraction order. The metasurfaces consist of a graded array of silicon posts arranged in a square lattice on a quartz substrate. We show full 2π phase control, and we experimentally demonstrate polarization-independent beam deflection at 532 nm wavelength. Our results open a new way for realizing efficient metasurfaces based on silicon for the technologically all-important display applications

Ultra-thin transmissive crystalline silicon high-contrast grating metasurfaces

Dielectric metasurfaces made from crystalline silicon, titanium dioxide, gallium nitride and silicon nitride have developed rapidly for applications in the visible wavelength regime. High performance metasurfaces typically require the realisation of subwavelength, high aspect ratio nanostructures, the fabrication of which can be challenging. Here, we propose and demonstrate the operation of high performance metasurfaces in ultra-thin (100 nm) crystalline silicon at the wavelength of 532 nm. Using optical beam analysis, we discuss fabrication complexity and show that our approach is more fabrication-tolerant than the nanofin approach, which has so far produced the highest performance metasurfaces, but may be difficult to manufacture, especially when using nanoimprint lithography

Tunable optical filters based on silicon nitride high contrast gratings

We report the use of a 500-nm-thick silicon nitride membrane as a high-reflectivity mirror in the orange-red spectral range. High contrast gratings based on semiconductors have already been used as high-reflectivity mirrors in the near-IR spectral range, but their use in the visible, which is essential for many types of biosensors, is much less explored. Our membrane is patterned with a high contrast grating of 560-nm period and forms part of a tunable Fabry-Pérot cavity. The cavity is tuned electrostatically and functions as a tunable optical filter. Three different designs of the membrane suspension are investigated to establish the effect of the arm geometry on the surface stress and the displacement of the membrane. By applying 9 V to the device, we observe a 13-nm wavelength shift of the spectral peak centered at 630 nm.</p