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Abstract
High refractive index makes silicon the optimal platform for dielectric metasurfaces capable of versatile control of light. Among various silicon modifications, its monocrystalline form has the weakest visible light absorption but requires a careful choice of the fabrication technique to avoid damage, contamination or amorphization. Presently prevailing chemical etching can shape thin silicon layers into two-dimensional patterns consisting of strips and posts with vertical walls and equal height. Here, the possibility to create silicon nanostructure of truly tree-dimensional shape by means of the focused ion beam lithography is explored, and a 300 nm thin film of monocrystalline epitaxial silicon on sapphire is patterned with a chiral nanoscale relief. It is demonstrated that exposing silicon to the ion beam causes a substantial drop of the visible transparency, which, however, is completely restored by annealing with oxidation of the damaged surface layer. As a result, the fabricated chiral metasurface combines high (50–80%) transmittance with the circular dichroism of up to 0.5 and the optical activity of up to 20° in the visible range. Being also remarkably durable, it possesses crystal-grade hardness, heat resistance up to 1000 °C and the inertness of glass.
Highlights
Dielectric metasurfaces exhibit an impressive combination of functional optical properties[1,2] including resonant electric and magnetic responses[3] and high optical chirality[4], and are capable of efficient control of the light phase, polarization and propagation direction within fractions of the free space wavelength[5,6]
While the chirality by itself requires the absence of mirror symmetry planes, rotational axes of the order of 3 and higher ensure that circularly polarized waves are the true eigenmodes and all polarization transformations during the transmission of normally incident light are caused by the metasurface chirality[42]
Its precise reconstruction by means of the atomic force microscopy (AFM) reveals the averaged unit cell relief map presented in Fig. 1(c), where the colour specifies the height as measured from the flat Si/Al2O3 interface
Summary
Dielectric metasurfaces exhibit an impressive combination of functional optical properties[1,2] including resonant electric and magnetic responses[3] and high optical chirality[4], and are capable of efficient control of the light phase, polarization and propagation direction within fractions of the free space wavelength[5,6]. For a metasurface to be non-diffracting, its unit cell has to be subwavelength with respect to the free space light, and, the dielectric resonators should possess large refractive index Their shape and size determine the optical performance while the functional efficiency in the most applications requires the dielectric absorption loss to be low. As most commercially available silicon-on-insulator (SOI) wafers designed for electronic applications are supported by thick silicon slabs, one has to perform extra preparation steps to obtain, e.g., a thin mono-c-Si layer glued to glass[6,17] In this regard, valuable simplicity is offered by the silicon on sapphire (SOS) platform with a thin mono-c-Si layer epitaxially grown on the R-cut surface of sapphire crystal. Poly-c-Si nanoparticles of more complex chiral crescent shapes have been fabricated by means of the gradient mask transfer technique[25]
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