Liquid-Crystal Metasurfaces Self-Assembled on Focused Ion Beam Patterned Polymer Layers: Electro-Optical Control of Light Diffraction and Transmission.

Self-assembling of liquid-crystal metasurfaces on polymer layers patterned by a focused ion beam manifests itself in distinctly colored optical transmission, as light from certain spectral bands is efficiently diffracted by the periodic liquid crystal modulations. We explore the metasurface electro-optics by applying voltage across the liquid crystal to straighten its director distribution and reroute the diffracted light into the direct transmission. We show that the characteristic times of switching from the diffracting to the transmitting state can be decreased down to a millisecond by increasing the driving voltage up to 6-8 V, while the main part of the relaxation back into the periodically deformed diffracting state occurs within about a few milliseconds, i.e., by an order of magnitude faster than the relaxation of the analogous homogeneous electro-optical liquid crystal cell. We explain the profound dynamics in terms of superimposed exponential modes governed by an interplay of the metasurface geometric parameters, the liquid crystal viscosity, and elasticity.

Precise local control of liquid crystal pretilt on polymer layers by focused ion beam nanopatterning.

Background: The alignment of liquid crystals by surfaces is crucial for applications. It determines the director configuration in the bulk, its stability against defects and electro-optical switching scenarios. The conventional planar alignment of rubbed polymer layers can be locally flipped to vertical by irradiation with a focused ion beam on a scale of tens of nanometers. Results: We propose a digital method to precisely steer the liquid crystal director tilt at polymer surfaces by combining micrometer-size areas treated with focused ion beam and pristine areas. The liquid crystal tends to average the competing vertical and planar alignment actions and is stabilized with an intermediate pretilt angle determined by the local pattern duty factor. In particular, we create micrometer-sized periodic stripe patterns with this factor gradually varying from 0 to 1. Our optical studies confirm a predictable alignment of a nematic liquid crystal with the pretilt angle continuously changing from 0° to 90°. A one-constant model neglecting the difference between the elastic moduli reproduces the results quantitatively correctly. Conclusion: The possibility of nanofabrication of polymer substrates supporting an arbitrary (from planar to vertical) spatially inhomogeneous liquid crystal alignment opens up prospects of "imprinting" electrically tunable versatile metasurfaces constituting lenses, prisms and q-plates.

Liquid crystal metasurfaces on micropatterned polymer substrates.

Formation of photonic liquid crystal metasurfaces on rubbed polyimide substrates patterned by focused ion beam is demonstrated. Modulation of the surface anchoring conditions with periods from 1 to 6 micrometers gives rise to periodic deformation of the nematic liquid crystal director field. The exact periodicity is confirmed by the light diffraction measurements. Distinct colors originating from the specific zero-order diffraction spectra are observed and qualitatively explained in terms of an analytical model within the one-constant approximation. Quantitatively accurate optical spectra are obtained by the full scale numerical simulations taking into account all relevant material parameters. The results pave the way for hybrid liquid-crystal-based metasurfaces with tunable optical transmission, diffraction, and lasing.