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

Abstract

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.

Superperiodic Liquid-Crystal Metasurfaces for Electrically Controlled Anomalous Refraction

“…The layer on the upper substrate promotes the vertical alignment of the adjacent LC director, while the one on the bottom substrate is specifically patterned to induce inhomogeneous alignment. Periodic patterns of parallel equally wide stripes are known to produce LC metasurfaces with the suppressed direct transmission in a certain wavelength range, as the light energy is symmetrically scattered in several oblique directions. , Here we explore the possibility to achieve strongly asymmetric diffraction effectively resulting in anomalous refraction by employing more complex patterns of stripes of different widths.…”

“…To fabricate the LC alignment layer according to the generated superperiodic designs, we use the previously developed method of FIB patterning of rubbed PI films ,− described briefly in Methods, Substrate Preparation and FIB Patterning. The superperiodic aligning patterns are imprinted as square-shaped areas filled with parallel stripes of different widths as is exemplified in Figure , where one can also see how close the fabricated patterns follow the theoretical superperiodic design.…”

“…While the pristine rubbed PI areas promote planar LC alignment along the rubbing direction, the areas treated with FIB align the adjacent LC vertically. Relatively simple metasurfaces assemble on periodic micrometer-scale stripe patterns and exhibit distinctly colored optical transmission due to strong scattering of light in several diffraction channels. , Subjecting PI surfaces to the FIB following a preprogrammed path allows imprinting almost any arbitrary submicrometer scale aligning patterns as, for example, those with variable filling factor that precisely induce the desired LC director pretilt at the surface, enabling local control of the transmitted light phase retardation …”

Tunable Metasurfaces: The Path to Fully Active Nanophotonics

“…The reorientation of nematic LC cells surrounding a nanostructure creates an effective change in the refractive index of the surrounding medium. This has been utilized in switchable applications, such as diffraction gratings [ 106,107 ] and lenses. [ 108,109 ] By combining nematic LCs with an all‐dielectric Huygens metasurface, an electrically switchable transparent display at an operating wavelength of 669 nm has been introduced (Figure 4b).…”

Liquid Crystal Devices for Beam Steering Applications

“…Here, some desired diffractive director profiles are induced by the NLC's enclosing surfaces, which can then be overwritten in bulk by a large applied voltage. The creation of diffractive blazed gratings has been achieved using SALs to continuously vary alignment pretilt [161,162] or photoalignment to vary the azimuthal or radial angles [41,163]. These devices can have reasonable efficiency-for example, Honma et al, publishing a 100 µm period blazed grating, created using SALs to steer light to the m = 1 order (0.19 • ) with 68% efficiency [161]; however, the angles are clearly small (similar to refractive devices).…”