This paper reports on the development and evaluation of a reconfigurable millimeter-wave reflectarray based on liquid-crystal (LC) substrate operating in the D-band (105-125 GHz). The reflectarray is composed of a high-impedance surface with a meta-array of 33 × 29 patches on a 2-mm-thick quartz substrate, separated from the ground plane by a 40 μm-thick LC layer. A novel LC composition with low dielectric losses (< 0.003) and high dielectric anisotropy (> 1.3) has been developed for operation at millimeter-waves. The results demonstrate a reflection phase tunability of 210 degrees and low insertion losses of 2.5 dB. Furthermore, the device was demonstrated as a proof of concept for 1D beam-steering applications, exhibiting an operational bandwidth of 12 GHz.

Mode hybridization is a unique way to manipulate the mode inside a fixed cavity or at interface. For example, Tamm plasmon-polariton at solid interface can be spectrally shifted without tuning the interface. Experimental implementation of tunable hybrid Tamm-microcavity modes is reported. The hybrid modes are excited in a one-dimensional photonic crystal bounded with a gold layer by attaching a nematic liquid crystal microcavity. Coupling between Tamm plasmon-polariton and microcavity modes leads to repulsion of their dispersion curves controlled by the refractive index of a liquid crystal and the polarization of incident light. Effective tuning of hybrid modes through heating or applying an external electric field to the liquid crystal layer is demonstrated. The experimentally measured strength coupling value between Tamm and microcavity modes was 20.7 meV.

A photonic crystal microcavity with a tunable quality factor (Q factor) has been implemented on the basis of a bound state in the continuum using the advanced liquid crystal cell technology platform. It has been shown that the Q factor of the microcavity changes from 100 to 360 in the voltage range of 0.6 V.

Orientational structures formed in nematic and cholesteric layers with tangential-conical boundary conditions have been investigated. LC cells with one substrate specifying the conical surface anchoring and another substrate specifying the tangential one have been considered. The director configurations and topological defects have been identified analyzing the texture patterns obtained by polarizing microscope in comparison with the structures and optical textures calculated by free energy minimization procedure of director field and finite-difference time-domain method, respectively. The domains, periodic structures and two-dimensional defects proper to the LC cells with tangential-conical anchoring have been studied depending on the layer thickness and cholesteric pitch.

The voltage dependences of polarization characteristics of light passed through the cholesteric layers with tangential-conical boundary conditions have been investigated. It has been shown that such layers allow turning the polarization azimuth more than 70 ∘ because of the unique untwisting effect of the cholesteric helix, due to free azimuthal rotation of the director on the substrate with conical anchoring. Optical cells under study have some advantages (low control voltage, smooth variation of polarization azimuth, simplicity of design) and can be used as an electrically controlled polarization rotator of white light.

A reorientation of cholesteric liquid crystal with a large helix pitch induced by the electrically controlled ionic modification of the surface anchoring has been studied. In initial state, the cholesteric helix is untwisted completely owing to the normal surface anchoring specified by the cations adsorbed at the substrates. As a result, the homeotropic director configuration is observed within the cell. Under the action of DC electric field, one of the substrates becomes free from the layer of surface active cations, therefore, setting the planar surface anchoring. The latter, in turn, leads to the formation of the hybrid chiral structure. The threshold value and dynamic parameters have been estimated for this process as well as the range of control voltages, which do not allow the electrohydrodynamic instabilities. The twisted hybrid director configuration observed in the experiment has been analysed by means of the simulation of polarisation change of light propagating through the cholesteric layer with asymmetric (planar and homeotropic) surface anchoring on the cell substrates. GRAPHICAL ABSTRACT