Stepan Ya. Vetrov

Voltage-induced defect mode coupling in a one-dimensional photonic crystal with a twisted-nematic defect layer.

Defect modes are investigated in a band gap of an electrically tunable one-dimensional photonic crystal infiltrated with a twisted-nematic liquid crystal. Their frequency shift and interference under applied voltage are studied both experimentally and theoretically. We deal with the case where the defect layer thickness is much larger than the wavelength (i.e., the Mauguin condition). It is shown theoretically that the defect modes could have a complex structure with elliptic polarization. Two series of polarized modes are coupled with each other and exhibit an avoided crossing phenomenon in the case of opposite parity.

Chiral Optical Tamm States: Temporal Coupled-Mode Theory

The chiral optical Tamm state (COTS) is a special localized state at the interface of a handedness-preserving mirror and a structurally chiral medium such as a cholesteric liquid crystal or a chiral sculptured thin film. The spectral behavior of COTS, observed as reflection resonances, is described by the temporal coupled-mode theory. Mode coupling is different for two circular light polarizations because COTS has a helical structure replicating that of the cholesteric. The mode coupling for co-handed circularly polarized light exponentially attenuates with the cholesteric layer thickness since the COTS frequency falls into the stop band. Cross-handed circularly polarized light freely goes through the cholesteric layer and can excite COTS when reflected from the handedness-preserving mirror. The coupling in this case is proportional to anisotropy of the cholesteric and theoretically only anisotropy in magnetic permittivity can ultimately cancel this coupling. These two couplings being equal result in a polarization crossover (the Kopp–Genack effect) for which a linear polarization is optimal to excite COTS. The corresponding cholesteric thickness and scattering matrix for COTS are generally described by simple expressions.

Enhanced light absorption with a cholesteric liquid crystal layer

We discuss the trapping features of cholesteric liquid crystal as a self-organized photonic crystal and suggest it for absorption improvement. The imperfection of the absorbing surface is manifested by the reflection originating from the indispensable impedance mismatch. The reflection can be suppressed by an additional surface coating to reflect the light back to the absorber once again. In the simple consideration this reflection loop is possible only twice. For a right-handed CLC coating, the boundary reflection switches the light polarization from left to right circular in the first loop and it switches in the second loop back to the left circular polarization before the emerging light finally escapes from the absorbing process. The enhancement of light absorption is possible in the frequency range as large as the photonic bang gap of the cholesteric layer, which can be adjusted as desired.

Localised optical states in a structure formed by two oppositely handed cholesteric liquid crystal layers and a metal

ABSTRACT A new model for observing localised optical states in a structure containing a cholesteric liquid crystal is proposed. The dependence of the mode frequency on the angle between directors at the interface of oppositely handed cholesterics is established. The effect of the metal layer thickness on localisation of light at the localised state frequency is studied. Polarisation features of the transmission spectrum are investigated. The possibility of experimental implementation of the proposed structure is discussed. GRAPHICAL ABSTRACT

Tunable hybrid Tamm-microcavity states

Spectral manifestations of hybrid Tamm-microcavity modes in a 1D photonic crystal bounded with a silver layer and containing a nematic liquid crystal layer working as a microcavity have been studied using numerical simulation. It is demonstrated that the hybrid modes can be effectively tuned owing to the high sensitivity of the liquid crystal to the temperature and external electric field variations. It is established that the effect of temperature on the transmission spectrum of the investigated structure is most pronounced at the point of the phase transition of the liquid crystal to the isotropic state, where the refractive index jump is observed.

Localized optical modes in a defect-containing liquid-crystal structure adjacent to the metal

The possibility of the existence of localized optical modes with maximum field intensity at the interface between the metal and cholesteric liquid crystal is theoretically grounded. The system comprises a defect-containing cholesteric liquid crystal and a metal layer. It is established that the localization occurs at almost any defect thickness rather than only at the half-wave thickness, at which the loss of the polarization dependence of diffraction reflection and the absence of field localization are observed for the structure without the metallic layer. It is demonstrated that the spectral properties can be controlled by external fields affecting the liquid crystal. At large thicknesses of the cholesteric liquid crystal between the defect and the metallic layer, the spectrum contains the pronounced peaks corresponding to the edge modes. The results obtained open new opportunities for controlling the transmittance spectrum, polarization, and localization of light in optoelectronic elements based on the investigated system.

Quasiperiodic one-dimensional photonic crystals with adjustable multiple photonic bandgaps

We propose an elegant approach to produce photonic bandgap (PBG) structures with multiple photonic bandgaps by constructing quasiperiodic photonic crystals (QPPCs) composed of a superposition of photonic lattices with different periods. Generally, QPPC structures exhibit both aperiodicity and multiple PBGs due to their long-range order. They are described by a simple analytical expression, instead of quasiperiodic tiling approaches based on substitution rules. Here we describe the optical properties of QPPCs exhibiting two PBGs that can be tuned independently. PBG interband spacing and its depth can be varied by choosing appropriate reciprocal lattice vectors and their amplitudes. These effects are confirmed by the proof-of-concept measurements made for the porous silicon-based QPPC of the appropriate design.

Optical Tamm states at the interface between a photonic crystal and a gyroid layer

The spectral properties of a one-dimensional photonic crystal bounded by an absorbing plasmonic gyroid layer are investigated. The gyroid material is a foam-like metallic three-dimensionally periodic curved film with zero mean curvature at every point. We calculate the transmittance and reflectance spectra at the normal and oblique light incidence. The possibility of the Tamm state formation at the interface between a photonic crystal and a plasmonic gyroid layer caused by the negative real part of the gyroid permittivity is demonstrated for the first time. Specific features of field localization at the Tamm state frequencies are discussed. It is shown that the spectral and polarization properties of the optical Tamm states are highly sensitive to the change in the angle of incidence and in the refractive index of a medium filling the gyroid cavities.