I. P. Pinkevych

Two-Beam Energy Exchange in a Hybrid Photorefractive Inorganic-Cholesteric Cell

We develop a theoretical model to describe two-beam energy exchange in a hybrid photorefractive inorganic-cholesteric cell. A cholesteric LC cell is placed between two inorganic photorefractive windows. Weak and strong light beams are incident on the LC cell. The interfering light beams induce a periodic space-charge field in the photorefractive windows. This penetrates into the LC, inducing a diffraction grating written on the LC director. The theory calculates the energy gain of the weak beam, as a result of its interaction with the pump beam within the diffraction grating. In the theory, the flexoelectric mechanism for electric field-director coupling is a more important than the LC static dielectric anisotropy coupling. The flexoelectric polarization in the bulk LC follows from the initial director pretilt at the cell substrates and is the main physical mechanism governing the magnitude of the director grating and the two-beam coupling. The LC optics is described in the Bragg regime. Theoretical results for exponential gain coefficients have been compared with experimental results for hybrid cells filled with cholesteric mixtures TL205/CB15 and BL038/CB15. In order to reconcile theory and experiment, we require that (a) the magnitude of the director grating must be cubic rather than linear in the space-charge field, and (b) near the cell surface, nematic ordering must dominate. Within this paradigm, we are able to fit experimental data to theory for both cholesteric mixtures, subject to the use of some fitting parameters.

Beam coupling in hybrid photorefractive inorganic-cholesteric liquid crystal cells: Impact of optical rotation

We develop a theoretical model to describe two-beam energy exchange in a hybrid photorefractive inorganic-cholesteric cell. A cholesteric layer is placed between two inorganic substrates. One of the substrates is photorefractive (Ce:SBN). Weak and strong light beams are incident on the hybrid cell. The interfering light beams induce a periodic space-charge field in the photorefractive window. This penetrates into the cholesteric liquid crystal (LC), inducing a diffraction grating written on the LC director. In the theory, the flexoelectric mechanism for electric field-director coupling is more important than the LC static dielectric anisotropy coupling. The LC optics is described in the Bragg regime. Each beam induces two circular polarized waves propagating in the cholesteric cell with different velocities. The model thus includes optical rotation in the cholesteric LC. The incident light beam wavelength can fall above, below, or inside the cholesteric gap. The theory calculates the energy gain of the weak beam, as a result of its interaction with the pump beam within the diffraction grating. Theoretical results for exponential gain coefficients are compared with experimental results for hybrid cells filled with cholesteric mixture BL038/CB15 at different concentrations of chiral agent CB15. Reconciliation between theory and experiment requires the inclusion of a phenomenological multiplier in the magnitude of the director grating. This multiplier is cubic in the space-charge field, and we provide a justification of the q-dependence of the multiplier. Within this paradigm, we are able to fit theory to experimental data for cholesteric mixtures with different spectral position of cholesteric gap relative to the wavelength of incident beams, subject to the use of some fitting parameters.

Cloaking by shells with radially inhomogeneous anisotropic permittivity

We model electromagnetic cloaking of a spherical or cylindrical nanoparticle enclosed by an optically anisotropic and optically inhomogeneous symmetric shell, by examining its electric response in a quasi-static uniform electric field. When the components of the shell permittivity are radially anisotropic and power-law dependent (ε~r(m)) whereris distance to the shell center, and m a positive or negative exponent which can be varied), the problem is analytically tractable. Formulas are calculated for the degree of cloaking in the general case, allowing the determination of a dielectric condition for the shells to be used as an invisibility cloak. Ideal cloaking is known to require that homogeneous shells exhibit an infinite ratio of tangential and radial components of the shell permittivity, but for radially inhomogeneous shells ideal cloaking can occur even for finite values of this ratio.

Surface plasmon absorption in MoS2 and graphene-MoS2 micro-gratings and the impact of a liquid crystal substrate

The absorption coefficients of a far-infrared wave are calculated at normal incidence for MoS2 and graphene-MoS2 micro-ribbon gratings placed between a nematic LC and an isotropic dielectric medium. Maxima in the absorption spectra, which are related to the excitation of the surface plasmons in micro-ribbons of these gratings, are observed. The spectral position of absorption maxima depends on the grating spacing, micro-ribbon width, and conductivity of the ribbons. The impact of the 2D electron concentration of the MoS2 ribbons on the plasmon bands is different for a MoS2-grating versus a graphene-MoS2 grating. The influence of the LC orientational state on the absorption spectra of the gratings enables the manipulation of the absorption peak magnitude.

Liquid crystal control of the plasmon resonances at terahertz frequencies in graphene microribbon gratings

We theoretically study the influence of the liquid crystal (LC) orientational state on the absorption, reflection, and transmission spectra of a graphene microribbon grating placed between a nematic LC and an isotropic dielectric substrate. We calculate the absorption, reflection, and transmission coefficients at normal incidence of a far-infrared transverse magnetic wave (THz) and show that control of the orientational state of the LC layer enables the manipulation of the magnitude of the absorption and reflection maxima. The influence the LC orientational state on the plasmonic resonance increases with increasing the isotropic substrate dielectric constant and the graphene microribbon width to grating spacing ratio.

Controlling hyperbolic metamaterials with a core-shell nanowire array [Invited]

We studied a system of a core-shell nanowire array in the liquid crystal (LC) matrix 5CB supposing that the nanowire shell permittivity is anisotropic with radially power-law dependent components. The nanowire core material is either silver or gold. We calculated the effective permittivity tensor of the core-shell nanowire array in the LC matrix, and show that the system under consideration can possess two hyperbolic metamaterial (HMM) areas: where the real part of the effective permittivity component parallel to the nanowire is negative, and the real part of the effective permittivity component perpendicular to the nanowire is negative. Dependence of the frequency position for these areas on the core-shell nanowire parameters and the LC matrix orientational state is studied.

Two-wave energy exchange in photorefractive hybrid cell with bent-core liquid crystal

ABSTRACT Energy gain of weak signal interacting with a strong pump at the director grating in photorefractive hybrid cell is studied. The cell is filled by bent-core cholesteric LC with small twist/bend elastic constants. The director grating is written by the space-charge field penetrating into LC from the photorefractive substrate. Due to a decreased twist/bend elasticity of the LC, the grating magnitude increases causing an increase of the energy gain. Influence of the elastic constants ratio, flexoelectric coefficients, and cholesteric pitch is studied. We show that the gain coefficient can reach higher values in bent-core cholesterics compared to conventional ones.

Electro-optical effect in a planar nematic cell with electric field sensitive boundary conditions

ABSTRACT Dynamics of planar-planar director reorientation under an electrostatic field is studied in nematic cell with the director easy axis gliding. Two cases of the electric field interaction with easy axis at the cell substrate is assumed, linear and quadratic in the electric field. In the case of quadratic interaction the director instability has a threshold. Dependence of the director dynamics on the anchoring energy and the electric field interaction with the easy axis are studied. We calculated the light transmittance of the cell and show that it reflects the director dynamics.

Two beam energy exchange in hybrid liquid crystal cells with photorefractive field controlled boundary conditions

We develop a theory describing energy gain when two light beams intersect in a hybrid nematic liquid crystal (LC) cell with photorefractive crystalline substrates. A periodic space-charge field induced by interfering light beams in the photorefractive substrates penetrates into the LC layer and reorients the director. We account for two main mechanisms of the LC director reorientation: the interaction of the photorefractive field with the LC flexopolarization and the director easy axis at the cell boundaries. It is shown that the resulting director grating is a sum of two in-phase gratings: the flexoelectric effect driven grating and the boundary-driven grating. Each light beam diffracts from the induced gratings leading to an energy exchange between beams. We evaluate the signal beam gain coefficient and analyze its dependence on the director anchoring energy and the magnitude of the director easy axis modulation.

Liquid Crystal Control of Surface Plasmon Resonance Sensor Based on Nanorods

We analyze a theoretical model of the five layer nanorod-mediated surface plasmon resonance (SPR) sensor comprising a nematic LC layer. The light propagation through the layered system is studied solving the Fresnel equations under SPR conditions. We calculate the light reflectance angular spectrum and show that control of the orientational state of the LC layer enables us to manipulate position of the reflectance curve minimums, their depth and sensitivity. It allows one to choose an interval of the light incidence angles convenient for work without need in angular tuning of sensor at replacing of the analyte.