M. V. Gorkunov

Structural tunability in metamaterials

We propose an efficient approach for tuning the transmission characteristics of metamaterials through a continuous adjustment of the lattice structure and confirm it experimentally in the microwave range. The concept is rather general and applicable to various metamaterials as long as the effective medium description is valid. The demonstrated continuous tuning of a metamaterial response is highly desirable for a number of emerging applications of metamaterials, including sensors, filters, and switches, realizable in a wide frequency range.

Metamaterial tuning by manipulation of near-field interaction

We analyze the near-field interaction between the resonant sub-wavelength elements of a metamaterial, and present a method to calculate the electric and magnetic interaction coefficients. We show that by adjusting the relative configuration of the neighboring split ring resonators it becomes possible to manipulate this near-field interaction, and thus tune the response of metamaterials. We use the results of this analysis to explain the experimentally observed tuning of microwave metamaterials.

Self-tuning mechanisms of nonlinear split-ring resonators

We study both theoretically and experimentally the dynamic tunability of the magnetic resonance of a single nonlinear split-ring resonator with varactor diode at microwave frequencies. We demonstrate different tuning regimes with and without an inductive coil in parallel with the varactor. We show that the coil changes the sign of the nonlinearity and eliminates the memory effect caused by charge accumulation across the varactor. In addition, at higher powers the nonlinear response of the split-ring resonator becomes multivalued, paving a way for creating bistable tunable metamaterials.

Effect of microscopic disorder on magnetic properties of metamaterials.

We analyze the effect of microscopic disorder on the macroscopic properties of composite metamaterials and study how weak statistically independent fluctuations of the parameters of the structure elements can modify their collective magnetic response and left-handed properties. We demonstrate that even a weak microscopic disorder may lead to a substantial modification of the metamaterial magnetic properties, and a 10% deviation in the parameters of the microscopic resonant elements may lead to a substantial suppression of the wave propagation in a wide frequency range. A noticeable suppression occurs also if more than 10% of the resonant magnetic elements possess strongly different properties, and in the latter case the defects can create an additional weak resonant line. These results are of a key importance for characterizing and optimizing novel composite metamaterials with the left-handed properties at terahertz and optical frequencies.

Mean-field theory of a nematic liquid crystal doped with anisotropic nanoparticles

In the framework of molecular mean-field theory we study the effect of nanoparticles embedded in nematic liquid crystals on the orientational ordering and nematic–isotropic phase transition. We show that spherically isotropic nanoparticles effectively dilute the liquid crystal medium and decrease the nematic–isotropic transition temperature. At the same time, anisotropic nanoparticles become aligned by the nematic host and, reciprocally, improve the liquid crystal alignment. The theory clarifies the microscopic origin of the experimentally observed shift of the isotropic–nematic phase transition and an improvement of the nematic order in composite materials. A considerable softening of the first order nematic–isotropic transition caused by strongly anisotropic nanoparticles is also predicted.

Enhanced parametric processes in binary metamaterials

We suggest double-resonant (binary) metamaterials composed of two types of magnetic resonant elements, and demonstrate that in the nonlinear regime such metamaterials provide unique possibilities for phase-matched parametric interaction and enhanced second-harmonic generation.

Tunability of wire-grid metamaterial immersed into nematic liquid crystal

We propose electrically tunable hybrid metamaterial consisting of special wire grid immersed into nematic liquid crystal. The plasmalike permittivity of the structure can be substantially varied due to switching of the liquid crystal alignment by external voltages applied to the wires. Depending on the scale of the structure, the effect is available for both microwave and optical frequency ranges.

Molecular model for de Vries type smectic-A-smectic-C phase transition in liquid crystals

We develop both phenomenological and molecular-statistical theory of smectic- A -smectic- C phase transition with anomalously weak smectic layer contraction. Using a general mean-field molecular model, we demonstrate that a relatively simple interaction potential suffices to describe the transition both in conventional and de Vries type smectics. The theoretical results are in excellent agreement with experimental data. The approach can be used to describe tilting transitions in other soft matter systems.

Extreme optical activity and circular dichroism of chiral metal hole arrays

We report extremely strong optical activity and circular dichroism exhibited by subwavelength arrays of four-start-screw holes fabricated with single-pass focused ion beam milling of freely suspended silver films. Having the fourth order rotational symmetry, the structures exhibit the polarization rotation up to 90° and peaks of full circular dichroism and operate as circular polarizers within certain ranges of wavelengths in the visible. We discuss the observations on the basis of general principles (symmetry, reciprocity, and reversibility) and conclude that the extreme optical chirality is determined by the chiral localized plasmonic resonances.

Phase separation effects and the nematic–isotropic transition in polymer and low molecular weight liquid crystals doped with nanoparticles

Properties of the nematic–isotropic phase transition in polymer and low molecular weight liquid crystals doped with nanoparticles have been studied both experimentally and theoretically in terms of molecular mean-field theory. The variation of the transition temperature and the transition heat with the increasing volume fraction of CdSe quantum dot nanoparticles in copolymer and low molecular weight nematics has been investigated experimentally and the data are interpreted using the results of the molecular theory which accounts for a possibility of phase separation when the system undergoes the nematic–isotropic transition. The theory predicts that the nematic and isotropic phases with different concentrations of nanoparticles may coexist over a broad temperature range, but only if the nanoparticle volume fraction exceeds a certain threshold value which depends on the material parameters. Such unusual phase separation effects are determined by the strong interaction between nanoparticles and mesogenic groups and between nanoparticles themselves.