Vladimir A. Gunyakov

Magnetic-field tunable defect modes in a photonic-crystal/liquid-crystal cell

Light transmission spectrum of a multilayer photonic crystal with a central liquid-crystal defect layer placed between crossed polarizers has been studied. Transmittance was varied due to the magnetically induced reorientation of the nematic director from homeotropic to planar alignment. Two notable effects were observed for this scheme: the spectral shift of defect modes corresponding to the extraordinary light wave and its superposition with the ordinary one. As a result, the optical cell allows controlling the intensity of interfering defect modes by applied magnetic field.

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.

Domain Structures in Nematic Liquid Crystals on a Polycarbonate Surface

Alignment of nematic liquid crystals on polycarbonate films obtained with the use of solvents with different solvations is studied. Domain structures occurring during the growth on the polymer surface against the background of the initial thread-like or schlieren texture are demonstrated. It is established by optical methods that the domains are stable formations visualizing the polymer surface structures. In nematic droplets, the temperature-induced transition from the domain structure with two extinction bands to the structure with four bands is observed. This transition is shown to be caused by reorientation of the nematic director in the liquid crystal volume from the planar alignment to the homeotropic state with the pronounced radial configuration of nematic molecules on the surface. The observed textures are compared with different combinations of the volume LC orientations and the radial distribution of the director field and the disclination lines at the polycarbonate surface.

Electric and Magnetic Field-Assisted Orientational Transitions in the Ensembles of Domains in a Nematic Liquid Crystal on the Polymer Surface

Using electro- and magneto-optical techniques, we investigated orientational transitions in the ensembles of domains in a nematic liquid crystal on the polycarbonate film surface under the conditions of competing surface forces that favor radial and uniform planar alignment of nematic molecules. Having analyzed field dependences of the intensity of light passed through a sample, we established the threshold character of the orientational effects, plotted the calculated intensity versus magnetic coherence length, and compared the latter with the equilibrium length that characterizes the balance of forces on the polymer surface.

Polarization exchange of optical eigenmode pair in twisted-nematic Fabry-Pérot resonator

The polarization exchange effect in a twisted-nematic Fabry-Pérot resonator is experimentally confirmed in the regimes of both uniform and electric-field-deformed twisted structures. The polarization of output light in the transmission peaks is shown to be linear rather than elliptical. The polarization deflection from the nematic director grows from 0^{∘} to 90^{∘} angle and exchanges the longitudinal and transverse directions. Untwisting of a nematic by a voltage leads to the rotation of the polarization plane of light passing through the resonator. The polarization exchange effect allows using the investigated resonator as a spectral-selective linear polarizer with the voltage-controlled rotation of the polarization plane.

Electrically controllable optical switch based on one-dimensional photonic crystal

Electrooptical switch within narrow spectral range has been demonstrated. The switch consisting of one-dimensional photonic crystal with a nematic liquid crystal defect layer was placed between two crossed polarizers. Principle of operation is based on the interference of the ordinary and extraordinary light waves passed across the device due to the electric-field induced coincidence of their wavelengths.

Defect modes in real photonic crystals

It is demonstrated experimentally that amplitudes of defect modes of one-dimensional photonic crystal have maximal value near edges of the photonic band gap while at the centre of the stop-band they are reduced, moreover than more number of layers in photonic crystal, the less the amplitude of defect mode at the center of the PBG. We explain such behavior of defect modes presence of losses at propagation of light in real photonic crystal structures.