V. A. Gunyakov

One-dimensional photonic crystals with a planar oriented nematic layer: Temperature and angular dependence of the spectra of defect modes

Transmission spectra of a one-dimensional photonic crystal (PC) formed by two multilayer dielectric mirrors and a planar oriented layer of 5CB nematic liquid crystal (LC) that is sandwiched between these mirrors and serves as a structure defect are investigated experimentally. Specific features of the behavior of the spectrum of defect modes as a function of the angle of incidence of light on the crystal are studied for two polarizations: parallel and perpendicular to the director of the LC; the director either lies in the plane of incidence or is perpendicular to it. It is shown that, for the configurations considered, the maxima of the defect modes shift toward the short-wavelength region as the tilt angle of incidence radiation increases; this tendency is more manifest for the parallel-polarized component, when the director lies in the plane of incidence. In the latter case, the width of the photonic band gap (PBG) appreciably decreases. The temperature dependence of the polarization components of the transmission spectra of a PC is investigated in the case of normal incidence of light. The spectral shift of defect modes due to the variation of the refractive index of the LC at the nematic-isotropic liquid phase transition point is measured. It is shown that, in real PCs, the amplitude of defect modes decreases when approaching the center of the band gap, as well as when the number of layers in the dielectric mirrors increases. Theoretical transmission spectra of the PCs calculated by the method of recurrence relations with regard to the decay of defect modes are in good agreement with experimental data.

Thermooptical Switching in a One-Dimensional Photonic Crystal

The temperature dependence of the optical transmission spectrum of a one-dimensional multilayer photonic crystal structure with a central defect layer has been studied. The defect was represented by a nematic liquid crystal (5CB) layer with a homeotropic orientation. It is shows that the defect modes exhibit a 10-nm spectral shift due to a change in the refractive index of the liquid crystal in the course of heating-induced transition to the isotropic phase. A comparison of the experimental data to the results of heating-induced transition to the isotropic phase. A comparison of the experimental data to the results of numerical analysis shows the importance of allowance for the decay of defect modes.

Electro- and magneto-optical switching of defect modes in one- dimensional photonic crystals

The transmission spectra of polarized light waves in a photonic crystal/liquid crystal (PC/LC) cell placed between crossed polarizers and controlled by an electric or magnetic field have been studied experimentally and theoretically. Electro- and magneto-optical switching based on the interference of polarized defect modes has been demonstrated. The transmission spectra of the PC/LC cell have been calculated as a function of the voltage applied to the LC layer and the magnetic field strength. The results of the calculations agree well with the experimental data.

Magnetic-field control of the transmission of a photonic crystal with a liquid-crystal defect

The method of modulation of transmittance of a multilayer photonic crystal with a nematic liquid-crystal defect upon the orientational transition from the homeotropic to the planar state is investigated. The orientation of the director of the nematic is controlled by a magnetic field in the B-effect mode. The method of recurrent relations is used for numerical simulation of transmission spectra of the photonic crystal structure under investigation.

Magnetic-field-assisted formation of alignment polymer coatings in liquid crystal cells

We describe a new method for obtaining liquid crystal (LC) layers with planar orientation in plane-parallel cells, which is based on the technology of LC-polymer interface formation in solution under the action of an applied magnetic field. The azimuthal anchoring energy of LC at the polymer surface has been determined by measuring the angle of orientation of the nematic LC director on the substrate surface as a function of the magnetic field. The LC orientation provided by the proposed method is stable, and the anisotropy of LC anchoring is comparable with that achieved using well-known methods of alignment polymer film preparation by rubbing.

Modulation of defect modes intensity by controlled light scattering in a photonic structure with a liquid-crystal component

We describe a new method of defect modes intensity modulation in a multilayer photonic structure containing liquid crystal (LC) inclusions. The proposed method is based on using the regime of electroconvective instability in a nematic LC, which leads to the appearance of a scattered mode sensitive to the polarization type in the optical response. The defect mode intensity is controlled by variation of the angle between the initial planar orientation of the director and the polarization plane of light normally incident onto the photonic structure.

Interference of polarized components of defect modes in a multilayered photonic crystal with an optically anisotropic defect

The specific features of propagation of light through a multilayered photonic crystal containing an optically anisotropic defect are considered in the case when the crystal is arranged between two crossed polarizers. The interference of polarized components of spectrally superposed defect modes is studied. The electrooptic effect of alternate amplification and quenching of the transmitted radiation depending on the difference between the numbers of interfering defect modes is suggested and experimentally demonstrated by the example of a multilayered photonic crystal with a nematic liquid crystal layer as a defect.

Optical bistability in a photonic crystal with a liquid-crystal defect

We investigate experimentally the nonlinear transmission of a photonic crystal with a nematic liquid-crystal defect upon reorientation of the director in a field of cw laser radiation propagating at a certain angle to the sample. At different detunings, we observed the regimes of bistability, differential gain, and optical limiting. The resonance transmission peaks were detuned from the laser radiation wavelength by changing the angle of light incidence onto the sample. The value of nematic nonlinear constant n2 determined from the experimental data is in good agreement with the theoretical estimate.

Orientational changes in the nematic liquid crystal structure on a polymer surface induced by phase separation in a magnetic field

The change in the domain texture of the 5CB nematic liquid crystal on the surface of a polyvinyl butyral gelatinous polymer solution cured in a magnetic field is studied optically. The alignment of the nematic is described by the effective order parameter S* of the polydomain cells. The anchoring energy of a liquid crystal with gelatinous polymer Wϕ = 1 × 10−3 erg/cm2 is determined.

Planar Photonic Crystal with Thermo-Optical Switching

Temperature dependence of the transmission spectrum of one-dimensional multilayer photonic crystal with a liquid crystal defect layer has been studied. Dielectric multilayers have been fabricated by deposition of an alternating stack of SiO2 and ZrO2 layers on the glass substrate. Homeotropic layer of nematic liquid crystal 5CB is introduced between two multilayers as a defect of the periodic structure. The photonic crystal under study forms the first photonic bandgap in the visible range of the transmission spectrum with a set of the localized modes which number and positions are determined by properties of the defect layer and multilayer dielectric mirrors. The red shift of the defect modes about 10 run has been observed at the nematic-isotropic phase transition. The defect modes positions calculated by the transfer matrix method are in a good agreement with experimental data