Photorefractive polymer-dispersed liquid crystals

We report observation of the photorefractive effect in functionalized polymer-dispersed liquid crystals. The photoconductive properties are provided by the polymer matrix, and the field-dependent refractive-index changes are generated by the dispersed nematic liquid-crystal droplets. A high diffraction efficiency (8%) and a high refractive-index modulation amplitude (Dn = 2 x 10(-3)) are obtained in 53- microm-thick samples with an applied field of 22 V/microm.     

Electrically switchable two-dimensional photonic crystals made of?polymer-dispersed liquid crystals based on?the?Talbot?self-imaging effect

Electrically switchable two-dimensional photonic crystals were demonstrated using polymer-dispersed liquid crystal materials based on the Talbot self-imaging effect of a single photomask. With the photomask subjected to a collimated Ar⁺ laser beam operating at 488 nm, a three-dimensional spatial light intensity pattern was created due to the Talbot self-imaging effect. The spatial light intensity pattern was then recorded inside a cell filled with the liquid crystal/prepolymer mixture to create photonic crystal structures. The surface morphology of the photonic crystals was examined by an atomic force microscopy. It showed square structures with a lattice constant of ∼0.9 μm. The diffraction and electro-optical properties were also presented. This approach shows a simple and fast fabrication.     

Optical edge modes in photonic liquid crystals

An analytic theory of localized edge modes in chiral liquid crystals (CLCs) is developed. Equations determining the edge-mode frequencies are found and analytically solved in the case of low decaying modes and are solved numerically for the problem parameter values typical for the experiment. The discrete edge-mode frequencies specified by the integer numbers n are located close to the stop-band edge frequencies outside the band. The expressions for the spatial distribution of the n’s mode field in a CLC layer and for its temporal decay are presented. The possibilities of a reduction of the lasing threshold due to the anomalously strong absorption effect are theoretically investigated for a distributed feedback lasing in CLCs. It is shown that a minimum of the threshold pumping wave intensity may be reached, generally, for the pumping wave propagating at an angle to the helical axis. However, for lucky values of the related parameters, it may be reached for the pumping wave propagating along the helical axis. The lowest threshold pumping wave intensity occurs for the lasing at the first low-frequency band-edge lasing mode and the pumping wave propagating at an angle to the spiral axis corresponding to the first angular absorption maximum of the anomalously strong absorption effect at the high-frequency edge of the stop band. The study is performed in the case of the average dielectric constant of the liquid crystal coinciding with the dielectric constant of the ambient material. Numerical calculations of the distributed feedback lasing threshold at the edge-mode frequencies are performed for typical values of the relevant parameters.     

Optical properties of mesoporous one-dimensional photonic crystals

The systematic features in the reflectance spectra of broadband optical radiation from the surface of one-dimensional photonic crystals based on mesoporous structures are presented. The dispersion dependence for electromagnetic waves in the mesoporous one-dimensional photonic crystal is established. On its basis, reflectance and transmittance spectra are calculated for particular structures, and the microscopic parameters of the samples under study are determined. The developed theory is used to explain the shape of reflectance spectra in mesoporous photonic crystals based on silicon, quartz, and aluminum oxide.     

Optical properties of photonic crystals filled with iodine vapor

Properties of electromagnetic waves in the photonic crystal with pores filled with iodine vapor are considered. The conditions of the permittivity resonance appearance in the visible region of the spectrum, caused by the electronic transition in iodine molecules, are determined. For the resonant photonic crystal under study, dispersion curves, group velocities of electromagnetic waves, and reflection spectra are calculated for various globule diameters.     

Tunable band gaps in electro-optical photonic bi-oriented crystals

Electrically induced birefringence is studied in photonic bi-oriented crystals in terms of molding lightflow in optical devices. In photonic bi-oriented crystals, misorientation of dielectric anisotropic grains results in a dielectric contrast at the grain boundaries. The translational periodicity of the optical constants depends upon a regular network of twisted dielectrics. Due to the anisotropy of the bicrystalline structure the direction of light propagation determines the dielectric contrast at the grain boundaries. In a specific crystallographic arrangement the optical properties of the bi-oriented crystal can be tuned by the electro-optical effect: the periodic dielectric contrast is electrically induced and photonic bandgaps are generated by applying external electric fields. The geometrical requirements for tunable photonic bicrystals are evaluated based on materials employed for electro-optical applications. Tunable photonic bi-oriented crystals may be candidates for fast optical switches, modulators and multiplexers in the optical communication network. Received: 5 July 2001 / Revised version: 3 August 2001 / Published online: 15 October 2001     

Optical properties of one-dimensional photonic crystals containing graded materials

The optical properties of an one-dimensional photonic crystal containing graded materials are studied theoretically. The graded layers have space dispersive permittivity and magnetic permeability which vary along the direction perpendicular to the surface of the layer. The gradation profiles of permittivity are studied in detail. We show that the structure possesses forbidden band gaps in its transmission spectra and the gradation profiles of permittivity affect the band gaps significantly. For the exponential gradation profile ε₁(x) = α e^βx, the number of the band gaps increases and the total frequency region corresponding to the gaps becomes large with increasing parameter β. On the other hand, the position of band gaps can be changed by the adjustment of the gradation profiles even if possessing same volume-average permittivity in the graded layers. Therefore, we can achieve suitable photonic band gaps by choosing gradation profiles of permittivity.     

Optical properties of one-dimensional soft photonic crystals with ferrofluids

We review the recent theoretical study on the optical properties of one-dimensional soft photonic crystals (1D SPCs) with ferrofluids. The proposed structure is composed of alternating ferrofluid layers and dielectric layers. For the ferrofluid, single domain ferromagnetic nanoparticles can align to a chain under the stimuli of an external magnetic field, thus changing the microstructure of the system. Meanwhile, nonlinear optical responses in ferrofluids are also briefly reviewed.     

Thermal polarization properties of thick anisotropic diffraction holograms recorded in polymer-dispersed liquid crystals

The diffraction and polarization properties of thick anisotropic holographic gratings recorded in polymer-dispersed liquid crystals are studied experimentally and theoretically. Such gratings are anisotropic and sensitive to temperature variations and also exhibit a strong dependence of the diffraction properties on the incident radiation polarization because of the presence of liquid-crystal molecules. The dependences of the diffraction efficiency and diffracted beam polarization orientation on the diffraction grating temperature and incident beam linear polarization are obtained for the case of Bragg incidence. The same dependences are derived using the known experimental temperature curves of the liquid crystal permittivity. The experimental data are in agreement with the theoretical calculations. The feasibility of controlling the diffraction characteristics of the grating in a wide temperature interval is demonstrated.     

Holographic diffraction gratings using polymer-dispersed ferroelectric liquid crystals

By controlling the morphology of holographic polymer-dispersed ferroelectric liquid crystals (FLCs), highly aligned FLC domains are obtained for diffractive optical applications. Rapid, thresholdless switching is observed for various grating pitch sizes between approximately 3 and approximately 12 microm. A simple phenomenological model is presented encompassing a distribution of domain sizes and an effective field that stabilizes the FLC domains to reflect the observed thresholdless switching and optical behavior.     

Optimal tunability of waveguides based on silicon photonic crystals infiltrated with liquid crystals

In this work we study the optimization of the tunability range in waveguides based on two-dimensional silicon photonic crystal infiltrated with liquid crystal. The analyzed structure consists of a two-dimensional silicon photonic crystal with a triangular lattice of circular holes where a line of scatterers in the direction Γ–K has been replaced by a line of circular holes with different radius infiltrated by E7 liquid crystal. To this end, we use the plane-wave expansion method considering anisotropy and modelling supercells to account for the lattice defects that define the waveguide. Finally we study the field distributions of the guided modes in order to analyze their symmetries and confinement.     

Orientational photorefractive effects observed in polymer-dispersed liquid crystals

We present a novel type of photorefractive polymer-dispersed liquid crystal (PDLC) that contains a polymer, low-molecular liquid crystals (LC's), and a photoconductive sensitizer. Photorefractivity is demonstrated by two-beam-coupling experiments with an external dc field. This material shows highly sensitive photorefractivity, and a purely refractive-index grating (Dn >/= 3.0 x 10(-3)) is generated by irradiation of interference light (lambda=632.8 nm) . The grating in the photorefractive PDLC's shows strong anisotropy, and the dynamics depends strongly on the composition. In the case of a 40:60 weight ratio of polymer:LC's, the photorefractive PDLC's present a stable memory effect.     

Optical properties of photonic crystals filled with metal quantum dots

The features of optical-range electromagnetic wave passage through the photonic crystal filled with metal quantum dots are studied. The possibility of localizing electromagnetic radiation within the photonic crystal with a nonequilibrium temperature increase in a small localization region to 10³–10⁶ K under femtosecond excitation by a laser pulse with an energy of 10^?3 J was shown.     

Optical properties of chiral photonic crystals with graded modulation parameters

The oblique transmission of light through a layer of a chiral photonic crystal with graded modulated parameters is considered. The problem is solved by the Ambartsumian’s layer addition method. Dependences of amplitude characteristics on the wavelength at different incident angels are presented for two cases: namely, for the case of a chiral photonic crystal with a linearly varied spatial period of modulation and in the case of a chiral photonic crystal with a linearly varied spatial depth of modulation. The case of a minimal influence of dielectric boundaries is considered. It is shown that in both cases a broadening of the photonic forbidden band gap takes place.     

Optical echo in photonic crystals

The dynamics of a photonic wavepacket in the effective oscillator potential is studied. The oscillator potential is constructed on the base of one-dimensional photonic crystal with a period of unit cell adiabatically varied in space. The structure has a locally equidistant discrete spectrum. This leads to an echo effect, i.e., the periodical reconstruction of the packet shape. The effect can be observed in the nonlinear response of the system. Numerical estimations for porous-silicon based structures are presented for a femtosecond Ti:sapphire laser pump. The text was submitted by the authors in English.     

Multimode lasing from the microcavity of an octagonal quasi-crystal based on holographic polymer-dispersed liquid crystals

An eightfold photonic quasi-crystal (PQC) sample is fabricated holographically using two-beam interference with multi-exposure based on polymer-dispersed liquid crystals. The transmission spectra from the finite-difference time-domain (FDTD) simulation prove the photonic stop band of the rotational symmetry structure of the sample. The resonant mode of the circular microcavity formed in the PQC is calculated. Amplified spontaneous emission and multimode lasing action are demonstrated from the pumped laser-dye-doped PQC microcavity using a Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) pulse laser.     

Acoustic properties of globular photonic crystals based on synthetic opals

Acoustic properties of globular photonic crystals based on synthetic opals and composed of closely packed SiO₂ globules about 200 nm in diameter are theoretically investigated. Dispersion characteristics of the investigated samples are numerically simulated, and the group velocity of acoustic waves and the effective mass of acoustic phonons are found. It is shown that phononic bandgaps in these photonic crystals are within the gigahertz frequency range. The effective mass of the acoustic phonons corresponding to the edges of the bandgaps is found, and a possibility that bound states of acoustic phonon pairs, biphonons, manifest themselves in the light scattering spectra is discussed.     

光子晶体的光学特性与近场光学测量表征

光子晶体具有独特而优异的光学特性及广泛的应用前景。介绍了光子晶体的概念、应用、发展,着重讨论了光子晶体的光子禁带、光子局域以及其它光学特性。从近场光学的原理和实验技术出发阐述了近场光学测量表征光子晶体的方法,对于光子晶体的研究具有重要的意义。     

Optical properties of chiral photonic crystals with an anisotropic defect layer

Properties of defect modes in chiral photonic crystals with an anisotropic defect are considered. The influence of the defect layer thickness, its location in the crystal and orientation of its optical axis, as well as of the chiral photonic crystal thickness on the properties of defect modes is studied. It is shown that at certain values of the defect layer thickness the medium loses its main property, namely, the polarization dependence of the diffraction reflection. At certain thicknesses of the defect layer this system transforms from the source of the right circular polarization into the source of the left circular polarization, if the layer position in the chiral photonic crystal changes.