Optical Bragg Reflection Research Articles

Giant light deflection via electro-mechanical modulation of liquid crystals

Liquid crystals (LCs) are matter with fluidity and anisotropy and have been used in various electro-optic devices for their capability to modulate the refractive index by voltage. Here, we show that LCs are capable of electro-mechanically modulating light to cause giant light deflection at low voltages (exceeding 64° at 1.0 V). We use a composite material where polymerized cholesteric LC particles that show optical Bragg reflection float in a nematic LC medium. The polymer-particles are elastically coupled with the host director through their surface molecular anchoring and rotate from a face-on to side-on configuration at the Frederik transition. Rigid-body rotation of the reflection plane causes light deflection, which is well reproducible and can be modelled theoretically. Our findings demonstrate the capability of LCs as a micro-electrical-mechanical system platform, which are potentially useful for large-area light-controlling applications.

Resonant Bragg structures based on III-nitrides

Abstract

Optical lattices of excitons in InGaN/GaN quantum well systems

Optical lattices of excitons in periodic systems of InGaN quantum wells with GaN barriers are designed, implemented, and investigated. Due to the collective interaction of quasi-two-dimensional excitons with light and a fairly high binding energy of excitons in GaN, optical Bragg reflection at room temperature is significantly enhanced. To increase the resonance optical response of the system, new structures with two quantum wells in a periodic supercell are designed and implemented. Resonance reflection of 40% at room temperatures for structures with 60 periods is demonstrated.

Modifying optical Bragg reflections from an antiferroelectric liquid crystal with photopolymer networks

By using an achiral monomer and a photoinitiator, we introduced polymer networks into the Sm-C* phase of an antiferroelectric liquid crystal that forms ferro-, ferri-, and antiferroelectric phases. We then investigated the temperature dependence of Bragg wavelengths selectively reflected from these samples and found that the reflection bands shift to shorter wavelengths with increasing polymer concentration. The intermediate and the Sm-C*(A) phases dominate over the Sm-C* phase for polymer concentration ≥4% by weight and the Sm-C* phase disappears completely for a 6% polymer sample.

Direct observation of columnar liquid crystal droplets

While the columnar liquid crystalline phase in suspensions of plate-like colloids is by now well-established, little is known about the pathway leading to the formation of this highly ordered, self-assembled structure. Here, we present direct observations of the morphology and structure of micrometer-sized droplets of the columnar phase formed in the nematic phase in suspensions of colloidal gibbsite plates. From polarized light microscopy and optical Bragg reflection measurements we deduce that these droplets consist of stacks of platelets in a hexagonal arrangement, forming a disk-shaped droplet. We discuss the relation of this droplet structure to the nucleation pathway of the columnar phase and to the anisotropic nematic–columnar interfacial tension.

Optical lattices of InGaN quantum well excitons

We demonstrate optical Bragg lattices formed by quasi-two-dimensional excitons in periodic systems of the InGaN quantum wells separated by the GaN barriers. When the Bragg resonance and exciton-polariton resonance are tuned to each-other, the medium exhibits an exciton-mediated resonantly enhanced optical Bragg reflection. The enhancement factor appeared to be larger than 2 for the system of 60 quantum wells. Owing to a high binding energy and oscillator strength of the excitons in InGaN quantum wells, the resonant enhancement was achieved at room temperature.

Optical Bragg Scattering from a Polymer-Stabilized Anti-Ferroelectric Liquid Crystal

We studied the effect of polymer networks on the temperature dependence of the optical Bragg reflections of an antiferroelectric liquid crystal which forms the chiral ferroelectric (SmC*), intermediate and antiferroelectric (SmC A *) phases. When polymerization is carried out in the SmC* phase, this phase is stabilized at the expense of the intermediate phases and the corresponding Bragg wavelengths follow a non-Landau description. When polymerization is in the SmA phase, the Bragg wavelengths of the SmC* and SmC A * phases are largely temperature independent. The intermediate phases are stabilized at the expense of the SmC* phase in non-radiated samples, which contain traces of the photo-polymer. The range over which the SmC A * is stable is unaffected by polymerization.

The Effect of Electric Fields on Selective Reflection in the SmC* Phase of Two Antiferroelectric Liquid Crystals

ABSTRACT The effects of electric fields on the temperature dependence of optical Bragg reflections in the SmC* phase of two chemically similar antiferroelectric liquid crystal were measured. These Bragg peaks are strong functions of temperature and applied fields resulted in Bragg peaks of reduced intensity and shifts to longer wavelengths. Relatively weak applied fields destroyed the Bragg peaks in both samples. The exact value of this applied field is strongly influenced by the magnitude of the dipole moment in each case.

The Effect of Polymer Stabilization on Phase Transitions in a Series of Antiferroelectric Heterocyclic Esters

ABSTRACT We used optical Bragg reflections to study the effects of photo-polymer networks on the phase transitions of four heterocyclic esters that exhibit various frustrated chiral smectic phases. When polymerization is carried out in the SmC* phase, this phase is stabilized at the expense of the intermediate phases. However, in non-radiated samples, which contained traces of the polymer, the intermediate phases are stabilized at the expense of the SmC* phase. The wavelengths selectively reflected by the SmC*A phase are longer than those reflected by the SmC* phase and the stability of the SmC*A is unaffected by polymerization.

Patterned loading of a Bose-Einstein condensate into an optical lattice

We have developed a technique to control the placement of atoms in an optical lattice by using a superlattice comprising two separately manipulated, periodic optical potentials with commensurate periods. We demonstrate selective loading of Bose-condensed ${}^{87}\mathrm{Rb}$ atoms into every third site of a one-dimensional optical lattice. Our technique provides atoms with wide separation yet tight confinement, useful properties for neutral-atom implementations of quantum computing. Interference of atoms released from the optical lattice and optical Bragg reflection from the atoms reveal the tight confinement and wide separation provided by the patterned filling.

Optical Bragg Reflections from a Series of Antiferroelectric Hetrocyclic Esters

Optical Bragg reflections were used to probe phase transitions of four heterocyclic esters that exhibit various frustrated chiral smectic phases. These transitions showed considerable hysteresis and were detected by the dicscontinuities of the Bragg peaks. For one of these esters, applied fields of ∼ 16 6 V/m completely destroyed the Bragg reflections. Smaller fields resulted in peaks of reduced intensity and shifts to longer wavelengths.

Electro-optic behavior of liquid-crystal-filled silica opal photonic crystals: effect of liquid-crystal alignment.

Photonic crystals made of nematic liquid crystal intercalated into the void space of close-packed silica spheres (synthetic porous opal) exhibit significant electric-field-induced shift of the optical Bragg reflection peak when the liquid crystal has the long molecular axis oriented parallel to the sphere surfaces. No such effect is observed for comparable fields when the long-axis orientation is normal to the sphere surfaces.

Optical Bragg reflection from the TGBA phase

We have studied theoretically the reflection band properties of the twist grain boundary A phase (TGBA). A 4 x 4 matrix approach for the electromagnetic wave propagation is used. At normal incidence, simple analytical expressions are found, while at oblique incidence numerical methods have to be used since no analytical solution is available. The chart of stability of the propagating waves and the reflection spectra are given and discussed. No evidence of the incommensurability properties of the TGBA phases can be found optically. However, the analysis of the output polarization within the Bragg reflection band, on thick enough samples, allows us to determine the physical parameters characterizing the TGBA phase. In particular conditions, we can get the rotation Delta Theta between two homogeneous slabs and the total optical phase phi of one slab. When Delta Theta and phi go to zero, the optical properties of the TGBA phase become identical to those of the cholesteric phase. The essential difference appears when Delta Theta and phi are increased; an infinite number of reflection bands forms each Bragg reflection order, due to the periodicity of the TGBA parameters.

Spatial dispersion of the refractive index in cubic liquid-crystalline blue phases in the vicinity of optical Bragg reflection.

An anomaly in the refractive index is observed in the cubic liquid-crystalline blue phases (BP's) I and II of several cholesteric-nematic mixtures in the vicinity of optical Bragg reflection. Because the reflection wavelength is of the order of the BP lattice constant, this phenomenon can be explained by spatial dispersion of the dielectric tensor.

Far-Field Diffraction Patterns of Scattered Light Arising from Interaction between Acoustic and Light Waves with Complex Amplitude Distributions

Optical Bragg reflection, when both acoustic and incident light waves have complex amplitude distributions, is treated mainly from theoretical point of view on the basis of the simplified integral method. As long as the scattered electric field is negligible compared with the incident electric field, the angular dependence of the electric field scattered by undamped or by damped acoustic waves is given by the product of the Laplace transform of the complex amplitude of the acoustic waves and that of the incident light waves. In the case of light scattering by undamped acoustic waves, these Laplace transforms are reduced to the directivity functions. Some explicit results of the angular dependence of the scattered electric field are given for the convenience of practical uses. A comparison between the theory and experiments is done for a simple case.