Photorefractive Crystal Research Articles

Coupled spatial periodic waves and solitons in the photovoltaic photorefractive crystals

The evolution of spatial solitons in the photovoltaic photorefractive crystal can be governed by the specific coupled nonlinear Schrodinger equations. Under the photovoltaic field with the external bias field, the coupled cn–sn-type periodic wave solution and the corresponding photorefractive bright–dark soliton pair were constructed to describe the evolution of beam. The influence of the external bias field on solitonic dynamics is analyzed. In the photovoltaic crystal, coupled sn–cn-type, sn–dn-type periodic wave solutions, solution constructed by products of elliptic functions and the corresponding dark–bright soliton pair and coupled double-peaked soliton solutions are found to describe the evolution of a spatial-phase-modulated photovoltaic soliton and a non-phase-modulated beam.

Influence of Photoelastic and Piezoelectric Effects on the Energetic Characteristics of Transmission and Reflection Holograms in a Photorefractive Crystal

The surface of the normal component of the inverse tensor of the dielectric permittivity of a cubic photorefractive piezoelectric Bi12SiO20 crystal was constructed. A volume phase holographic grating with a wave vector along the $$ \left[1\overline{1}1\right] $$ crystallographic axis was recorded in this crystal. It is established that the combined action of the photoelastic and piezoelectric effects leads to an increasing normal component of the inverse tensor of the dielectric permittivity along the $$ \left[1\overline{1}1\right] $$ direction and a decreasing normal component of the inverse tensor of the dielectric permittivity in the perpendicular direction. It is revealed that the anisotropy of the contribution of the photoelastic and piezoelectric effects causes an increase in the output energetic characteristics (optimized in the azimuth of polarization of light waves) for the transmission hologram and their decrease for the reflection hologram.

Photoinduced Absorption and Pulsed Recording of Dynamic Holograms in Bismuth Silicate Crystals

The dynamics of photoinduced absorption and holographic-grating recording in photorefractive crystals of bismuth silicate is studied. It is shown that, with the use of nanosecond laser pulses and of the intensity on the order of 1 MW/cm2 or higher, the induced absorption due to population of the short-lived trapping levels, with characteristic relaxation times about several milliseconds or tens of milliseconds, is the case. Recording of dynamic holograms has been realized in these conditions in bismuth silicate crystals. Two mechanisms of holographic-grating recording, with the lifetimes differing by three orders of magnitude, are established. At relatively low intensities, about 1 MW/cm2 or lower, the medium response is determined by a photorefractive mechanism of nonlinearity, with relaxation times of several seconds. At the intensities exceeding 5 MW/cm2, one can observe a fast (ms relaxation times) component that may be associated with population of the short-lived traps. It is shown that the contribution of each mechanism is greatly dependent on the intensity of laser radiation and, for the intensities above 10–15 MW/cm2, the short-lived traps having millisecond lifetimes play the decisive role.

Generation of regular optical patterns and photonic structures by a single Gaussian beam in a photorefractive LiNbO3:Fe crystal

We report 2D micrometric scale regular optical patterns formation induced by a laser beam propagating through a Fe doped lithium niobate (LN:Fe) crystal. A single mode CW laser radiation at 632.8 nm wavelength and beam powers from 0.5 to 10 mW and LN:Fe samples with 0.03 wt% Fe concentration and thicknesses of 4 mm and 10 mm were used in the experiments. The observed phenomenon of optical patterns formation in photorefractive crystal is explained with the light-induced complex refractive index variation with a central minimum (negative lens) and symmetric maxima (positive lenses) on each side due to photovoltaic effect in the crystal. Light-induced complex lenses produce beam defocusing from the beam center and focusing on the periphery. Mutual interference of the beams leads to a regular optical pattern formation. Regular optical pattern, in its turn, leads to the formation of quasi-crystalline photonic structure in LN:Fe crystal. Experimental results are supported by numerical simulations.

Generation and manipulation of spiral beams in photovoltaic photorefractive crystal twinning with mirroring diffusion management

The dynamics characteristics of spiral beams composed of two coherent solitons in photovoltaic photorefractive crystal twinning with mirroring diffusion management are investigated. The numerical simulations show that the orbits of spiral beams depend on the co-effect of the diffusion and coherence effects, and that spiral beams are forcedly rotated around the propagation axis set inside the twinning plane. Moreover, when the incident position of two coherent solitons relative to the twinning plane is reversed, the rotation direction of spiral beams is reversed accordingly. Furthermore, when the initial separation between (peak intensity of) two coherent solitons is adjusted, the rotation speed of spiral beams is adjusted accordingly.

Gap solitons supported by an optical lattice in biased photorefractive crystals having both the linear and quadratic electro-optic effect

Abstract We investigate the existence and stability of gap solitons supported by an optical lattice in biased photorefractive (PR) crystals having both the linear and quadratic electro-optic effect. Such PR crystals have an interesting interplay between the linear and quadratic nonlinearities. Gap solitons are predicted for the first time in such novel PR media. Taking a relevant example (PMN-0.33PT), we find that the gap solitons in the first finite bandgap are single humped, positive and symmetric solitons while those in the second finite band gap are antisymmetric and double humped. The power of the gap soliton depends upon the value of the axial propagation constant. We delineate three power regimes and study the gap soliton profiles in each region. The gap solitons in the first finite band gap are not linearly stable while those in the second finite band gap are found to be stable against small perturbations. We study their stability properties in detail throughout the finite band gaps. The interplay between the linear and quadratic electro-optic effect is studied by investigating the spatial profiles and stability of the gap solitons for different ratios of the linear and quadratic nonlinear coefficients.

Surface Waves at the Boundary of a Medium with a Refractive Index Switching and a Crystal with the Diffusion-Type Photorefractive Nonlinearity

It is formulated a model, within which, regularities for formation of nonlinear surface waves with a specific field structure are described. It is shown that there are two new types of waves of chosen polarization, which propagate along the contact of a photorefractive crystal with diffusion nonlinearity and a nonlinear optical medium with switching. Nonlinearity in this medium is modelled by a stepwise changing the refractive index depending on the field amplitude. The field structure in the obtained surface waves consists of three components in different regions of contacting media. The propagation of a localized light beam along the boundary at the near-boundary region results in formation of an area of a finite width, which has a refractive index that is different from the remaining medium. The field exponentially decreases in the region behind this domain when moving away from the near-boundary region. In a photorefractive crystal, the field can decrease with oscillations or without oscillations. The dependence of the propagation constant on optical characteristics of media and guiding parameters is derived in the explicit analytical form.

Generation and manipulation of bright spatial bound-soliton pairs under the diffusion effect in photovoltaic photorefractive crystals**Project supported by the National Natural Science Foundation of China (Grant Nos. 61875058, 11874018, 11974006, and 61378036).

The generation and propagation characteristics of bright spatial bound-soliton pairs (BSPs) are investigated under the diffusion effect in photovoltaic photorefractive crystals by numerical simulation. The results show that two coherent solitons, one as the signal light and the other as the control light, can form a BSP when the peak intensity of the control light is appropriately selected. Moreover, under the diffusion effect, the BSP experiences a self-bending process during propagating and the center of the BSP moves on a parabolic trajectory. Furthermore, the lateral shift of the BSP at the output face of the crystal can be manipulated by adjusting the peak intensity of the control light. The research results provide a method for the design of all-optical switching and routing based on the manipulation of the lateral position of BSPs.

Construction of photorefractive photonic quasicrystal microstructures by twisted square lattices.

A convenient method to fabricate two-dimensional photonic quasicrystal microstructures was experimentally demonstrated by using a rotatable four-wedge prism. Two-dimensional eightfold symmetric quasicrystal microstructures are formed by two groups of twisted square lattices in a photorefractive crystal. The experimental devices of this method are simple and stable without complicated optical adjustment equipment. Optical-induced quasicrystal microstructures are analyzed and verified by magnified imaging and far-field diffraction pattern imaging. The method can be extended to fabricate more complex quasicrystal and moiré lattice microstructures. We numerically demonstrate that this method can be used to fabricate other complex photonic microstructures by using different multi-wedge prisms and adjusting the rotation angle properly.

Light propagation in aperiodic photonic lattices created by synthesized Mathieu–Gauss beams

We investigate light propagation in a two-dimensional aperiodic refractive index lattice realized using the interference of multiple Mathieu–Gauss beams. We demonstrate experimentally and numerically that such a lattice effectively hinders linear light expansion and leads to light localization, compared to periodic photonic lattices in a photorefractive crystal. Most promisingly, we show that such an aperiodic lattice supports the nonlinear confinement of light in the form of soliton-like propagation that is robust with respect to changes in a wide range of intensities.

Slow light of dark pulses in a photorefractive crystal

This paper demonstrates how dark pulses may be decelerating in a photorefractive crystal at room temperature. The authors show the conditions that allow to achieve delay larger than the input dark pulse width. When the input pulse duration is close to the crystal response time, the fractional delay is close to unity.

Surface Waves at the Interface of a Photorefractive Crystal and a Medium with Positive Kerr Nonlinearity

The types of nonlinear surface waves with extraordinary polarization arising at the interface between a photorefractive crystal and a medium with positive Kerr nonlinearity are described. It has been shown that nonlinear surface waves of two types with an asymmetric profile can exist in such a system. Waves of the first type damp when moving away from the interface into the depth of the photorefractive and Kerr crystals without oscillations, while waves of the second type damp into the depth of a photorefractive crystal with oscillations. Dispersion relations have been obtained and the conditions for the existence of all described types of waves have been written depending on the optical characteristics of the crystals.

Defect solitons supported by two-dimensional lattice potentials in biased centrosymmetric photorefractive crystals with saturable self-focusing nonlinearity

We investigate the defect solitons (DSs) in square photonic lattices in biased centrosymmetric photorefractive crystals with saturable self-fousing nonlinearity. We reveal the existence and stable regions of DSs due to the changing defect strength; in particular, DSs appear to be unstable in the first bandgap with high power where the repulsive defect is, which is usually stable in non-centrosymmetric media. The DSs existence and stable regions reduce with the increase of the attractive defect strength in the semi-infinite bandgap. For a repulsive defect, DSs can be profoundly affected by their defect potentials and power.

Influence of Inverse Piezoelectric Effect, Photoelasticity, and Optical Activity on the Diffraction Efficiency of Transmissing Holograms in Photorefractive Crystal Bi12SiO20

The results of a theoretical study of the dependence of the diffraction efficiency of a transmission hologram formed in a sample of a Bi12SiO20 photorefractive crystal on the orientation angle, specific rotation of the crystal, and its thickness are presented. The theoretical model takes into account linear electrooptical, inverse piezoelectric and photoelastic effects. It is shown that with a change in the sign of the specific rotation of the crystal, there can be a shift of the maximums of diffraction efficiency relative to the orientation angle.

Effect of the Dark Illumination Intensity on the Characteristics of Surface Waves Propagating Along the Interface between Photorefractive and Nonlinear Kerr Crystals

Effect of the Dark Illumination Intensity on the Characteristics of Surface Waves Propagating Along the Interface between Photorefractive and Nonlinear Kerr Crystals

Laser adaptive holographic hydroacoustic intensimeter

A new type of a vector hydrophone, a laser adaptive hydroacoustic intensimeter, is investigated. Two identical, spatially separated coil-type sensors are used as a primary receiver of the acoustic signal. The signals received at the sensor output are phase demodulated in a two-channel adaptive holographic interferometer based on two dynamic holograms multiplexed in a CdTe photorefractive crystal. Using a laser intensimeter, the acoustic field formed in a limited volume is studied. The performance characteristics of the intensimeter are determined experimentally; its threshold sensitivity, according to the acoustic field intensity, constitutes 0.1 × 10−13 W m−2.

Polaron approach for nondestructive readout of holograms in photorefractive LiNbO3:Fe crystal in an external magnetic field

A novel method of magnetic field assisted nondestructive optical probing of photonic lattice structures in photorefractive crystals is demonstrated experimentally. High contrast annular symmetry micrometric scale refractive lattice structures were recorded by nondiffracting Bessel beam technique with the use of 532 nm, 17 mW power laser beam in Fe doped lithium niobate crystal. The stability of recorded lattices against erasure during readout by probe Gaussian beam at the recording wavelength has been studied. Time evolutions of the diffraction efficiency of the lattices for readout beam power of 17 mW in zero magnetic field and in external magnetic field of 0.85 T were measured. The measurements showed up to 3.4 times increase of erasure constant for magnetic field assisted readout of stored lattices. The physical model based on the approach of small polarons in external magnetic field to explain the experimental results is discussed.

Sensitivity Analysis of Adaptive Holographic Fiber-Optic Acoustic Emission Sensors during the Registration of Acoustic Waves In a Plate

Paper presents the results of studying the features of recording acoustic emission waves by fiber-optic sensors. Fiber-optic sensors are used as an elastic medium that receives acoustic emission waves. Optical fibers were built between the layers of the polymer composite material during the manufacturing process. The measuring system is constructed on the basis of an adaptive interferometer. Optical fibers are part of the interferometer optical scheme. The elastic wave acting on an optical fiber changes its optical density, changing the interference pattern of the optical beam. The work of the holographic interferometer based on the fact that the interference pattern, formed by the object and reference light beams, is a dynamic hologram that is constantly recorded in the photorefractive crystal during the elastic action on the fiber. Sensitivity of fiber-optic sensors based on an adaptive interferometer allows recording acoustic emission waves. The acoustic emission waves were generated by the Hsu-Nielsen source. A significant difference in the speed of sound is explained by the anisotropy of the properties of the composite material. The structure and density of the material’s fibers in different directions are different. The wavelet transformation was used to form the signal front.

Optical induction of two-dimensional photorefractive photonic microstructures using six wedge prisms array

We experimentally demonstrate a convenient method to fabricate two-dimensional photonic microstructures in photorefractive crystal. Three kinds of photonic microstructures have been fabricated in iron-doped lithium niobate crystal using six wedge prisms array. The experimental setup of our method does not require complicated optical alignment components. Induced two-dimensional triangular lattice photonic microstructures are analyzed and validated by plane wave guiding, far-field diffraction pattern imaging, Brillouin-zone spectroscopy, and angle-dependent transmission spectrum. The method can be easily extended to fabricate other complex photonic microstructures by designing different wedge prisms array and adjusting polarization and phase delay properly.

Director grating and two-beam energy exchange in a hybrid photorefractive cholesteric cell with a helicoidal polymer network

We develop a theory describing two-beam energy exchange in a cholesteric liquid crystal (CLC) stabilized in the planar state by a helicoidal polymer network. The CLC layer is placed between photorefractive and non-photorefractive substrates and illuminated by two intersecting coherent light beams. An interference pattern created by the incident beams induces a spatially periodic space-charge electric field in the photorefractive substrate. The field penetrates into the adjacent CLC layer and interacts with the charges trapped on the polymer fibrils forcing the fibrils to move along the helicoidal axis. At new positions, the fibrils reorient the CLC director and, therefore, induce a director grating. The light beams propagating across the cell couple within the grating. We calculate the energy exchange between the coupled beams and the gain of the weak light beam. We analyze the dependence of the gain coefficient on the parameters of the polymer network and the CLC and show that it can reach values greater than those obtained in typical solid photorefractive crystals.