Photorefractive Crystal Research Articles

Waveguide properties of interface separating a photorefractive crystal with diffusion nonlinearity and an exponential graded-index medium

Waveguide properties of the planar interface between the photorefractive crystal with diffusion type of nonlinearity and exponential graded-index medium are described theoretically. The wave equation with complicated refractive index spatially distributed and intensity-dependent is proposed and its exact solutions are found, which describe the transverse magnetic waves of two types. Waves of first type decay monotonically into the media at the both sides from interface, and waves of the second type decay with oscillations into the photorefractive crystal. It is possible to achieve such localization of the wave by choosing the values of the parameters in such a way that the oscillation period becomes sufficiently large. It is shown that narrow localization of the second type waves can be achieved by choosing the values of the parameters in such a way that the oscillation period becomes sufficiently large. The waveguide modes, the order of which is determined by the number of the intensity maxima in the exponential graded-index medium, are obtained. The waveguide mode decays with oscillations in an exponentially gradient medium. It is shown that the transition between modes is carried out by increasing the characteristic thickness of the exponential graded-index layer. The total power flow is calculated and the possibility of redistributing wave energy between contacting media is investigated with its using.

Modulated waves patterns in the photovoltaic photorefractive crystal

Modulated waves patterns in the photovoltaic photorefractive crystal

Surface waves propagating along the interface between parabolic graded-index medium and photorefractive crystal with diffusion nonlinearity

The new types of surface waves propagating along the interfaces between photorefractive crystal and parabolic graded-index medium are described theoretically. The optical parameters of contacting media are chosen that the depth of field localization is less than the thickness of the graded-index layer. Exact solutions to the boundary problem in different range of the effective refractive index are obtained. The surface waves attenuating with oscillations into the photorefractive crystal can propagate with arbitrary values of the effective refractive index and with its fixed values corresponding to the discrete spectrum. The effect of the optical characteristics of the crystals on field distribution is analyzed. The surface wave corresponding to the ground mode of the discrete spectrum monotonically attenuating on both sides of the interface is found. The angle of incidence need not be small to excite surface waves with a non-oscillating profile. The period of spatial oscillations must be greater than the field penetration depth into the photorefractive crystal to excite the surface waves monotonically attenuating in the depth of the crystal.

Fully controllable multichannel waveguides induced by counterpropagating Bessel beams

We theoretically analyze the waveguiding structures photo-induced by two incoherent counter-propagating Bessel beams (BBs) in a biased photorefractive crystal. We demonstrate that the cross-coupling of two BBs enables adressable channels and tunability of the forming guiding structures. The truncation parameter of the BBs, their Bessel orders and the misalignment between the two beams are all key parameters for tailoring the characteristics of the photo-induced waveguides such as the number of outputs, the output intensity levels and the distance between each output channel. Accordingly, we optimized the different parameters for designing not only a fully tunable Y-coupler but also optical splitters with up to five outputs and even more complex star couplers for all-optical interconnect applications. Finally, we report on the stability behavior of the photo-induced platform. The stability threshold depends on the nonlinearity parameter beyond which the beams display time-periodic, quasi-periodic and turbulent dynamics where spatially localized instabilities can be observed. All these results suggest more opportunities for fully controllable complex waveguiding structures and new all-optical solutions for active components in optical telecommunication and innovative ways of performing optical computing based on spatiotemporal chaos.

Temperature effects on Manakov solitons in biased photorefractive crystals exhibiting the linear and quadratic electro-optic effect simultaneously

We investigate the effect of temperature on Manakov solitons in biased photorefractive crystals having both linear and quadratic electro-optic effects simultaneously. Two different physical quantities, namely, the dielectric constant and the dark irradiance, have a temperature dependence which in turn affects the Manakov solitons. We consider three realizations of the Manakov solitons for our present analysis, namely, the bright–bright, dark–dark and bright–dark Manakov soliton pair. The existence curve and its modification due to temperature has been studied in detail for each of the Manakov soliton pair. A comprehensive investigation of the functional dependence of the soliton width with temperature is undertaken for different parameters like beam intensity and external electric field for each of the Manakov soliton pair. The tuning of photorefractive soliton characteristics by changing of temperature has potential practical applications in optical networks and optical waveguiding.

Bending optical soliton-induced waveguide channels in a photorefractive LiNbO3 crystal

Bending optical soliton-induced waveguide channels in a photorefractive LiNbO3 crystal

Intense Wave Formation from Multiple Soliton Fusion and the Role of Extra Dimensions.

We experimentally and numerically explore the role of dimensionality in multiple (three or more) soliton fusion supported by nonreciprocal energy exchange. Three-soliton fusion into an intense wave is found when an extra dimension, with no broken inversion symmetry, is involved. The phenomenon is observed for 2+1D spatial waves in photorefractive crystals, where solitons are supported by a spatially local saturated Kerr-like self-focusing and fusion is driven by the leading nonlocal correction, the spatial analog of the nonlinear Raman effect.

Scattered Image Reconstruction at Near-Infrared Based on Spatial Modulation Instability

We present a method of near-infrared image reconstruction based on spatial modulation instability in a photorefractive strontium barium niobate crystal. The conditions that lead to the formation of modulation instability at near-infrared are discussed depending on the theory of modulation instability gain. Experimental results of scattered image reconstruction at the 1064 nm wavelength show the maximum cross-correlation coefficient and cross-correlation gain are 0.57 and 2.09 respectively. This method is expected to be an aid for near-infrared imaging technologies.

Dependence of gain coefficient and response time on the applied electric field in LiNbO3:Fe crystal

In this paper, the characteristics of two-wave mixing in photorefractive crystal LiNbO3:Fe are theoretically and experimentally analyzed. The relationship between the gain coefficient and the external dc electric field is investigated. The effects of the external dc electric field, incident intensity and density of electron donor on the response time are analyzed It has been found that the gain coefficient can be increased by applying the external electric field when the incident intensity and beam crossing angle are 12 mW/cm2 and 15°, respectively. The response time of two-wave mixing can be reduced by enhancing the external electric field incident intensity or density of electron donor. Experimental results show that when the external electric field is increased from 0 to 8 kV/cm, the response time of LiNbO3:Fe crystal drops from 87 s to 72 s. Furthermore, the response time decreases rapidly and then slowly with the enhancement of the incident intensity. The response time reduces from 180 s to 42 s with the increase of the incident intensity when the external electric field is constant at 10 kV/cm. The response time with an external electric field is slightly shorter than that without an external electric field. The experimental results are consistent with the theoretical simulation. The present work can provide application guidance for the research of photorefractive crystals.

Three-Dimensional All-Optical Switching Using a Single Diffracting Bessel Beam

A single Bessel beam (BB) enables the scalable optical writing of waveguide devices with multiple inputs and outputs in a nonlinear medium. For this purpose, we experimentally propagate a diffracting zero-order BB in a strontium barium niobate photorefractive crystal. Such a configuration can be used for the self-writing of complex waveguiding structures. By tailoring different key parameters such as the diffraction phenomenon through the BB size, the nonlinearity strength through the applied electric field, and the photoinduced refractive-index modulation depth through input beam intensity, our optical platform enables not only the generation of optical splitters but also an all-optical control of the output intensity levels. We also analyze the stability in space and time of the different photoinduced channels. We show that adding a background illumination onto the medium permits a steady-state photoinduced refractive-index modulation with up to nine output channels. These results enrich the research on scalable optical writing techniques for the realization of complex interconnects and enlarge further possibilities for all-optical switching.

High-contrast phase imaging based on nonlinear holographic hot image model

Imaging for weak-phase objects is a challenging issue in the linear imaging process. Here, we demonstrate a high-contrast phase imaging method based on a nonlinear holographic hot image model. Due to the nonlinear Kerr effect, the holographic hot image can transform a weak phase into strong amplitude as a signal amplifier. The phase information is iteratively obtained from the light field distribution of the holographic hot image. The strong signal-to-noise ratio helps improve the imaging contrast. Using a tunable photorefractive crystal, we numerically and experimentally demonstrate the advantage of this method for imaging weak-phase objects. For the determined sample, our method doubles the imaging contrast. As far as we know, this is the first report using the nonlinear holographic hot image for imaging technology. This study can provide a potential strategy to achieve high-contrast imaging for various weak-phase objects applied in biomedical imaging, semiconductor metrology, and photolithography.

Two dimensional Airy beam soliton

We demonstrate the formation of a two dimensional Airy beam soliton in a photorefractive crystal. By simply varying the nonlinearity strength we identify several scenarios showing the coexistence between an Airy beam and the emerging soliton. The soliton output profile behaves according to the theoretical soliton existence curve and can be tailored by the nonlinearity strength even without modifying the input Airy beam shape. This last feature makes this Airy soliton distinct from the Gaussian beam generated photorefractive soliton.

INFLUENCE OF INVERSE PIEZOELECTRIC AND PHOTOELASTIC EFFECTS ON THE INDEX SURFACE OF THE NORMAL COMPONENT INVERSE TENSOR DIELECTRIC PERMEABILITY OF THE Bi12TiO20 CRYSTAL

The index surface of the normal component of variation of the inverse tensor dielectric constant of the photorefractive Bi12TiO20 crystal of 23 symmetry class is constructed and analyzed. The extremal values of the index surface of the normal component for Bi12TiO20 crystal with formed holographic grating are determined. It is shown that the combined action of the photoelastic and inverse piezoelectric effects leads to a change in the maximum and minimum values of the normal component of variation of the inverse tensor dielectric constant of the crystal with the wave vector of the holographic grating oriented along the crystallographic direction.

Spatial modulation spectroscopy of semiconductors using dynamic gratings

Subject of study. Several characterization methods that utilize recording and reading out of dynamic gratings to analyze functional materials are proposed. Method. The methods are based on pulsed recording of thin and volume dynamic gratings followed by reconstruction using continuous laser emission to facilitate the detection of the formation and relaxation dynamics of the recorded diffraction grating. Nonlinear optical, thermo-optic, and kinetic properties of condensed media were obtained based on the measured kinetics of the diffracted signals. Main results. The performance characteristics of a dynamic grating method were analyzed for characterization and parameter measurements of functional materials. In particular, the efficacy of the measurement of the kinetic properties for pulsed recordings of thin and volume holographic gratings was demonstrated. The lifetimes of the short- and long-lived trap levels involved in recording of the dynamic grating were determined in a photorefractive bismuth silicate crystal and the dependence of the diffraction efficiency of the gratings on the intensity of the recording radiation was determined. The advantages of using an additional grating (homodyne) to enable the separation of the contributions of different simultaneously manifesting nonlinearity mechanisms were demonstrated. This method was applied to monocrystalline germanium. A method for assessing the thickness of a film on the surface of a sample based on the amplitude of an oscillating component related to a sound wave propagating in air in the vicinity of the film’s surface was also proposed. A silicon dioxide film was considered as an example. The application of the diffraction to different orders on volume dynamic gratings was proposed for the measurement of the nonlinear optical susceptibility of the fifth and higher orders. The proposed approach was evaluated on photorefractive crystals and semiconductor media. The kinetic and thermo-optic properties, including the time of the population of trap levels, thermo-optic coefficient, and thermal diffusivity, of the media were determined. Practical significance. The proposed characterization methods facilitate the isolation of different nonlinearity mechanisms involved in the formation of dynamic gratings and the measurement of nonlinear optical, thermo-optic, and kinetic properties (optical susceptibilities of different orders, thermo-optic coefficient, thermal diffusivity, and lifetimes of free-charge carriers and trap levels).

Weak Incoherent Optical Signal Amplification Based on Modulation Instability for Imaging Through Fog

Weak optical signal processing based on nonlinear effects offers new approaches for imaging through scattering media. A novel incoherent optical signal amplification method based on spatial modulation instability is proposed for imaging through fog. We experimentally demonstrated the amplification and recovery of degraded weak incoherent optical image signals after passing through dense fog in a photorefractive crystal. Our experimental results indicate that the intensity profiles of the output images can be redistributed from disordered to ordered when the nonlinear strength exceeds the threshold of incoherent modulation instability, which shows that the partially disordered incoherent probe light intensities are orderly transferred to enhance the signal intensity profiles and the residuals become a uniform background. The restored nonlinear output images with high visibility were observed for a proper optical thickness of fog, and weak optical imaging from undetectable to detectable with relatively poor visibility for a larger optical thickness was also realized in the experiment. This incoherent optical signal amplification method based on modulation instability has a potential application for image recovery in atmospheric scattering imagings.

The surface waves propagating along the contact between the layer with the constant gradient of refractive index and photorefractive crystal

We consider the photorefractive crystal with diffusion nonlinearity mechanism contacting with the linearly graded-index medium. The new types of the surface waves propagating along the contact are found and their properties are described. The transition between two types of the surface waves can be controlled by the waveguide temperature. The light field penetrates into the photorefractive crystal much deeper than into the graded-index medium. The intensity maxima monotonically increase with an increase in the propagation constant, and monotonically decrease with an increase in the temperature. The localization length of the surface waves decreases with an increase in the characteristic distance the graded-index medium and with a decrease in the propagation constant and the temperature. The discrete spectrum of the propagation constant for the particular waveguide mode corresponding to the case of exponential damping of intensity maxima of the surface waves with an increasing temperature is obtained. It is shown that the intensity maxima in the photorefractive crystal depend parabolic on the temperature at low temperature waveguide mode.

Image encryption using spatial nonlinear optics

Optical technologies have been widely used in information security owing to its parallel and high-speed processing capability. However, the most critical problem with current optical encryption techniques is that the cyphertext is linearly related with the plaintext, leading to the possibility that one can crack the system by solving a set of linear equations with only two cyphertext from the same encryption machine. Many efforts have been taken in the last decade to resolve the linearity issue, but none of these offers a true nonlinear solution. Inspired by the recent advance in spatial nonlinear optics, here we demonstrate a true nonlinear optical encryption technique. We show that, owing to the self-phase modulation effect of the photorefractive crystal, the proposed nonlinear optical image encryption technique is robust against the known plaintext attack based on phase retrieval. This opens up a new avenue for optical encryption in the spatial nonlinear domain.

Slowdown of nanosecond light pulses with photorefractive two-wave mixing

In the demonstrations of slow light in photorefractive (PR) materials, the delays are only reported for long pulse durations (ms to s) that are in discrepancy with typical ns-pulse length in data processing applications. Here we demonstrate that the slowdown of light in a PR crystal can be also achieved in the nanosecond regime at room temperature. We experimentally show that the use of a pulsed laser with a high peak intensity may indeed reduce the ${\mathrm{Sn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$ crystal response time to a scale of ns and achieve a two-wave mixing (TWM) gain of $7.9\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}1}$. We show the slowdown of light pulses whose duration ranges from 10 to 100 ns with a fractional delay up to 0.3. These results are similar to those obtained in optical fibers, except that the slow light with PR TWM is achieved through smaller propagation lengths and at wide wavelength ranges. This could reveal potential applications of PR slow light systems and have a significant impact for future optical communications systems.

Recording the dynamic polarisation gratings in pure nematic liquid crystal

We report on experimental realisation of polarisation dynamic gratings recording in pure nematic liquid crystal. This work is devoted to the experimental implementation of writing polarisation dynamic gratings in a pure nematic liquid crystal. The registration mechanism is based on the photorefractive effect in a cell filled with a pure nematic crystal under the action of an applied constant electric field. This phenomenon does not require the presence of photoexcited charge carriers in the liquid crystal and is explained by the extremely high anisotropy of the liquid crystal. The article provides an elementary theoretical model of this process. The dependences of the grating writing process on the spatial frequency of the interference field and the intensity of the recording beams are investigated. It was found that the spatial grating does not coincide with the interference field – the response of the medium is non-local. However, we do not associate this property with that in photorefractive crystals. In our case, a mechanism of purely geometric mismatch is more likely due to the peculiarity of the recording mechanism. One of the fundamental properties of the described processes is their independence from the wavelength of the recording radiation, which confirms the model of the recording mechanism we have chosen.

Nonlinear coupling and modulation of the guided modes in photorefractive slab waveguide

Owing to its excellent electro-optic, ferroelectric and nonlinear optical properties, photorefractive crystal (PRC) provides a versatile platform for developing nonlinear waveguides, which has a great potential in integrated optics components. In this paper, we theoretically study nonlinear coupling of optical guided modes in photorefractive waveguides by the spatial-partition fast Fourier transform beam propagation method (SP FFT-BPM). The modulation of applied external electrical field on the guided modes is investigated, and demonstrated as a new method to modulate the guided modes without artificially refabricating physical configuration or structure of the waveguide.