Photorefractive System Research Articles

Polarization-Dependent Large Photorefractive Effect In A Wide Bandgap 2D Metal Halide Ferroelectric.

Photorefractive effect of ferroelectrics refers to the light-induced change of refractive index, which is an optical controlling avenue in holographic storage and image processing. For most ferroelectrics, however, the small photorefractive effect (10-5 -10-4 ) hinders their practical application and it is urgent to exploit new photorefractive system. Here, for the first time, strong photorefractive effects are achieved in a 2D metal-halide ferroelectric, [CH3 (CH2 )3 NH3 ]2 (CH3 NH3 )Pb2 Cl7 (1), showing large spontaneous polarization (≈4.1 µC cm-2 ) and wide optical bandgap (≈3.20eV). Notably, under light irradiation, 1 enables a large variation of refractive indices up to ≈ 1× 10-3 , being one order higher than the existing materials and comparable to the state-of-the-art inorganic ferroelectrics. This intriguing photorefractive behavior involves with the sharp variation of polarization caused by photo-pyroelectricity. As the first report of 2D metal-halide photorefractive ferroelectric, this work sheds light on optical controlling of physical properties in electric-ordered materials.

Improved slow light performances using photorefractive two-wave mixing.

We experimentally observe an ultralow effective group velocity of 0.9cm/s of light pulses using the two-wave mixing process in an Sn2P2S6 (SPS):Te crystal at a visible wavelength. The time delay can be controlled through the nonlinear photorefractive gain and the input pulse duration. By optimizing the nonlinearity strength, we achieve a maximum fractional delay of 0.79 for a pulse duration of 100ms. Our photorefractive slow light system allows combining low group velocity with large delay-bandwidth product for pulse durations spanning three orders of magnitude.

Tribal Odisha Eye Disease Study # 4: Accuracy and utility of photorefraction for refractive error correction in tribal Odisha (India) school screening.

Purpose:To compare the photorefraction system (Welch Allyn Spot™) performance with subjective refraction in school sight program in one Odisha (India) tribal district.Methods:In a cross-sectional study school students, aged 5–15 years, referred after the preliminary screening by trained school teachers received photoscreening and subjective correction. The photoscreener was compared to subjective refraction in the range of +2D to −7.5D. Statistical analysis included Friedman nonparametric test, Wilcoxon signed-rank test, linear regression, and Bland–Altman plotting.Results:The photoscreener was used in 5990 children. This analysis included 443 children (187 males, 256 females, and the mean age was 12.43 ± 2.5 years) who received both photorefraction and subjective correction, and vision improved to 6/6 in either eye. The median spherical equivalent (SE) with spot photorefraction was 0.00 D (minimum −5.0D; maximum +1.6 D), and with subjective correction was 0.00D (minimum −6.00 D; maximum +1.5 D). The difference in the SE between the two methods was statistically significant (P < 0.001) using Friedman nonparametric test; it was not significant for J 45 and J 180 (P = 0.39 and P = 0.17, respectively). There was a good correlation in linear regression analysis (R2 = 0.84) and Bland–Altman showed a good agreement between photorefraction and subjective correction in the tested range.Conclusion:Photorefraction may be recommended for autorefraction in school screening with reasonable accuracy if verified with a satisfactory subjective correction. The added advantages include its speed, need of less expensive eye care personnel, ability to refract both eyes together, and examination possibility in the native surrounding.

16-3 フォトレフラクション法を用いた眼球焦点調節測定装置の試作

16-3 フォトレフラクション法を用いた眼球焦点調節測定装置の試作

Determining the relative contribution of retinal disparity and blur cues to ocular accommodation in Down syndrome

Individuals with Down syndrome (DS) often exhibit hypoaccommodation alongside accurate vergence. This study investigates the sensitivity of the two systems to retinal disparity and blur cues, establishing the relationship between the two in terms of accommodative-convergence to accommodation (AC/A) and convergence-accommodation to convergence (CA/C) ratios. An objective photorefraction system measured accommodation and vergence under binocular conditions and when retinal disparity and blur cues were removed. Participants were aged 6–16 years (DS n = 41, controls n = 76). Measures were obtained from 65.9% of participants with DS and 100% of controls. Accommodative and vergence responses were reduced with the removal of one or both cues in controls (p < 0.007). For participants with DS, removal of blur was less detrimental to accommodative responses than removal of disparity; accommodative responses being significantly better when all cues were available or when blur was removed in comparison to when proximity was the only available cue. AC/A ratios were larger and CA/C ratios smaller in participants with DS (p < 0.00001). This study demonstrates that retinal disparity is the main driver to both systems in DS and illustrates the diminished influence of retinal blur. High AC/A and low CA/C ratios in combination with disparity-driven responses suggest prioritisation of vergence over accurate accommodation.

Controlling and synchronizing the spatiotemporal chaos of photorefractive ring oscillators with coupling

We present the control and synchronization of spatiotemporal chaos in the photo-refractive ring oscillator systems with coupling technology. First, we realize the synchronization of spatiotemporal chaos in the two photorefractive ring oscillator systems via mutual coupling by choosing a suitable coupling strength. With the mutual coupling strength enlarging, the two mutual coupling photorefractive ring oscillator systems are controlled into periodic state, period number differs on account of the coupling strength and lattice coordinates. By increasing the coupling strength, the photorefractive ring oscillator is converted into period 8, subsequently it is converted into periods 4 and 2, periodic synchronization of the photorefractive ring oscillator systems is achieved at the same time. Calculation results show that period 1 is impossible by mutual coupling technology. Then, we investigate the influence of noise and parameter deviation on chaotic synchronization. We find that mutual coupling chaotic synchronization method can synchronize two chaotic systems with the weak noise and parameter deviation and has very good robustness. Given that the weak noise and parameter deviation have a slight effect on synchronization. Furthermore, we investigate two dimension control and synchronization of spatiotemporal chaos in the photorefractive ring osillator systems with coupling technology and get successful results. Mutual coupling technology is suitable in practical photorefractive ring oscillator systems.

Trying to see, failing to focus: near visual impairment in Down syndrome.

The majority of individuals with Down syndrome (DS) do not exhibit accurate accommodation, with the aetiology of this deficit unknown. This study examines the mechanism underlying hypoaccommodation in DS by simultaneously investigating the ‘near triad’ – accommodation, vergence and pupillary response. An objective photorefraction system measured accommodation, pupil size and gaze position (vergence) under binocular conditions while participants viewed an animated movie at 50, 33, 25 and 20 cm. Participants were aged 6–16 years (DS = 41, controls = 76). Measures were obtained from 59% of participants with DS and 99% of controls. Accommodative response was significantly less in DS (p < 0.001) and greater accommodative deficits were associated with worsening visual acuity (p = 0.02). Vergence responses were as accurate in DS as in controls (p = 0.90). Habitual pupil diameter did not differ between groups (p = 0.24) but reduced significantly with increasing accommodative demand in both participants with and without DS (p < 0.0001). This study is the first to report simultaneous binocular measurement of the near triad in DS demonstrating that hypoaccommodation is linked to poor visual acuity. Vergence responses were accurate indicating that hypoaccommodation cannot be dismissed as a failure to visually engage with near targets, but rather is a consequence of underlying neurological or physiological deficits.

Accommodation and pupil responses to random-dot stereograms

We investigated the dynamics of accommodative and pupillary responses to random-dot stereograms presented in crossed and uncrossed disparity in six visually normal young adult subjects (mean age=25.8±3.1 years). Accommodation and pupil measures were monitored monocularly with a custom built photorefraction system while subjects fixated at the center of a random-dot stereogram. On each trial, the stereogram initially depicted a flat plane and then changed to depict a sinusoidal corrugation in depth while fixation remained constant. Increase in disparity specified depth resulted in pupil constriction during both crossed and uncrossed disparity presentations. The change in pupil size between crossed and uncrossed disparity conditions was not significantly different (p>0.05). The change in pupil size was also accompanied by a small concomitant increase in accommodation. In addition, the dynamic properties of pupil responses varied as a function of their initial (starting) diameter. The finding that accommodation and pupil responses increased with disparity regardless of the sign of retinal disparity suggests that these responses were driven by apparent depth rather than shifts in mean simulated distance of the stimulus. Presumably the need for the increased depth of focus when viewing stimuli extended in depth results in pupil constriction which also results in a concomitant change in accommodation. Starting position effects in pupil response confirm the non-linearity in the operating range of the pupil.

Fast Response Beam Coupling in Dye Doped Liquid Crystal Cells Sandwiched with ZnSe Coated Substrates

ABSTRACTIn this paper we present fast response processes in dye doped liquid crystal cell with ZnSe layers on electrodes. Two-wave mixing and grating monitoring experiments were performed to characterize this liquid crystal system. With the help of photoconductive layers, the response time was reduced to a few milliseconds, which improved further the response rate in liquid crystal photorefractive system. A peak in response time versus applied voltage and unexpected oscillations in diffraction efficiency dynamics were observed, which implies unconventionally complex charge dynamics associated with SPPs excited at one of the photoconductive layer/liquid crystal interfaces.

Spatial rogue waves in a photorefractive pattern-forming system.

We have experimentally analyzed pattern formation in an optical system composed of a bulk photorefractive crystal subjected to a single optical feedback. In a highly nonlinear regime far above the modulational instability threshold, we are reporting on turbulent spatiotemporal dynamics that leads to rare, intense localized optical peaks. We have proven that the statistics and features of those peaks correspond to the signatures of two-dimensional spatial rogue events. These optical rogue waves occur erratically in space and time and live typically the same amount of time as the response time of the photorefractive material.

Effect of external electric field and background illumination on the intensity distribution of optical surface waves in the metal – photorefractive crystal system

The influence of the external electric field and background illumination on the intensity distribution of optical photorefractive surface waves at the metal – photorefractive crystal interface has been numerically simulated. The simulation is performed for a strontium – barium niobate (SBN) crystal using the parameters corresponding to the experimental data. The replacement of a real metal with an ideal one and the choice of the corresponding boundary conditions (depending on the wave power) in the numerical simulation have been substantiated. The calculation results have shown good agreement with the previously published experimental data on the effect of background illumination and a significant discrepancy for the data on the effect of the external electric field. It is found that the effect of the external electric field can be significantly enhanced by reducing the optical power of the photorefractive wave to values close to the threshold ones.

Hybrid liquid crystal photorefractive system for the photorefractive coupling of surface plasmon polaritons

We present a detailed investigation of a photorefractive surface plasmon polariton system capable of coupling energy between two predefined surface plasmon modes with efficiencies up to 25%. We have investigated the dependence of the diffraction efficiency on the energy, the initial and final wavevectors of the surface plasmon modes, and the cell parameters. We have also developed numerical simulations of the system based upon the defect-free Q-tensor approach and rigorous diffraction theory, which fit the experimental data very well and have allowed us to develop a good theoretical understanding of the performance of these cells. On the basis of the experimental results and theory we discuss the prospects that a hybrid liquid crystal photorefractive system could lead to photorefractive gain for surface plasmon polaritons.

Pattern formation using optical vortices in a photorefractive single feedback system

We analyze numerically the dynamics of optical patterns in a photorefractive single feedback system where a vortex beam is used as the input beam. Depending on the topological charge of the vortex and on the nonlinear photorefractive coupling strength, we observe different pattern geometries with a rotating dynamics.

Photoconducting Polymers for Photorefractive 3D Display Applications

Photorefractive composites derived from photoconducting polymers offer the advantage of dynamically recording holograms without the need for processing of any kind. Thus, they are the material of choice for many cutting edge applications, such as updatable 3D displays and imaging through a scattering medium. This article reviews the basic properties of photorefractive polymer systems and the inherent advantages that have attracted much attention. The chemistry and physics relevant for the design of the high-performance guest−host composite are discussed and recent advances emphasized. In particular, a charge transporting polymer with high mobility and history-independent response times is highlighted, as well as polymer systems useful for holographic displays and the material considerations necessary to develop high-speed, large-sensitivity composites.

Lattice-controlled modulation instability in photorefractive feedback systems

We study the modulation instability in a two-dimensional nonlinear single feedback system with a photonic lattice and reveal a sharp transition in the instability regimes as the lattice strength is increased. For a shallow lattice, the instability modes are enhanced parallel to the lattice wave vector, while in stronger lattices, these modes are suppressed.

Temperature effect on the evolution and stability of rigid screening spatial soliton in biased photorefractive crystal

Based on the temperature dependence of two-wave mixing, temperature effects on the dynamical evolution and stability of rigid screening (RS) bright solitons in a photorefractive dissipative system based on two-wave mixing have been investigated numerically. Our results indicate that the stability of bright RS solitons depends strongly on the crystal temperature. The RS solitons are stable to small temperature perturbations. They will not evolve into stable rigid screening solitons, however, and their intensity and width vary with the propagation distance if the temperature deviation is large enough. The potential applications of the temperature properties of these RS solitons in optical attenuators or repeaters are discussed.

Noise- and dynamics-sustained patterns in a nonlinear photorefractive system

We analyze experimentally the dynamics of optical patterns emerging from a photorefractive two-wave mixing geometry where a tilted single-feedback mirror generates an advectionlike effect. Depending on the nonlocal coupling (introduced by the tilting angle) between the two counterpropagating beams, the strength of the nonlocal response of the nonlinear photorefractive bulk medium, and the distance between the mirror and the crystal, we observe the initiation of different pattern geometries, the inversion of pattern transverse phase velocity, and the bifurcation from convective to absolute instabilities.

Feedback method of the noise suppression in wave-mixing amplifiers based on non-linear materials with photorefractive response in a reflection grating configuration

In the photorefractive wave-mixing system, fluctuation in the signal beam intensity of the photorefractive output with a reflection grating has been analyzed by employing pump feedback method. In this method, fluctuations of the photorefractive wave-mixing process not only induce the intensity fluctuation of the mixing waves but also induce phase fluctuation of the mixing waves. Thus, the phase of the pump and signal beams at the output surface fluctuates in time around a mean value. Using such a positive feedback method of a pump beams, the relative fluctuation in the photorefractive output signal beam intensity with respect to its mean intensity can be minimized significantly without reducing its mean intensity. The factors that control the fluctuation in the signal beam intensity, such as the phase fluctuation of the output pump beam, absorption strength of the material and the feedback reflectivity of the cavity mirrors, on the relative fluctuation of the output signal intensity in the photorefractive wave-mixing systems have been studied in detail. It has been found that the fluctuation of the output signal intensity relative to its mean intensity in the photorefractive wave-mixing system can be suppressed to larger extent by taking lower value of feedback reflectivity of the cavity mirrors which could exist at a higher value of absorption strength of the photorefractive materials.

Holographic solitons in centrosymmetric photorefractive dissipative systems

It is shown theoretically that the signal beam alone can evolve into steady-state bright and dark holographic solitons in a centrosymmetric photorefractive dissipative system that includes two balances, i.e., loss is exactly compensated by gain and diffraction is compensated by nonlinearity resulting from both self-phase modulation self-focusing and holographic focusing. These solitons have fixed amplitude and width. When the absorption of the system is neglected and the pump beam is switched to a background light, these solitons can become the “traditional” photorefractive solitons with arbitrary amplitude.

Theory of photorefractive resonance for localized beams in two-carrier photorefractive systems

This paper extends the existing theory of two-carrier photorefractivity resonance, which is generally applied to iron doped indium phosphide (InP:Fe), to the case of low nonharmonic illumination. The space charge field profile is computed, and the variations of its amplitude, width and position are determined as functions of the background intensity. The effect of photorefractive resonance on these quantities is evidenced, contributing to the understanding of published experimental results in InP:Fe.