María Aguilar

High‐efficiency Stark‐geometry photorefractive quantum wells with intrinsic cladding layers

Output diffraction efficiencies approaching 40% and input diffraction efficiencies approaching 3% have been achieved in photorefractive p‐i‐n quantum well diodes operating in the longitudinal Stark geometry. The device structure consists of a low‐temperature‐grown multiple quantum well isolated from the doped contacts by an intrinsic standoff layer of Al0.5Ga0.5As. All charge trapping and screening occurs within the quantum wells without the need for trapping in specialized buffer layers used in previous designs. This new design operates at lower voltages (18 V) and lower fields (6 V/μm) than previously demonstrated.

Photorefractive nonlinearity of periodically poled ferroelectrics

We study theoretically the dependence of the photorefractive response of a periodically poled ferroelectric on the spatial frequency of light fringes, on the period of the domain inversion, and on its mark-to-space ratio. The photovoltaic effect, the drift of photoelectrons under an applied field, and their diffusion are considered as charge-transport mechanisms. Strong suppression of the photorefractive properties at low spatial frequencies (reduction of optical damage) together with retention of these properties in a region of short-fringe spacing are shown. We also found that the periodicity of the domain structure can produce a new kind of photorefractive parametric wave coupling. This coupling is characterized by a large gain factor for the waves propagating at angles that depend on the domain-inversion period. Finally, we discuss promising photorefractive experiments in periodically poled lithium niobate.

Optimization of selective erasure in photorefractive memories

A theoretical analysis for the optimization of the selective-erasure process in holographic photorefractive memories is presented. It is based on the solution of the standard material equations under a linear approximation (low modulation depths). Specific expressions for the optimum phase shifts and erasure rates are obtained. The approach includes all transport processes and so applies to photovoltaic materials such as LiNbO3. The different behavior with regard to nonphotovoltaic materials is discussed. Some additional strategies to improve the overall erasure process are proposed.

Photorefractive Stark-geometry quantum wells: diffraction nonlinearities and displacement currents

Abstract An equivalent electronic circuit is formulated to illustrate the importance of displacement current and diffraction nonlinearities in the time-evolution of diffracted signals from Stark-geometry photorefractive quantum wells. Quadratic electro-optic effects and quadratic diffractin efficiencies mix the spatial and temporal harmonics of space-charge electric fields generated during dynamic holography. The electro-optic and diffraction nonlinearities invalidate superposition, but small-signal analysis remains useful. Lateral transport that limits device spatial resolution can be included in an equivalent circuit with lateral circuit elements and transverse electric fields.

Holographic recording in photorefractive thin films: Edge effects

The initial holographic recording rate has been analytically studied for a photorefractive thin slab. The finite thickness is explicitly taken into account by using a two‐dimensional formulation (coordinates x and z parallel and perpendicular to the slab faces, respectively). It is shown that for a thickness comparable to the grating period, the solution appreciably departs from the usual one‐dimensional result, i.e., edge effects are remarkable. In particular, significant space‐charge fields perpendicular to the slab faces are generated. These effects may substantially modify the diffraction properties of the recorded photorefractive gratings, as exemplified for a GaAs film.

Unified two‐dimensional model for grating dynamics in photorefractive materials

A two‐dimensional treatment suitable both for perpendicular (Pockels readout optical modulators, etc.) and for parallel (image amplifiers, etc.) photorefractive devices is presented. A partial differential equation of the third order, valid for both families of devices, is set up and solved analytically for the relevant initial and boundary conditions in the full time domain. The results are illustrated in a few examples showing the relative significance of bulk and surface charges, and some new features of the solution for the perpendicular configuration.

Threshold for pattern formation in a medium with a local photorefractive response

Threshold equations for the instability of counterpropagating waves against spontaneous initiation of small-angle light patterns are analyzed for a medium with a local photorefractive response and various boundary conditions. These equations take into account all actual contributions to the reflection phase gratings. The form of the threshold equation is directly related to the spatial symmetry of the problem. We study the dependence of the threshold thickness of the crystal and the angle of light emission on experimentally controllable parameters and discuss the prospects for experimental studies of pattern formation in LiNbO3 crystals.

Linear electroabsorption in semi-insulating GaAs/AlGaAs asymmetric double quantum wells

Electroabsorption has been investigated in semi-insulating asymmetric GaAs/AlGaAs double quantum wells presenting high linear Stark responses, adequate for photorefractive applications. We have used the envelope function approximation to calculate the linear Stark shifts of the energy levels and select a suitable structure for the experimental study. The experimental data indicate that the response to the applied field critically depends on a complicated interplay of effects that compete or cooperate to suppress or enhance the electroabsorption. For positive field polarity, the competing contributions of the overlapping e1–hh1 and e1–hh2 transitions partially cancel the electroabsorption despite large linear Stark shifts. On the other hand, small negative fields induce large electroabsorption because the Stark shifts of the two transitions have opposite signs.

Short-time photorefractive recording in multiple quantum wells: longitudinal geometry

A two-dimensional model to describe the photorefractive behavior of a semiconductor multiple-quantum-well structure operating in the longitudinal geometry has been formulated. It takes into account longitudinal and transverse transport as well as both components of the space-charge field. The rate equations have been solved in the short-time regime, and the role of the various geometrical and physical parameters has been investigated. Special attention has been paid to clarifying the effects of transverse transport.

Mechanism of self‐organized light‐induced scattering in periodically poled lithium niobate

It is shown that the photorefractive grating produced by a pair of plane waves in periodically poled lithium niobate includes an additional set of spatial harmonics related to the periodic domain structure. This results in new schemes for photorefractive wave coupling. Using the modified phase matching conditions and the concept of optical oscillation we accurately describe the position of the diffraction peaks and explain the main characteristics of self‐organized photoinduced scattering reported recently.