Transport modeling of multiple-quantum-well optically addressed spatial light modulators

Abstract

Optically addressed spatial light modulators are essential elements in any optical processing system. Applications such as optical image correlation, short pulse auto-correlation, and gated holography require high speed, high resolution devices for use in compact, high throughput systems. Other important device criteria include ease of fabrication and operation. In this work we study the transport dynamics of a new kind of optically addressed spatial light modulator that uses semi-insulating or intrinsic quantum-well material to produce high performance devices without the need for pixellation or complicated device design.In response to an incident intensity pattern, basic device operation occurs through the screening of an applied voltage via the optical generation, transport, and trapping of photocarriers. A field pattern which mimics the incident intensity pattern is produced by the screening process. This generates strong index and absorption holograms via the quantum confined Stark effect. These holograms can be read out simultaneously with a probe beam to provide dynamic read/write operation.Overall device performance is determined by the transport of photocarriers during the field screening process. We have developed a transient, two-dimensional drift-diffusion model to describe both free and well-confined carrier transport as well as nonlinear effects such as velocity saturation and field-dependent carrier emission from quantum wells. Various analytical and numerical results for the internal carrier and space charge distributions, different screening regimes, and relative carrier contributions to the screening process aregiven. An experimental characterization of a GaAs/AlGaAs device using optical IV transmission and photocurrent techniques is also presented and used to verify the main results of the transport model.Analytical and numerical analyses of various transport effects that limit the resolution are also given. We show that, contrary to the results of earlier device models, both the speed response and resolution can be simultaneously optimized using appropriate device design. Using realistic device parameters, frame rates of 100 kHz at 10 mW/cm 2 intensity with 7 \im device resolution are predicted.First, I would like to thank my advisor, Bert Hesselink, for his support and encouragement over these past several years. He gave me a great deal of freedom not only in choosing this topic, but in determining the course of research. He has allowed me to pursue a number of interests, both directly and not so directly, related to this project. It is an opportunity that I have greatly appreciated. I would also like to thank two researchers from AT&T Bell Laboratories who have been of great help: William Coughran who got me started on semiconductor device modeling, and Afshin Partovi who fabricated and generously provided the samples used in our experiments.One of the great things about being in a group as large as ours is that there are always interesting and useful people around. I would especially like t...

Diffraction response of a low-temperature-grown photorefractive multiple quantum well modulator

“…The presence of a large density of trapping centers in the LTG material will inevitably produce a distribution of trapped charges within the MQW region, unlike STG MQWs where the trapped charges are concentrated at the MQW/cladding layer interfaces. 11 In this section, the space-charge field in the bright and dark regions is analyzed using a numerical simulation involving the standard continuity equations 12 for semiconductor materials in the presence of traps. It is shown that the space-charge field grows nonuniformly along the MQW growth direction.…”

“…13 Asgrown samples with excess arsenic show evidence of a large density of point defects that coalesce into As precipitates with annealing. We use the standard photorefractive picture 11 to model the traps phenomenologically as deep, donor-like defects with a volume density N D that are partially compensated by shallow acceptors of density N A . The shallow traps are assumed for charge neutrality and play no role in the carrier dynamics.…”

“…The thickness of the MQW region is d while the symmetric cladding layers each have thickness l . The MQW region is treated as a bulk material with an effective free carrier mobility 11 and the simulated device operates in transmission.…”

The resolution of distributed photorefractive multiple quantum well devices

“…9 Lateral drift within the MQW region was added heuristically. Figure 1 shows the division of the DPD in terms of SPD subdevices, each of which is split, as before, into an electro-optic region ͑containing MQWs where photocarriers are generated͒ sandwiched between two LTG layers, with an accumulation layer 14,15 at each MQW region/ LTG layer interface.…”

Optical grating recording in anisotropic photorefractive thin film: Dimensional resonance

“…A number of publications were devoted to the analysis of transient grating formation in twodimensional ͑2D͒ geometry in the so-called perpendicular configuration ͑PERC͒ when the grating vector is normal to an externally applied electric field. [1][2][3][4] In these articles the emphasis was on the influence of field fringing and screening on the resolution of an optically addressed spatial light modulator based on a thin PR film. 2D studies of PR thin films were also done in the parallel configuration ͑PARC͒ when the external field is parallel to the grating vector, the configuration being useful for dynamic hologram enhancement.…”