K. M. Kwolek

Holographic storage and high background imaging using photorefractive multiple quantum wells

We report holographic, real time, depth‐resolved image acquisition, storage, and reconstruction in photorefractive GaAs/AlGaAs multiple quantum wells under high background radiation conditions. Reconstructed images of 50 μm transverse and depth resolution have been achieved using this device as a coherence gate to image through 9 mean free paths of turbid scattering medial

Photorefractive p‐i‐n diode quantum well spatial light modulators

We demonstrate the performance of all‐semiconductor photorefractive p‐i‐n diodes operating in the longitudinal quantum‐confined Stark geometry. Low‐temperature‐grown shallow quantum wells provide high‐mobility vertical transport, and potential steps incorporated into the semiconductor buffer layers increase the transit time across the buffer and therefore increase the quantum efficiency for trapping of charge before it is swept out to the doped p‐type and n‐type contacts. The buffer design and the doped contacts both make all‐semiconductor photorefractive devices possible, with peak transient output diffraction efficiencies approaching 3%, but without the need for dielectric insulating layers. We also redefine device speed by making a distinction between transient rise times and frequency response, showing that in these p‐i‐n devices the update rate is an order of magnitude slower than the inverse rise time.

Depth-resolved holography through turbid media using photorefraction

A technique based on photorefractive holography for imaging objects obscured by a scattering medium is presented. Using ultrashort pulse illumination, depth-resolved whole-field images of three dimensional objects embedded in scattering media have been obtained. Bulk photorefractive crystals and photorefractive multiple quantum-well (MQW) devices have been investigated as the hologram recording element. Images have been obtained through media of up to 16 scattering mean free paths with a system based on bulk rhodium-doped barium titanate (Rh:BaTiO/sub 3/). Using MQW devices, a real-time image acquisition (<0.4 ms) has been demonstrated when imaging through eight scattering mean free paths. The relative merits of photorefractive holography are discussed, including its potential to provide a higher dynamic range of detection than traditional photographic film based or electronic holography. This could be important for in vivo imaging through biological tissue.

Photorefractive asymmetric Fabry–Pérot quantum wells: Transverse‐field geometry

Photorefractive asymmetric Fabry–Perot quantum‐well structures yield significantly enhanced diffraction during four‐wave mixing by employing the sensitive amplitude and phase control of multiple‐beam interference within the device. We present an Al0.1Ga0.9As/GaAs photorefractive quantum‐well device with a near optimal input diffraction efficiency of 0.36% and an AlAs/GaAs quantum‐well device with an output diffraction efficiency of 200%.

Diffraction from a short-cavity Fabry-Pérot : application to photorefractive quantum wells

Abstract Diffraction efficiencies from absorption and index gratings in short-cavity Fabry-Perot multiple quantum well structures are strong functions of film thickness. Diffraction efficiencies of transmitted and reflected waves vary by an order of magnitude by adjusting the device thickness to bring the Fabry-Perot resonance condition close to the excitonic absorption. Asymmetric Fabry-Perot quantum-well reflection modulators produce the largest diffraction efficiencies, with input diffraction efficiencies approaching 6%. Analytic expressions for the diffraction efficiencies are derived in the Raman-Nath regime for a single thin film and for multilayer structures. These results apply in general for dynamic holography and transient dielectric gratings in thin films, but is illustrated specifically for photorefractive semiconductor quantum wells.

Direct-to-video holographic 3-D imaging using photorefractive multiple quantum well devices.

Due to an oversight during the revision process, one of the authors was not mentioned in this paper. The author list should read: R. Jones, M. Tziraki, D. Parsons Karavassilis, P. M. W. French, K. M. Kwolek, D. D. Nolte and M. R. Melloch.

Dynamic holography in a broad-area optically pumped vertical GaAs microcavity

A broad-surface-area vertical GaAs microcavity was operated as an adaptive holographic film. The cavity mirrors were transparent to high-energy (millijoules per square centimeter) hologram writing pulses at a wavelength of 730 nm that generated optically pumped gain gratings in a 1-µm-thick active layer of GaAs. The gain gratings were probed with a low-intensity (mW) tunable laser at wavelengths near the GaAs band edge in the high-reflectance bandwidth of the cavity Bragg mirrors. When the structure was designed with low mirror reflectances [(R1R2)1/2=90%] to operate below the lasing threshold, the cavity resonance bandwidth was sufficiently broad to permit homogeneous hologram readout over a large (several square millimeters) area. Diffraction efficiencies of approximately 10% were predicted and approached experimentally. These results represent a first step toward the realization of a holographic vertical-cavity surface-emitting laser structure.

Photorefractive semiconductor nanostructures

Publisher Summary This chapter provides an introduction to photorefractive semiconductor nanostructures. They are nonlinear optical devices that alter their optical properties in response to time- and space-varying light intensity patterns. They are used to perform dynamic holography under ultra low-light intensities much smaller than those used for traditional nonlinear optical materials. These devices contain nanometer-scale features that enhance their optical properties and alter their electronic transport relative to bulk behavior. They also contain high densities of deep level defects that make the devices semi-insulating. These defects trap and store photogenerated carriers that accumulate into space-charge densities that match the intensity patterns. The trapped charge produces electric fields that modify the optical properties of the nanostructure. The translation of temporally and spatially varying light intensity patterns into physical changes in the optical properties of the material, matching the intensity patterns, is called the photorefractive process. There are several ways that electric fields can be applied to quantum-well structures, and several ways that optical beams can be incident on the devices to write holograms. The various configurations can be summarized by three basic fields and grating geometries. Two of the structures are transmission structures with the optical beams incident on the same face, and one is reflection geometry with the beams incident from opposite faces.

Chapter 12 – Photorefractive semiconductor nanostructures

Publisher Summary This chapter provides an introduction to photorefractive semiconductor nanostructures. They are nonlinear optical devices that alter their optical properties in response to time- and space-varying light intensity patterns. They are used to perform dynamic holography under ultra low-light intensities much smaller than those used for traditional nonlinear optical materials. These devices contain nanometer-scale features that enhance their optical properties and alter their electronic transport relative to bulk behavior. They also contain high densities of deep level defects that make the devices semi-insulating. These defects trap and store photogenerated carriers that accumulate into space-charge densities that match the intensity patterns. The trapped charge produces electric fields that modify the optical properties of the nanostructure. The translation of temporally and spatially varying light intensity patterns into physical changes in the optical properties of the material, matching the intensity patterns, is called the photorefractive process. There are several ways that electric fields can be applied to quantum-well structures, and several ways that optical beams can be incident on the devices to write holograms. The various configurations can be summarized by three basic fields and grating geometries. Two of the structures are transmission structures with the optical beams incident on the same face, and one is reflection geometry with the beams incident from opposite faces.

Holographic vertical-cavity surface-emitting laser

Summary form only given. We present holographic diffraction from a broad-area thin-film holographic laser. The holographic laser is similar to a vertical-cavity surface-emitting laser (VCSEL) and consists of a 1 /spl mu/m thick GaAs active layer sandwiched between two 61.6 nm Al/sub 0.1/Ga/sub 0.9/As-AlAs 72.8 nm Bragg stacks, centered at 870 nm, with top and bottom periods of 5 and 10, respectively. No post-growth processing or fabrication is required.