M. R. Melloch

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.

Pre-illumination to control the active trap density in a semi-insulating MQW device

The trap density in a photorefractive semi-insulating quantum-well (QW) device can be dynamically adjusted for optimum sensitivity by passivating some of its traps with photocarriers from a pre-illumination pulse. By reducing the effective trap density with pre-illumination, we demonstrate that the carrier drift length can increase, causing more than an order of magnitude increase in device sensitivity by increasing the number of QWs screened per carrier. Too much pre-illumination, on the other hand, decreases four-wave mixing diffraction efficiency, due to lateral drift. The optimum trap density enables higher sensitivity while maintaining high-resolution operation. Pre-illumination can also be used to speed up grating erasure by increasing the electron-hole recombination probability.

Real-time 3-D imaging using structured illumination and photorefractive holography, including with fluorescence

Summary form only given. We report real-time 3-D imaging using structured illumination with photorefractive MQW media. This produces optically-sectioned reflected light or fluorescence images with no post-processing and the photorefractive effect discriminates against a diffuse (scattered light) background.

High speed 3-D imaging using photorefractive holography with novel low coherence interferometers

Summary form only given. Photorefractive holography is a whole-field coherence-gating technique for 3-D imaging, including through turbid media, that offers a unique mechanism to discriminate against a background of diffuse light. In contrast to confocal scanning techniques such as optical coherence tomography, it does not need transversal scan and so may be implemented at very high frame rates and may utilize light sources of almost arbitrary spatial coherence, including low-cost LEDs and broad stripe multi-mode diode lasers. We demonstrate photorefractive holography with diverse alternative sources to the modelocked laser and discuss design considerations for the different imaging configurations. Photorefractive holography can exploit high power LEDs.

Optoelectronic properties of nonstoichiometric heterostructures

We review the optoelectronic properties of nonstoichiometric heterostructures, including the contribution to material absorption by arsenic precipitates in GaAs and the electroabsorption of quantum confined excitons in LTG quantum wells. These materials form the basis of adaptive holographic quantum well films used for adaptive optics applications. The optical applications rely on space-charge trapped in the deep energy levels which we calculate for spheroidal mesoscopic defects using an extension of the many-electron model of Haldane and Anderson. With electroabsorption spectroscopy we are able to establish details of vacancy migration and vacancy decay mechanisms during annealing.

Enhancement of narrow-band terahertz radiation from large aperture photoconductors through multiple pulse excitation

Summary form only given. We present the saturation properties of the THz emission from biased, large aperture photoconductors excited by trains of amplified fs optical pulses. Our studies demonstrate avoidance of saturation effects through multiple pulse excitation, and hence a strong enhancement of the peak power spectral density, for emitters with carrier lifetimes shorter than the interpulse spacing. Such an enhancement was previously observed for photoconductive dipole antennas excited by pulses from a mode-locked oscillator; the current experiments extend this to the much higher THz field regime available using large aperture emitters and amplified laser pulses. Our results also confirm that the saturation recovery time depends on the photocurrent lifetime, consistent with previous theoretical results.

Temporally and spatially incoherent holography using photorefractive multiple quantum well devices

Summary form only given. Near infrared radiation offers the potential for medical diagnostic and functional imaging in biological tissue since the absorption of a typical tissue in this region is at a minimum. However, the scattering of light in tissue in this spectral region is a significant problem and much research has been motivated by the desire to recover image information in the presence of this scattered light. In particular, various schemes have been devised to form images using ballistic (or unscattered) light, which is normally obscured by the presence of the highly diffuse background. These techniques include spatial filtering, time gating and coherence gating. Photorefractive holography is a coherence gating technique like OCT but it differs in that it is a whole field imaging technique that acquires all the pixel information in parallel. When imaging in the reflection geometry using backscattered light, coherence gating can also provide depth-resolved (time-of-flight) image information. The coherence length of the light source primarily determines the depth resolution. Photorefractive MQW devices have been used to record spectrally resolved holograms, as well as recording images direct-to-video through scattering media. The authors are motivated by the desire to develop an imaging technique for applications such as skin cancer detection. For this task imaging through solid scattering media is an important issue.

Observation of the transition from the near-field to the far-field region for broadband terahertz radiation

Summary form only given. In the antenna theory it has long been well known that the space surrounding an antenna can be divided into the near- and far-field region. But in the terahertz (THz) optoelectronics community, this had not received much attention until recently when Budiarto et al. (1998) observed the near-field effect in the measurement of THz radiation from a large aperture photoconductive transmitter. In their experiment, due to the very large aperture size (3 cm), all the measurements were limited to the near-field region, where the waveforms keep changing as they propagate away from the emitter. In this work, we observed the transition of the broadband THz radiation from the near-field regime to the far-field regime where the waveform does not depend on the distance between the emitter and detector and where the amplitude of the field follows the 1/r rule. We also observed that the transition distance depends on the aperture size of both the transmitter and receiver. We believe that these observations help to elucidate the nature of the broadband electromagnetic wave propagation and the transmitter/receiver system.

Comparison of terahertz waveforms measured by EO-sampling and a photoconductive dipole antenna

We directly compared the measured terahertz radiation taken from free space electro optic sampling (FSEOS) and photoconductive (PC)-sampling. We observed significant difference between the two types of waveforms. We find that the waveforms measured by the PC-antenna can be derived from the FS-EOS waveforms in conjunction with the carrier lifetime of the PC-antenna material and the frequency dependent antenna response.

Laser-based ultrasound with linear detection in photorefractive multiple quantum wells

Laser-based ultrasound is a promising NDE technique for remote sensing, manufacturing diagnostics, and in-service inspection. Nonadaptive homodyne and heterodyne reference beam interferometers could not operate effectively with speckle, and time-delay interferometers (such as the confocal Fabry-Perot) require path-length stabilization to a fraction of an optical wavelength. We use dynamic holography in a photorefractive quantum well, which acts as an adaptive beam splitter to compensate for wave-front distortions. These devices operate close to the quantum noise limit in the presence of speckle and do not require path-length stabilization. Most important, the photorefractive quantum wells have a unique ability to achieve linear homodyne detection regardless of the value of the photorefractive phase shift.