Within the last several years, the field of terahertz science and technology has changed dramatically. Many new advances in the technology for generation, manipulation, and detection of terahertz radiation have revolutionized the field. Much of this interest has been inspired by the promise of valuable new applications for terahertz imaging and sensing. Among a long list of proposed uses, one finds compelling needs such as security screening and quality control, as well as whimsical notions such as counting the almonds in a bar of chocolate. This list has grown in parallel with the development of new technologies and new paradigms for imaging and sensing. Many of these proposed applications exploit the unique capabilities of terahertz radiation to penetrate common packaging materials and provide spectroscopic information about the materials within. Several of the techniques used for terahertz imaging have been borrowed from other, more well established fields such as x-ray computed tomography and synthetic aperture radar. Others have been developed exclusively for the terahertz field, and have no analogies in other portions of the spectrum. This review provides a comprehensive description of the various techniques which have been employed for terahertz image formation, as well as discussing numerous examples which illustrate the many exciting potential uses for these emerging technologies.

https://iopscience.iop.org/article/10.1088/0034-4885/70/8/R02/pdf

Imaging with terahertz radiation

An optical coherence tomography system is described which can image up to video rate. The system utilizes a high power broadband source and real time image acquisition hardware and features a high speed scanning delay line in the reference arm based on Fourier-transform pulse shaping technology. The theory of low coherence interferometry with a dispersive delay line, and the operation of the delay line are detailed and the design equations of the system are presented. Real time imaging is demonstrated in vivo in tissues relevant to early human disease diagnosis (skin, eye) and in an important model in developmental biology (Xenopus laevis).

In vivo video rate optical coherence tomography.

5.3.2. Traps and Grating Dark Decay 3288 5.3.3. Other Concerns 3288 6. New Photorefractive Materials 3289 6.1. Polymer Composites 3289 6.2. Organic Amorphous Glasses 3292 6.3. Fully Functionalized Polymers 3294 6.4. Polymer-Dispersed Liquid Crystals 3295 6.5. Other Liquid Crystal-Containing Materials 3296 6.6. Near-Infrared-Sensitive Materials 3297 6.7. Other Materials Directions 3298 6.7.1. Hybrid Organic−Inorganic Composites and Glasses 3298

Organic photorefractives: mechanisms, materials, and applications.

Semiconductors provide some of the most promising materials for nonlinear optics, because of large resonant nonlinearities, control of recombination time (from milliseconds to femtoseconds), well-developed fabrication technologies, and compatibility with other optoelectronic devices. The paper reviews some of the concepts and results that have come from the study of nonlinear optics in semiconductors. The emphasis is on nonlinear absorption and the refractive index that arises from the photo-induced excitation of free carriers. Mechanisms described include state-filling, carrier transport, and photorefractivity. Devices include optical bistability, reflective asymmetric Fabry-Perot all-optical absorptive switches, optically addressed spatial light modulators, and real-time holography. The paper's approach is to provide a basic engineering understanding of the principles, some of the historical details, and a snapshot of the state of the field today.

Resonant optical nonlinearities in semiconductors

This review covers a spectrum of optoelectronic properties of and uses for semi-insulating semiconductor heterostructures and thin films, including epilayers and quantum wells. Compensation by doping, implantation, and nonstoichiometric growth are described in terms of the properties of point defects and Fermi level stabilization and pinning. The principal optical and optoelectronic properties of semi-insulating epilayers and heterostructures, such as excitonic electroabsorption of quantum-confined excitons, are described, in addition to optical absorption by metallic or semimetallic precipitates in these layers. Low-temperature grown quantum wells that have an arsenic-rich nonstoichiometry and a supersaturated concentration of grown-in vacancies are discussed. These heterostructures experience transient enhanced diffusion and superlattice disordering. The review discusses the performance of optoelectronic heterostructures and microcavities that contain semi-insulating layers, such as buried heterostructure stripe lasers, vertical cavity surface emitting lasers, and optical electroabsorption modulators. Short time-scale applications arise from the ultrashort carrier lifetimes in semi-insulating materials, such as in photoconductors for terahertz generation, and in saturable absorbers for mode-locking solid state lasers. This review also comprehensively describes the properties and applications of photorefractive heterostructures. The low dark-carrier concentrations of semi-insulating heterostructures make these materials highly sensitive as dynamic holographic thin films that are useful for adaptive optics applications. The high mobilities of free carriers in photorefractive heterostructures produce fast dielectric relaxation rates that allow light-induced space-charge gratings to adapt to rapidly varying optical fringe patterns, canceling out environmental noise during interferometric detection in laser-based ultrasound, and in optical coherence tomography. They are also the functional layers in high-sensitivity dynamic holographic materials that replace static holograms in Fourier imaging systems and in experimental Tbit/s optical systems. Semi-insulating heterostructures and their applications have attained a degree of maturity, but many critical materials science issues remain unexplored.

/pdf/semi-insulating-semiconductor-heterostructures-s0zw42kvzr.pdf

Semi-insulating semiconductor heterostructures: Optoelectronic properties and applications

We demonstrate a laser-based adaptive ultrasonic homodyne receiver using dynamic holography in AlGaAs/GaAs photorefractive multiple quantum wells. The dynamic hologram acts as an adaptive beamsplitter that compensates wavefront distortions in the presence of speckle and requires no path-length stabilization. The photorefractive quantum wells have the unique ability to achieve maximum linear homodyne detection regardless of the value of the photorefractive phase shift by tuning the excitonic spectral phase. We achieve a root mean square noise-equivalent surface displacement of 6.7×10−7 A(W/Hz)1/2.

/pdf/laser-based-ultrasound-detection-using-photorefractive-39f08a4d7i.pdf

Laser-based ultrasound detection using photorefractive quantum wells

The photorefractive response to an applied electric field is measured in a photorefractive quantum well, providing evidence in favor of the nonlinear transport in the device due to the hot electrons. The reduced mobility of the hot electrons limits the drift length, and thereby limits fringe overshoot. Thus the nonlinear transport prevents the slowing down of the grating writing rate for increasing fields which is common in bulk photorefractives. The photorefractive phase shift in transverse-field photorefractive quantum wells is measured as a function of the frequency offset between two laser writing beams that generate moving gratings. The two-wave mixing passes through a maximum at an optimum frequency which depends on the magnitude and the sign of the applied dc electric field. The phase shift associated with the moving grating adds or subtracts from the static phase shift induced by hot-electron transport in the semiconductor quantum wells, depending on the sign of the field and the sign of the dominant photocarriers. We observe a linear relationship between the roll-off frequency and the power of the writing beams.

Two-wave-mixing dynamics and nonlinear hot-electron transport in transverse-geometry photorefractive quantum wells studied by moving gratings

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.

/pdf/photorefractive-p-i-n-diode-quantum-well-spatial-light-5ckezcq5r3.pdf

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

Sharp quantum‐confined excitons in semi‐insulating low‐temperature‐growth AlAs/GaAs quantum wells with 15 ps carrier lifetimes are demonstrated. High‐quality well‐barrier interfaces can be grown at low substrate temperatures and annealed up to temperatures of 700 °C, beyond which interface mixing broadens the exciton transitions. Electroabsorption from the quantum‐confined Stark effect in as‐grown modulators approaches 10 000 cm−1, which is comparable to traditional high‐temperature growth quantum wells. The low‐temperature growth quantum well structures eliminate the need for postgrowth processing, such as ion implantation for photorefractive quantum wells, ultrafast saturable absorption, or electro‐optic sampling applications.

Ultrafast-lifetime quantum wells with sharp exciton spectra

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.

I. Lahiri

Papers

Laser-based ultrasound detection using photorefractive quantum wells

Two-wave-mixing dynamics and nonlinear hot-electron transport in transverse-geometry photorefractive quantum wells studied by moving gratings

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

Ultrafast-lifetime quantum wells with sharp exciton spectra

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