R. M. Brubaker

Photorefractive quantum wells: transverse Franz–Keldysh geometry

The photorefractive properties of semi-insulating AlGaAs–GaAs multiple quantum wells are described for the transverse Franz–Keldysh geometry with the electric field in the plane of the quantum wells. Combining the strong electroabsorption of quantum-confined excitons with the high resistivity of semi-insulating quantum wells yields large nonlinear optical sensitivities. The photorefractive quantum wells have effective nonlinear optical sensitivities of n2 ≈ 103 cm2/W and α2/α0 ≈ 104 cm2/W for applied fields of 10 kV/cm. Photorefractive gains approaching 1000 cm−1 have been observed in two-wave mixing under dc electric fields and stationary fringes. The transverse Franz–Keldysh geometry retains the general transport properties and behavior of conventional bulk photorefractive materials. The resonant excitation of free electrons and holes in the quantum wells leads to novel behavior associated with electron–hole competition. We demonstrate that under resonant excitation of electrons and holes the device resolution is fundamentally limited by diffusion lengths but is insensitive to long drift lengths.

Femtosecond pulse shaping by dynamic holograms in photorefractive multiple quantum wells

Femtosecond pulses can be shaped in the time domain by diffraction from dynamic holograms in a photorefractive multiple quantum well placed inside a Fourier pulse shaper. We present several examples of shaped pulses obtained by controlling the amplitude or the phase of the hologram writing beams, which modifies the complex spectrum of the femtosecond output.

Bandwidth-limited diffraction of femtosecond pulses from photorefractive quantum wells

The diffraction of 100-fs pulses from the static gratings of photorefractive quantum wells (QWs) produces diffracted pulses that are nearly transform-limited, despite the strong dispersion near the quantum-confined excitonic transitions. This quality makes the QWs candidates for use in femtosecond pulse shaping, although the currently limited bandwidth of the quantum-confined excitonic transitions broadens the diffracted pulses. Femtosecond electric-field cross correlation and spectral interferometry techniques completely characterize the low-intensity pulses diffracted from stand-alone photorefractive QWs, and from QWs placed inside a Fourier-domain femtosecond pulse shaper.

Two‐wave mixing in Stark geometry photorefractive quantum wells using moving gratings

Nonreciprocal energy transfer between two coherent laser beams during two‐wave mixing requires a photorefractive phase shift between the interference pattern and the resulting space charge. The symmetry of the Stark geometry photorefractive quantum wells forbids a phase shift using either dc or ac fields. We present the first experimental demonstration of nonreciprocal energy transfer during two‐wave mixing using moving gratings. Optical energy exchange is observed in photorefractive p‐i‐n quantum well diodes with photorefractive gains approaching 1000 cm−1 and exhibiting complex spatio‐temporal dynamics.

Photorefractive phase shift induced by nonlinear electronic transport

A photorefractive phase shift can be generated under dc applied fields if the dominant photocarriers have a nonlinear velocity-field dependence with a vanishing differential mobility. Phase shifts as large as pi/2 are possible when velocity saturation disables dielectric relaxation while still permitting large drift rates. The inability of the space-charge field to relax leads to a saturated trap density that mimics trap-limited behavior. All direct-gap photorefractive semiconductors have strong velocity saturation from hot-electron transport effects, most widely known for the origin of the Gunn effect. Previous photorefractive trap-limited-field studies may have to be reevaluated in the context of transport nonlinearity.

Femtosecond pulse shaping by dynamic holographyin photorefractive multiple quantum wells

Femtosecond pulses can be shaped in the time domain by diffraction from dynamic holograms in a photorefractive multiple quantum well placed inside a Fourier pulse shaper. We present several examples of shaped pulses obtained by controlling the amplitude or the phase of the hologram writing beams, which modifies the complex spectrum of the femtosecond output.