J. L. Shultz

Photorefractive Spatial Solitons

Photorefractive crystals are electro-optic dielectrics that host a small amount of photosensitive impurities. Light propagation leads to the generation of outof-equilibrium mobile charge, that, in order to reach a stable electro-static configuration, redistributes throughout the crystal. The ensuing space-charge field, modifying electro-optically the crystal index of refraction, changes the trajectory of the ionizing light distribution, altering, in turn, the original charge-equilibrium conditions [1]. This feedback mechanism gives rise to a variety of nonlinear effects that go under the generic term of photorefractive nonlinear optics [2] [3] [4] [5]. For confined optical beams, nonlinear beam dynamics leads to two basic qualitatively different phenomena: beam fanning and self-lensing, connected, respectively, to the two basic charge transport mechanisms, diffusion and drift. Photorefractive spatial solitons emerge when beam self-focusing exactly balances diffraction, and are thus generally connected to regimes in which charge drift plays a fundamental role [6] [7] [8][9]. In a nonlinear beam perspective, spatial beams self-trap when the light-space-charge feedback mechanism finds its dynamic equilibrium point in a nondiffracting slab or needle of light, corresponding to an appropriate waveguide-like refractive index distribution. In what follows, we discuss such nonlinear phenomena, concentrating in particular on the basic theory and phenomenology.

Strain-driven facet formation on self-assembled InAs islands on GaAs (311)A

The shape of InAs three-dimensional islands grown on GaAs(311)A substrates by molecular-beam epitaxy was investigated by in situ scanning tunneling microscopy. The island is found to be laterally surrounded by (111)A and {110} facets together with a convex curved region close to the (100) facet. The top ridge of the islands is atomically resolved to be the most recently discovered high-index surface {11,5,2}. This observation points to the importance of the study of nanostructure growth on high-index surfaces and their characterization.

GaAs(311) templates for molecular beam epitaxy growth: surface morphologies and reconstruction

Morphologies of GaAs(311) surfaces grown by molecular beam epitaxy were investigated by in situ reflection high-energy electron diffraction and scanning tunnelling microscope. In addition to the (8×1) reconstruction, two surface phases, GaAs(311)A-(4×1) and GaAs(311)B-(2×1) were observed. Both of these surfaces are characterized by wider, atomically smooth terraces with much lower structural anisotropy, when compared to the (8×1) reconstructed GaAs(311) surfaces. The observed surfaces have potential as templates for the growth of organized quantum dots, wires, and wells.

One-dimensional postwetting layer in InGaAs/GaAs(100) quantum-dot chains

Long chains of quantum dots formed in InGaAs∕GaAs(100) multiple layers have been systematically investigated by scanning electron, transmission electron, and atomic force microscopies. In addition to the usual two-dimensional wetting layer involved in the Stranski-Krastanov growth, we have directly observed a one-dimensional postwetting layer along the [01−1] direction that strings together the quantum dots in each chain. In sharp contrast with the two-dimensional wetting layer, which exists before the quantum-dot chains form, this one-dimensional postwetting layer develops during the GaAs capping of the existing dot chains. This one-dimensional layer forms through the anisotropic surface diffusion of In atoms that accompanies the change in strain profile during capping and therefore produces the steady-state material distribution that includes a one-dimensional postwetting layer as a result.

Morphology evolution during strained (In,Ga)As epitaxial growth on GaAs vicinal (100) surfaces

Molecular-beam-epitaxy growth of strained (In,Ga)As on GaAs vicinal (100) surfaces is investigated by scanning tunneling microscopy. Surface roughing as the consequence of step bunching driven by strain is explored. By tuning the In content over the range from 0.05 to 0.2, the step bunching is observed to exhibit considerable uniformity and order. These results experimentally demonstrate that strain-driven step bunching is a viable approach to provide templates for nanostructure growth.

Manifestation of circular photogalvanic current by dynamic holography in BaTiO3

We report the first measurement of the photo-galvanic circular current antisymmetric tensor component in BaTiO3: Co. The measurement gives a value of this coefficient, for extraordinary beam amplification, of 4×10−9 A/W using a nonstationary measurement technique at a wavelength of 0.632 μm.

Color imaging in photorefractive crystals

Color phase-conjugate imaging is demonstrated using a multi-colored laser beam. Speed of response, size of the image, clarity of the image, and the intensity of the image are investigated. Color images are stored and recalled without crosstalk between different colors.

Enhancing the photorefractive effect

Abstract The transient response for beam fanning in photorefractive tungsten bronze crystals with an applied electric field and a focused laser beam is studied. Response times on the order of 1 ms for an incident power of 1 mW observed for incident beams focused to a 30 μm diameter in the crystal. This improved response time is accomplished without the significant reduction in the magnitude of the photorefractive effect normally observed for focused beams.

Experimental observation of chirped continuous pulse-train soliton solutions to the Maxwell-Bloch equations

A frequency chirped continuous wave laser beam incident upon a resonant, two-level atomic absorber is seen to evolve into a Jacobi elliptic pulse-train solution to the Maxwell-Bloch equations. Experimental pulse-train envelopes are found in good agreement with numerical and analytical predictions.

Holographic interferometry and image processing on reflection and transmission gratings in photorefractive materials

We present both a theoretical and experimental analysis of dynamic holographic interferometry using a reflection grating geometry in Bi12TiO20.