Ming-Feng Shih

Photorefractive screening solitons of high and low intensity

We analyze self-trapping of optical beams in photorefractive media for low irradiances that are typical of cw lasers and high irradiances that are typical of short-pulse lasers and obtain bright and dark soliton solutions in one transverse dimension. Whereas in the low-intensity regime photorefractive screening solitons are trapped by the refractive-index change that is related to the electric field associated with ionized donors or acceptors, in the high-intensity regime the index is related to the field that is also associated with free carriers. Since the time necessary to form the soliton scales inversely with the intensity, one expects a very fast response time in the high-intensity regime, and this suggests attractive applications in optical switching and reconfigurable optical waveguides. We show that throughout the entire intensity range dark solitons require lower voltages and less energy per pulse for applications than do bright solitons.

Two-dimensional steady-state photorefractive screening solitons

We study experimentally steady-state photorefractive screening solitons trapped in both transverse dimensionsand measure their beam profiles as they propagate throughout the crystal. The solitons are observed to be axially symmetric, and they self-bend. We characterize the soliton dependence on the optical intensity, appliedelectric-field strength, and beam diameter.

INCOHERENT COLLISIONS BETWEEN TWO-DIMENSIONAL BRIGHT STEADY-STATE PHOTOREFRACTIVE SPATIAL SCREENING SOLITONS

We report the observation of incoherent collisions between two-dimensional bright photorefractive screening solitons. The solitons remain intact and do not exchange energy whenever the collision angle exceeds thecritical angle for guidance in the waveguide that each soliton induces, which is, in turn, fully controlled by the soliton parameters. When the collision angle is much smaller than the critical angle the solitons fuse to form a single beam.

STEADY-STATE DARK PHOTOREFRACTIVE SCREENING SOLITONS

We present an experimental study of steady-state dark photorefractive screening solitons trapped in a bulk strontium barium niobate crystal. We compare experimental measurements with theoretical calculations of the soliton properties and find good agreement between theory and experiments. We confirm the shapepreserving behavior of the dark soliton by measuring its beam profile as it propagates throughout a specially cut crystal and by guiding a beam of a different wavelength.

Directional coupler with soliton-induced waveguides

We demonstrate a directional coupler that employs two waveguides induced by two mutually incoherent photorefractive solitons propagating in parallel at close proximity. Efficient coupling from one waveguide to the other is achieved for probe beams at wavelengths much longer than that of the solitons. We study the mutual coupling as a function of distance between solitons.

CIRCULAR WAVEGUIDES INDUCED BY TWO-DIMENSIONAL BRIGHT STEADY-STATE PHOTOREFRACTIVE SPATIAL SCREENING SOLITONS

We report what we believe is the first experimental observation of bright-soliton-induced two-dimensional waveguides. The waveguides are induced by two-dimensional photorefractive screening solitons, and their guiding properties, i.e., whether the waveguides are single mode or multimode, are controlled by adjustment of the soliton parameters.

SECOND-HARMONIC GENERATION IN WAVEGUIDES INDUCED BY PHOTOREFRACTIVE SPATIAL SOLITONS

We propose and demonstrate experimentally second-harmonic generation in waveguides induced by photorefractive solitons, and show that the conversion efficiency is improved considerably.

Coherent collisions of photorefractive solitons

We present an experimental study of coherent collisions between one-dimensional bright photorefractive screening solitons in a bulk strontium barium niobate crystal.

Steady-state vortex-screening solitons formed in biased photorefractive media.

We report the observation of steady-state photorefractive vortex-screening solitons. As a singly charged circular vortex nested on a broad beam propagates through a biased strontium barium niobate crystal, it self-traps in both transverse dimensions despite the inherent anisotropy of the photorefractive nonlinearity. When the vortex beam is a doughnut-shaped narrow beam, it breaks up into two elongated slices (with a self-defocusing nonlinearity) or into two focused filaments (with a self-focusing nonlinearity). We demonstrate the optical guidance of a probe beam in a circular waveguide induced by the self-trapped vortex.

Optically induced photovoltaic self-defocusing-to-self-focusing transition

We show theoretically and experimentally that the photovoltaic nonlinearity that gives rise to spatial solitons can be switched from self-defocusing to self-focusing (or vice versa) by use of background illumination. This raises the possibility of bright photovoltaic solitons in LiNbO(3) .