Galen Duree

Dimensionality and size of photorefractive spatial solitons.

We study experimentally self-trapping of optical beams in photorefractive media and show that the trapping is inherently asymmetric with respect to the two (transverse) trapping dimensions. We also present experimental results that show how the sizes of the resultant photorefractive spatial solitons are independent (within their range of existence) of the amplitude of the externally applied electric field used to generate them.

Stability of photorefractive spatial solitons.

We present a theoretical analysis of the stability of photorefractive spatial solitons along with experimental results that show that the solitons are stable for small-scale perturbations but break down when the perturbations exhibit a transverse scale comparable with the soliton size (cross section).

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.

Photorefractive self-focusing and defocusing as an optical limiter

Focusing and defocusing of laser light has been observed for many years. Kerr type materials exhibit this effect but only for high intensities. We show experimental evidence that photorefractive materials can also produce dramatic focusing and defocusing. Whereas Kerr materials produce this effect for high intensities, photorefractive materials produce these effects independent of intensity indicating that this effect would be ideal for an optical limiter. We compare the characteristics of Kerr and photorefractive materials, discuss the physical models for both materials and present experimental evidence for photorefractive defocusing. Self-focusing and defocusing was observed for any incident polarization although the effect was more pronounced using extraordinary polarized light. In addition, self-focusing or defocusing could be observed depending on the direction of the applied electric field. When the applied field was in the same direction as the crystal spontaneous polarization, focusing was observed. When the applied field was opposite the material spontaneous polarization, the incident laser light was dramatically defocused.

The use of applied electric fields the photorefractive tungsten bronze ferroelectrics

The traditional method of determining the photorefractive effective charge density is to plot the photorefractive space charge field versus the crossing angle in a two-beam coupling experiment. The difficulty with this traditional measurement technique is that the apparatus must be moved several times in order to obtain data over the sufficient number of crossing angles needed for an accurate fit with theory. Moreover, with small crossing angles the overlap between the two crossing beams can easily extend over the entire crystal, while with larger crossing angles the overlap between the two beams becomes less certain. In this paper we demonstrate an alternative method of determining the photorefractive charge density. In this approach we measure the phase shift between the optical intensity pattern in the crystal and the resulting index pattern, as a function of the magnitude of an applied d.c. field. By comparing the measured value of the d.c. field which produces a minimum phase shift with that predicted by theory the photorefractive effective charge density is found. In this case, only the magnitude of the applied field is varied and the apparatus remains fixed. The result is obtained quickly and with little error.

Photorefractive spatial solitons-theory and experiments

The existence of photorefractive (PR) spatial solitons has been predicted by us some two years ago/sup (1,2)/. The self-trapping effects occur when diffraction is exactly balanced by self-scattering (two-wave mixing) of the spatial (plane wave) components of the soliton beam.<<ETX>>