Stable oscillating nonlinear beams in square-wave-biased photorefractives

Abstract

We demonstrate experimentally that in a centrosymmetric paraelectric non-stationary boundary conditions can dynamically halt the intrinsic instability of quasi-steady-state photorefractive selftrapping, driving beam evolution into a stable oscillating two-soliton-state configuration.The propagation of light in a photorefractive crystal gives rise to intense beam self-action that, in its most generic manifestation, causes fanning and anisotropic scattering [1]. The application of an external bias field to the crystal can drastically change this behaviour, and allow spatial self-trapping and soliton formation: the nondiffracting propagation of micron-sized optical beams [2]. In a biased system this occurs only in a transient regime, for a finite time window, and the resulting nonlinear waves are called quasi-steady-state-solitons [3]. By increasing the natural dark conductivity, this regime can be made stable, giving rise to steady-state screening solitons [4].In this paper we investigate, for the first time, a fundamentally different stabilization process connected to beam behaviour in a non-stationary external bias field [5]. In particular, we study beam evolution in the presence of an alternating field in centrosymmetric potassiumlithium-tantalate-niobate (KLTN) [6], a material known to support a rich variety of nonlinear beam phenomena [7] [8] [9] [10].Results indicate that, for appropriate conditions, the beam self-trapping process, that leads to transient quasisteady-state solitons for stationary conditions, can be driven into a stable self-trapped regime, formed by an alternating oscillation between two beam trajectories, in the absence of enhanced dark conductivity. This phenomenon, in our understanding, is made possible by the fact that single optical trajectories, corresponding to the two alternate states of the bias field, non-coincident due to asymmetric diffusion-seeded bending and electro-optic read-out effects, engender the simultaneous formation, through the quadratic electro-optic response of the crystal in the paraelectric phase, of two trapping index patterns, that form two back-to-back specular double layers of charge that halt runaways charge buildup.Experiments are carried out in samples of zero-cut centrosymmetric photorefractive KLTN, a composite perovskite doped with Copper and Vanadium impurities, with a set-up that is similar to those generally used in photorefractive soliton studies [7] [8], apart from the absence of background illumination and the use of an alternating external voltage source. The sample temperature is kept at a given value T by means of a stabilized current controlled Peltier-junction. A λ=514nm continuous-wave TEM 00 beam, from an argon-ion laser, is first expanded and then focused onto the input facet of the sample, and launched along the crystal principal axis z. Focusing is obtained either with a cylindrical y-oriented lens, giving rise to a one-dimensional beam confined in the x transverse direction, for investigation of slab-solitons, or a spherical lens for ful...

Molding and Stretched Evolution of Optical Solitons in Cumulative Nonlinearities

“…Here, experiments for conditions in which the incoherent/multimodal self-trapping supersedes specklelike filamentation indicate a more elaborate inertial smearing of spacecharge fluctuations, that mediate self-action, even before an averaged response is reached [9]. Further evidence of this can be found in the appearance of multipattern oscillation, quite different from a time-averaged behavior, when the driving bias field is appropriately modulated in time [10], and more recently in the identification of spacetime effects in modulation instability [11]. Whereas the model of the dynamics has been in part formulated [12,13], the phenomenology and, hence, the physical behavior of the transient formation stage of solitons has remained hereto unknown.…”