Spontaneous Self-Trapping of Optical Beams in Metastable Paraelectric Crystals

Abstract

We report on the observation of a new mechanism for self-trapping of optical beams: selftrapping that stems from spontaneous creation of ferroelectric crystalline clusters, seeded by a weak photorefractive diffusion field. This is an evident observation of the highly nonlinear aspects of propagation in a thermodynamically metastable system, including optically driven crystalline ordering in a medium undergoing a phase transition. PACS numbers: 42.65.Jx, Spontaneous symmetry breaking in systems undergoing a phase transition is one of the most intriguing subjects of statistical physics. In this regime, fluctuations are exhalted and ultimately allow for a macroscopic dynamical change in the properties of a system. In this Letter, we investigate light propagation in a metastable system undergoing a phase transition, and show that, when the propagating entity (light) is coupled to the out-ofequilibrium host (matter), the spontaneous material response can lead to considerable nonlinear propagation effects and the light induces crystalline ordering. We launch a very narrow optical beam into a photorefractive potassium-lithium-tantalate-niobate (KLTN) crystal [1] and study its diffraction. When the crystal is at a temperature slightly above the paraelectric-ferroelectric transition, the beam diffracts and is initially strongly distorted. After a temporal transient, the beam self-focuses and eventually self-traps: exhibiting nondiffracting propagation. This occurs in both one ͑1 1 1͒D and two ͑2 1 1͒D transverse dimensions. The resultant self-trapping mechanism of the beam is extremely insensitive to the beam's parameters, and is attributed to spontaneous alignment of dipolar clusters induced by the weak photorefractive diffusion field. We refer to this mechanism as "spontaneous self-trapping" to underline the fact that the interaction is a seeded thermodynamic relaxation.Optical beam propagation at the vicinity of the Curie temperature ͑T c ͒ in media undergoing a phase transition has been recently addressed in the context of photorefractive media [2]. However, no observation has been carried out in the highly nonlinear metastable phase-transition regime itself. Typically, the medium is optically opaque at T T c , as the strong density fluctuations severely scatter light. Numerous other effects were observed in such systems. For example, in supersaturated aqueous urea solutions, light induces crystallization and prenucleation [3]. In some atomic/molecular systems, laser induces clustering [4]. In paraelectric KLTN, the screening of space-charge separation induces metastable ferroelectric clustering [5]. In nematic liquid crystals, laser heating generates isotropic "holes," giving rise to strong selfphase modulation [6], and in metals at the melting point, an enhanced cubic nonlinearity has been reported [7]. Thus far, optical propagation effects have been studied only in critical binary liquid mixtures [8], and even there only theoretically. Here we study nonlinear beam propagation in very close proximity to the thermody...

Soliton electro-optic effects in paraelectrics

“…Although results suggest a means of obtaining all-optical functionality, actual implementation is hampered by the generally slow nonlinear response [6], that can be "accelerated" only at the expense of stringent intensity requirements [7]. In contrast, non-dynamic guiding structures have been observed by fixing a screening soliton [8], or in relation to the observation of spontaneous selftrapping during a structural crystal phase-transition [9]. One possible method of obtaining acceptable dynamics is to make directly use of the electro-optic properties of the ferroelectrics involved, in combination with the internal photorefractive space charge field deposited by the soliton.…”

Fiber-launched ultratight photorefractive solitons integrating fast soliton-based beam manipulation circuitry

“…Involving more tenable values of ⌬n, 6 this approach shifts the integration issue one step further, i.e., collecting the transmitted signal into the outgoing fiber, thwarting the single-mode assembly. The stringent ultratight structure, which we have demonstrated for visible beams, for standard long-haul telecommunication systems, involves a considerably higher soliton index pattern; two to three times higher than that reported here: A matter that implies operation at still higher values of ⑀ r ͑activating near-transition effects͒, 8 or adopting other soliton formation schemes. Moreover, the nonlinear nature of the link involves an initial mode adaptation that self-consistently attains the adiabatic passage from the single-mode fiber output, associated with the stepindex structure, to the needle soliton wave form.…”

“…7 Our achievement is mediated by the emergence of buttcoupled ultratight ͓3-4 m intensity at full width half maximum ͑FWHM͔͒ needles that appear only in the very proximity of the crystal dielectric anomaly, at temperatures slightly above the ferroelectric phase-transition temperature T C , in a condition in which charge diffusion and spontaneous polarization play a negligible role. 8 We are able to implement this achievement to demonstrate a soliton-based switch and router on a single-mode backbone, for a longer nonphotorefractive wavelength. 5 The experimental setup is shown in Fig.…”

Fixing multiple waveguides induced by photorefractive solitons: directional couplers and beam splitters

“…Although the self-induced and easily erased nature of photorefractive soliton-induced waveguides is attractive for dynamic applications, for many applications it is advantageous to impress waveguides into the crystalline structure permanently, that is, to have the induced waveguide last indefinitely without an applied field, even under intense illumination. Recently, Klotz et al 19 and DelRe et al 20 demonstrated how to transform the "real-time" screening soliton into a permanent waveguide by means of ferroelectric domain reversal. 19,20 Here, we experimentally demonstrate the f ixing of multiple photorefractive solitons into permanent two-dimensional single-mode waveguides that also act as directional couplers and multiple beam splitters.…”