Soliton electro-optic effects in paraelectrics

Abstract

The combination of charge separation induced by the formation of a single photorefractive screening soliton and an applied external bias field in a paraelectric is shown to lead to a family of useful electro-optic guiding patterns and properties.Apart from their inherent interest as peculiar products of nonlinearity, spatial solitons hold the promise of allowing viable optical steering in bulk environments [1] [2]. Photorefractive screening solitons differ from other known manifestations of spatial self-trapping for their peculiar ease of observation and versatility [3], and recent experiments in photorefractive strontium-bariumniobate (SBN) and potassium-niobate (KNbO 3 ) have demonstrated two conceptual applications of their guiding properties. In the first case, a tunable directional coupler was realized making use of two independent slabsolitons [4]; in the second, self-induced phase-matching was observed to enhance second-harmonic-generation [5]. 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. Since photorefractive charge-activation is wavelength dependent, one can induce charge separation in soliton-like structures at one active wavelength (typically visible), and then read the electrooptic index modulation at a different, nonphotorefractive, wavelength (typically infrared) [10] [11]. For noncentrosymmetric samples (such as the above mentioned crystals) that typically host screening soliton formation, the electro-optic index of refraction modulation is proportional to the static crystal polarization P, and thus to the electric field (linear electro-optic effect). For these, no electro-optic modulation effects are possible: for whatever value of external constant electric field E ext , the original soliton supporting guiding pattern remains unchanged. In centrosymmetrics, such as photorefractive potassium-lithium-tantalate-niobate (KLTN), solitons are supported by the quadratic electro-optic effect [12] [13] [14] [15]. In this case, the "nonlinear" combination of the internal photorefractive field with an external electric field can give rise to new and useful soliton-based electro-optic phenomena, which we here study for the first time.The basic mechanism leading to screening soliton formation is the following: a highly diffracting optical beam ionizes impurities hosted in the lattice of an electro-optic crystal. An externally applied electric field makes these mobile charges drift...

Anisotropic charge displacement supporting isolated photorefractive optical needles

“…Needles, in their richer higher-dimensional environment, have led to a substantial advance in our phenomenological investigation of nonlinear dynamics, expanding the scope of possible soliton-based applications [4] [5].…”

“…This is made possible by the quadratic electro-optic response of paraelectrics, that allows the electro-holographic separation of optical phenomenology from the underlying space-charge field [5].…”

“…One basic consequence of these findings is that the anisotropy underlying a photorefractive needle leads not to one, but to three spatially separated index structures, that can be made to alternatively guide light depending on the applied external voltage in the read-out phase. This would not have been possible had the nonlinear response been local, as in the one-dimensional case [5]. The electroholographic read-out would have implied a transition from a localized single mode structure (the needle) to a delocalized "doughnut-like" guiding pattern.…”

Electro-optic beam manipulation through photorefractive needles

“…Studies of one-dimensional slab solitons show that electro-optic functionality can be exploited to couple, route, and modulate light. 12 An obstacle to direct extension of this functionality to needles is the distorted and highly elliptical nature of the underlying anisotropic lobular patterns. 13 In this Letter we describe experiments that led to what is to our knowledge the f irst demonstration of practical electro-optic beam manipulation based on the electro-optic functionality associated with a pair of photorefractive needles of opposite electroholographic charge.…”

Stable oscillating nonlinear beams in square-wave-biased photorefractives

“…Concerning the formation of the twostate oscillation, we note that, although in a purely driftlike configuration free photoexcited charge under the influence of a zero average square-wave alternate field E sq , with T sq ≪ τ 1 , cannot give rise to any space-charge separation, asymmetric charge diffusion components can seed two intensity distributions I + and I − , corresponding to the two alternate states of external field, that separate during propagation along the z axis, and thus allows a non-zero photorefractive response [12]. The final state is a product of beam charge separation during one electric field polarity, combined with electro-holographic effects during the opposite polarity phase [13]. To break down the process, consider the formation of a single quasi-steady-state soliton with a constant voltage V=V + (equal to the positive value of the alternating field in the oscillating case)(see Fig.(4a-b)).…”