High-intensity nanosecond photorefractive spatial solitons

Abstract

We report the observation of high-intensity solitons in a bulk strontium barium niobate crystal. The solitons are observed by use of 8-ns optical pulses with optical intensities greater than 100 MW͞cm 2 . Each soliton forms and attains its minimum width after roughly ten pulses and reaches e 21 of the steady-state width after the first pulse. We find good agreement between experimental observations and theoretical predictions for the soliton existence curve. 4,5 For the most part these demonstrations generally require high intensities in the megawatt range. Interestingly, although Kerr solitons form as a result of the presence of a refractive-index change that is proportional to the optical intensity, it is precisely this dependence that prevents the existence of stable two-dimensional (2D) bright Kerr solitons. Recently, a new type of spatial soliton 1 based on the photorefractive effect was predicted and observed both in a quasi-steadystate regime 2,3 and more recently in the steady-state regime. 4 -10 Compared with those of Kerr 11 -14 spatial solitons, the most distinctive features of photorefractive spatial solitons are that they are observed at low light intensities [in the milliwatts per square centimeter ͑mW͞cm 2 ͒ range] and that robust trapping occurs in both transverse dimensions. Both of these attributes make photorefractive solitons attractive for applications and for fundamental studies involving the interaction between spatial solitons. 15 -22 One transverse-dimension theory of photorefractive screening solitons 5 -7 predicts a universal relationship among the width of the soliton, the applied electric field, and the ratio of the soliton intensity to the sum of the equivalent dark irradiance and a uniform background intensity. We refer to this curve as the soliton existence curve. This curve is important because experiments show that considerable deviations (ϳ20% or more) from the curve lead to instability and breakup of the soliton beam, 10,22 whereas much smaller deviations are typically tolerated and are arrested by the soliton stability properties. In the case of a low-intensity photorefractive soliton beam, i.e., a beam with an intensity in the mW͞cm 2 to kW͞cm 2 range, recent 1D experiments have known good agreement with this universal relationship. 10,20Although the low-intensity feature of photorefractive spatial solitons is attractive for applications, high-intensity ͑MW͞cm 2 to GW͞cm 2 ͒ photorefractive solitons are also interesting, since the speed with which the steady-state screening soliton forms is inversely proportional to the optical intensity. As we show below, solitons in strontium barium niobate (SBN) can form at nanosecond speeds for GW͞cm 2 intensities, which implies that for photorefractive semiconductors, which have mobilities 100 -1000 times larger than those of the photorefractive oxides, soliton formation should occur at picosecond time scales for similar intensities. For these intensities, however, the excited free-carrier density is no longer smaller than that o...

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