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Observation of the commutation between bright and dark spatial optical solitons

We report a new experimental technique on generation of both dark and bright optical spatial solitons in liquid crystals doped with organic dyes. In our recent works we have demonstrated that in some liquid crystals (LC's) the effective nonlinear refraction index (γ) is polarization dependent. Moreover, under certain conditions it is possible to realize inversion of γ, from γ > 0 to γ < 0, by proper adjustment of incident lightwave polarization. In this work we present experimental evidence that this LC is capable of supporting both bright and dark spatial solitons. The experimental setup used for characterization of solitons used two CCD cameras, which allowed us to analyze the longitudinal and transverse propagation characteristics of solitons in a glass cell containing a LC. It was possible to observe a clear correlation between the phase and the polarization state of incident light and the characteristic profile of the dark or bright soliton. The extreme values of the effective nonlinear refraction index were measured to be γ = -4.68 × 10-10 cm2/W (dark spatial soliton case), and γ = 2.80 × 10-10 cm2/W (bright spatial soliton). The key parameters of spatial solitons were estimated by using the nonlinear Schrödinger equation.