Nonlocal bi-color vector solitons in liquid crystals

We investigate a novel class of vector solitons (VS), two-color spatial solitons in a highly nonlocal and anisotropic Kerr-like medium. While each color experiences a different birefringence and walk-off, the generated nonlocal VS is a self-localized breather with a unique Poynting vector. With reference to the experiments in nematic liquid crystals (NLC), we refer to a reorientational response, adopting the paraxial approximation along each Poynting vector (s) and assuming incoherent interactions owing to non-locality. The derived set of coupled Schrodinger equations with parabolic potentials nonlinearly depending on the beam amplitudes describes an induced waveguide with index distribution longitudinally varying through cross-phase modulation; hence, eigenmodes and eigenvalues change continuously along s, giving rise to different VS breathing at the two colors and to aperiodic propagation. We carried out the experiments in the limit of single walk-off using NLC in a highly nonlocal geometry, launching extraordinarily-polarized Gaussian red and infrared beams in a collinear configuration. Figure 1 shows acquired (left) and computed (right) intensities in the propagation plane of the cell for input powers of P_R=1.6mW at 632.8nm, and 1.064mum powers P_NIR =0 (a, b), 0.7 (c, d) and 2.4mW (e, f), respectively. Clearly, both colors contribute to self-localization. Fig. 2 (left) maps the acquired red peak intensity I_R(s) versus propagation s and total excitation P_R+P_NIR (left axes) for a fixed Pr. The observed breathing and its dependence on total excitation are in agreement with predictions (right) for Pr =0.1 (a, e), 0.4 (b, f), 1.0 (c, g) and 1.6mW (d, h), respectively. While aberrations and scattering upon light injection in the cell cause a slight shift of the breathing to the right with respect to the simulations, the agreement between the coupled NLSE model and data is excellent.