Steering and guiding light with light in a nanosuspension

Abstract

We experimentally demonstrate guiding of a low-power probe beam (633 nm wavelength) by means of a lightinduced waveguide generated by the self-focusing of a strong pump beam (532 nm wavelength) in an artificial nonlinear medium, constituted by a colloidal suspension of dielectric nanoparticles. We also demonstrate optical steering of the probe beam by controlling the direction of propagation of the pump beam. The distance over which guiding is demonstrated (5 mm . Qualitatively, the nonlinearity can be understood as a consequence of the existence of an optical gradient force, which attracts (repels) nanoparticles to (from) the regions of highest optical intensity when their refractive index (n p ) is higher (lower) than that of the surrounding medium (n m ). This leads to spatial variations of the effective refractive index directly related with the intensity distribution of the incident beam. Although the phenomenon was initially described in terms of a pure Kerr nonlinearity [4], it was recognized later that this assumption is an oversimplification. In fact, it is well known that a Kerr nonlinearity does not yield stable soliton propagation in the 2 1D case [5]. This fact renewed the interest on the subject and motivated the development of several theoretical models intended to elucidate the right kind of nonlinearity produced by the nanosuspensions. The general idea is to consider the medium as a gas of particles [6][7][8][9], where diffusion is driven by the optical gradient force until reaching equilibrium conditions. This leads to diverse expressions for the position-dependent particle density ρr, determined by the incident light spatial distribution and the characteristics of the nonlinear response of the medium.In the slowly varying envelope approximation, the propagation of a light beam in the medium is described bywhere the optical field is given by Er; t ψr exp ik 0 n b z − ωt, with k 0 being the wavenumber of the light in vacuum, ω its frequency, and n b the refractive index of the medium in absence of light. Fρ represents a complex function of the particle density (or concentration), whose real and imaginary parts are associated with the refractive index and Rayleigh scattering loss, respectively. The equation describing the interaction between the incident light and the medium, which is specified for each theoretical model, forms a coupled system with Eq.(1). In order to explain the observation of stable propagation of OSS in 2 1D, different considerations and assumptions have been made, including for example, the presence of nonlinear Rayleigh losses [7], hard-sphere potential interactions [8], or screened Coulomb repulsions ("van der Waals" gas) [9], which lead to different kinds of nonlinearities, from Kerr to exponential. With the aim of testing the different theoretical approaches, new experiments were conducted recently [10].In this Letter, we present experimental results on the guiding of a weak probe beam (wavelength λ r 633 nm) by means of the waveguide induced by an intense pump beam (wav...

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