Thermo-optic soliton routing in nematic liquid crystals

Abstract

We demonstrate thermo-optic control on the propagation of optical spatial solitons in nematic liquid crystals. By varying the sample temperature, both linear and nonlinear optical properties of the reorientational material are modulated by acting on the refractive indices, the birefringence, and the elastic response. As a result, both the trajectory and transverse confinement of spatial solitons can be adjusted, demonstrating an effective means to tune and readdress self-induced optical waveguides.

Temperature control of nematicon trajectories

“…Despite this approximation, the average of ∆(u * u X − uu * X ) is too involved to calculate, even if f e is assumed to be a Gaussian (15). We note that the non-dimensional conductivity µ is large, O(100), for typical experiments, as found by using the parameter values of [35] and the NLC thermal parameter values of [37]. Therefore, the width w of the light beam is much narrower than the width of the temperature response of the medium, in much the same manner in which the NLC re-orientational response is nonlocal [6].…”

“…The light beam directly heats the medium, while the whole material is held at a fixed temperature. As the refractive indices n and n ⊥ are temperature dependent [35], the walkoff ∆ and the optical anisotropy ǫ a are also temperature dependent.…”

“…The only difficulty is the averaging of the walkoff term ∆(u * u X − uu * X ), as the walkoff ∆ depends on (X, Y ) through the temperature (17). To enable the averaging integral to be calculated, we note that the change due to temperature of the refractive indices n and n ⊥ is small compared with the initial values [35]. We then expand the walkoff ∆ in a Taylor series in τ…”

“…As we are only interested in the nematicon trajectory, we shall assume constant values for the nematicon amplitude a and width w, as well as the amplitude a d and width w d of the molecular director distribution. Taking the beam and director amplitudes and widths as constants, variations (30) and (31) based on the experimental data of [35].…”

“…In this paper, we model the routing of nematicons due to thermo-optic changes in the material properties, i.e., variations of the optical parameters of the dielectric due to the ambient temperature, as well as the (localized) heating caused by light absorption as the beam travels through the bulk medium. As recently demonstrated, such thermo-optical effects can alter the degree of nematicon confinement, its oscillatory behaviour in propagation and its trajectory [33][34][35]. Hereby, we adopt modulation theory [36] and develop a simple mathematical model describing temperature control of nematicon trajectories in undoped nematic liquid crystals, which includes the role of external temperature, input beam power and one-photon absorption.…”