Quenching nematicon fluctuations via photo-stabilization

Abstract

Abstract-Light localization into optical spatial solitons can be achieved by launching optical beams in nonlocal nonlinear nematic liquid crystals. Such solitons often undergo undesired fluctuations of their trajectories. We demonstrate that partial polymerization in monoacrylate-doped nematic liquid crystals is effective in quenching such fluctuations in transverse space.Spatial nonlocality in the optical response of dielectrics entails a number of benefits when dealing with nonlinear optics for signal processing and switching. It was recognized in the early days of nonlinear integrated optics that a diffusive hence nonlocal response, such as in thermo-optic materials, could lead to longitudinal optical feedback even in the absence of resonances, with consequent hysteresis versus input beam excitation [1][2][3][4]. More recently, a nonlocal nonlinear response was found fit to stabilize beam self-confinement into twodimensional (2D) spatial solitons which, in local Kerr media with an intensity-dependent refractive index, are otherwise unstable and prone to filamentation and catastrophic collapse [5][6][7][8][9][10]. In the highly nonlocal limit in space, spatial optical solitons tend to mutually attract and exhibit long-range interactions, [11][12] as well as reduced modulational instability [13][14], incoherent character [15][16], and breathing oscillations [17][18]. The synergy of local and nonlocal effects in time was indicated as a route towards spatiotemporal light bullets [19].Nematic liquid crystals (NLC) are molecular liquids with a substantial degree of orientational order, so much so that they exhibit a macroscopically (positive) uniaxial response, a reorientational nonlinearity, a large electrooptic effect as well as high nonlocality [20][21]. When an intense light beam with electric field E wave propagates through NLC, the induced microscopic dipoles tend to rotate under the action of torque:with  0 the dielectric susceptibility of vacuum, n e and n o the refractive index eigenvalues for electric fields parallel and orthogonal to the optic axis n, respectively. The resulting increase of the orientation angle  between the wavevector k||z and n yields self-focusing through an increase in the extraordinary (e)-refractive index ruled by   When self-focusing compensates for diffraction, a stable spatial soliton (a nematicon) is formed, with an associated graded-index waveguide wider that the beam generating it [5,22]. All-optical reorientation enables nematicon generation even at sub-mW power levels [23][24], but such self-trapped beams are usually subject to dynamic instabilities due to the interplay between optical torque, thermal agitation, fluid convection [25], anisotropy [26], and anchoring conditions at the boundaries.Transverse light localization through all-optical reorientation, in fact, is understood to locally increase the order parameter of this soft matter; this local field (alloptical) effect combines/competes with the bulk reorientation of the optic axis (molecular director) due to ela...