September 2015
Spotlight Summary by Yaroslav V. Kartashov
Observation of stable-vector vortex solitons
The exploration of new nonlinear materials and geometries where stable self-sustained excitations in the form of vortex solitons can exist is one of the most attractive routes in modern nonlinear photonics. The authors of this Optics Letters paper provide an elegant experimental proof of the generation of such states in the form of vector complexes, where a red-light vortex soliton component is stabilized by nonlinear coupling to a green-light fundamental soliton state in a nonlocal medium.
While many nonlinear media with focusing nonlinearities are capable of supporting fundamental bell-shaped spatial solitons that keep their shapes upon propagation over considerable distances, the examples of uniform materials supporting stable vortex solitons are really rare. The primary reason for this fact is that vortex solitons are excited states of the system with bright ring-like shapes, carrying specific staircase phase distribution with a dislocation in the center. Being excited states, vortex solitons are usually prone to strong dynamical instabilities that tend to amplify local azimuthal modulations of the initial ring shape and split it into fragments flying away from the initial vortex ring.
A nonlocality of the nonlinear response of the medium may strongly influence the propagation of such excited states. The nematic liquid crystals used by the authors of this Optics Letters paper are widely utilized in experiments on self-trapping of laser radiation thanks to their strong reorientational nonlinearities. Upon illumination of these crystals with an intense beam a final refractive index distribution is established when equilibrium is achieved between the torque imposed on molecules by the light field and elastic forces present through intermo-lecular links. Therefore the width of the induced refractive index defect may extend far beyond the actual illumination area and, moreover, any local modulations of the beam shape that may occur in the course of propagation will tend to smooth out in the broader refractive index landscape induced by the beam, a mechanism that may lead to stable propagation.
In this work the authors make use of the nonlocal reorientational nonlinearity of liquid crystals for a first experimental demonstration of composite two-color vector solitons, where one of the components carries an optical vortex with unit topological charge. The coupling with the fundamental soliton avoids the astigmatic transformations of the input vortex component into spiraling dipole states that is unavoidable in this anisotropic medium when a vortex-carrying beam propagates alone. Remarkably, the composite vector soliton was observed for comparable powers of the red-light and green-light components, indicating a strong nonlinear coupling between them – a signature of the formation of a truly vector vortex soliton.
The results presented in this paper may stimulate the generation of till-now elusive types of composite solitons, such as multipole or multiring complexes and their periodic dynamical transformations/oscillations. The great potential of possible settings for controllable stable routing of vortex-carrying excitations and the information encoded in their nontrivial phase distributions should be properly appreciated.
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While many nonlinear media with focusing nonlinearities are capable of supporting fundamental bell-shaped spatial solitons that keep their shapes upon propagation over considerable distances, the examples of uniform materials supporting stable vortex solitons are really rare. The primary reason for this fact is that vortex solitons are excited states of the system with bright ring-like shapes, carrying specific staircase phase distribution with a dislocation in the center. Being excited states, vortex solitons are usually prone to strong dynamical instabilities that tend to amplify local azimuthal modulations of the initial ring shape and split it into fragments flying away from the initial vortex ring.
A nonlocality of the nonlinear response of the medium may strongly influence the propagation of such excited states. The nematic liquid crystals used by the authors of this Optics Letters paper are widely utilized in experiments on self-trapping of laser radiation thanks to their strong reorientational nonlinearities. Upon illumination of these crystals with an intense beam a final refractive index distribution is established when equilibrium is achieved between the torque imposed on molecules by the light field and elastic forces present through intermo-lecular links. Therefore the width of the induced refractive index defect may extend far beyond the actual illumination area and, moreover, any local modulations of the beam shape that may occur in the course of propagation will tend to smooth out in the broader refractive index landscape induced by the beam, a mechanism that may lead to stable propagation.
In this work the authors make use of the nonlocal reorientational nonlinearity of liquid crystals for a first experimental demonstration of composite two-color vector solitons, where one of the components carries an optical vortex with unit topological charge. The coupling with the fundamental soliton avoids the astigmatic transformations of the input vortex component into spiraling dipole states that is unavoidable in this anisotropic medium when a vortex-carrying beam propagates alone. Remarkably, the composite vector soliton was observed for comparable powers of the red-light and green-light components, indicating a strong nonlinear coupling between them – a signature of the formation of a truly vector vortex soliton.
The results presented in this paper may stimulate the generation of till-now elusive types of composite solitons, such as multipole or multiring complexes and their periodic dynamical transformations/oscillations. The great potential of possible settings for controllable stable routing of vortex-carrying excitations and the information encoded in their nontrivial phase distributions should be properly appreciated.
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Article Information
Observation of stable-vector vortex solitons
Yana Izdebskaya, Gaetano Assanto, and Wieslaw Krolikowski
Opt. Lett. 40(17) 4182-4185 (2015) View: Abstract | HTML | PDF