Propagation And Control Of Spatial Solitons In Nonlocal Medium

Spatial optical solitons are light beams that keep their shapes upon propagation be-cause of the balance between di?raction and nonlinearity. Spatial optical solitons have po-tential applications for all-optical signal processing and routing due to their particle-likenature on propagations and interactions. Recently, the study on nonlocal spatial solitonsbecomes a research hotspot in the soliton area due to their numerous novel properties. Incontrast to local media, in nonlocal media, the nonlinear refractive index response at aparticular point is not determined solely by the wave intensity at that point, but also de-pends on the wave intensity in its vicinity. Nonlocal media exhibit many features whichfacilitate the formation, stabilization and propagation of spatial optical solitons. Thus thenonlocal spatial solitons ensure larger ?exibility than their local counterparts on solitoncontrol. The main objective of this thesis is to investigate of the new propagation dynam-ics and the feasibility of being optical control scheme for nonlocal solitons.Based on the phenomenological propagation model combining competing focusingand defocusing nonlinearities with di?erent nonlocal length, the features of fundamentaland dipole solitons are investigated. It is shown that the mismatch between nonlocallengths of the two nonlinear components can not only largely modify the existence andstability properties of the dipole solitons, but also change the characters of the in-phasesoliton interaction from attraction to repulsion. There exists an unstable composite solitonserving as a intermediate state of such interaction-behavior transformations.Based on the Mihalache'phenomenological model for nonlocal medium featuringcompeting cubic-quintic nonlinearities, the properties of dark-type solitons, includingdark solitons and dark-like bright solitons, are investigated. It is revealed that nonlocalitydrastically modifies the shapes, velocity, existence and stability properties of the dark-typesolitons and allows the formation of bound states. The stability of the single dark-typesolitons exactly obeys the stability criterion. At suitable parameter regions, nonlocalitycould impose strong restrictions on soliton existence or exhibit remarkable destabilizingaction on dark-type solitons. Collisions between the bound states can exhibit interestingscenarios.Due to the fact that thermal materials can only support stable surface multipole soli-tons with less than three poles, it is proposed three di?erent approaches on the stabiliza- tion of higher order multipole surface solitons, including vector coupling stabilization,linear refractive index ramp stabilization and defect stabilization approaches. It is shownat suitable parameter regions, all the three approaches could stabilize multipole surfacesolitons of arbitrary orders.The properties of solitons supported by optical lattice without/with a defect in un-biased centrosymmetric photorefractive crystals featuring weakly nonlocality are inves-tigated. In the uniform lattice case, such solitons can only exist in the finite gaps of thelattice spectrum below a finite saturation parameter. The defect can strongly a?ect theproperties of such solitons: no matter positive or negative the defect is, the solitons ex-hibit enhanced stability and can come to exist for unlimited saturation parameters if theybifurcate from the defect modes; on the other hand, the defect imposes strong restric-tions on soliton existence and exhibits remarkable destabilizing action on solitons whichbifurcate from the band edges.The properties of the domain solitons and composite domain solitons in optical lat-tice with large-scale lower-index defect imprinted in defocusing nonlocal media are inves-tigated, and the corresponding Bragg-type coupler is also discussed. It is shown that thenonlocality slightly shrinks the existence regions of domain solitons, while it can largelyreduce the existence regions of composite domain solitons. The higher order domain soli-tons, which are robust in the local media, become unstable in large part of their existenceregions. The corresponding Bragg-type coupler exhibits a multi-mode feature and canachieve satisfactory nonlocality-controlled switching properties.The switching properties of X junctions and couplers based on nonlocal soliton in-teractions are investigated. It is revealed that the out-of-phase bright solitons and the darksolitons are promising candidates in constructing X junctions. Nonlocality drasticallymodifies the switching properties of the soliton X junctions: for the bright solitons case,nonlocality tends to enhance the transmission ability, while for the dark solitons case,nonlocality tends to reduce the transmission ability, but the reduction is a non-monotonicrelation with nonlocal length. The switching parameter regions of the X junctions couldbe largely enhanced by suitable section of the wavelength of the signal light. Moreover,two co-propagating out-of-phase bright solitons could server as a coupler whose couplerlength can be adjusted by nonlocal lengths and input energy ?ow.