Influence Of Competing Nonlinearity On Beam Propogation In Nematic Liquid Crystals

Spatial soliton refers to the self-trapped beam due to the balance between diffraction and nonlinear effects in nonlinear media.Solitons have great potential usage in many fields,such as all-optical devices,all-optical calculation,optical information processing,and other fields,because of their ability to maintain amplitude and phase unchanged as they propagate,and the elastic collision between multi-soliton is similar to the interaction of particles.Most of the materials in nature are belong to the nonlocal media,such as nematic liquid crystal and photorefractive materials.The refractive index induced by light propagation in the nonlocal media is not only related to the light intensity at one point but also related to the light intensity in the surrounding area,which may result in the formation of complex solitons in noncal media.In recent decades,spatial solitons in nonlocal medium with competing nonlinearities have attracted lots of attention.Recently,it is found that the spatial solitons in media with competition nonlinearity between thermal effect and reorientation effect have many unique properties.In this paper,we analytically investigate light propagation in liquid crystals with competing nonlocal nonlinearities.The main research content is as below:Firstly,we use the model which is proposed by Jung et al to study the modulation instability of plane waves in liquid crystals.We find that the competition between thermal and reorientational nonlinearities lead to unique modulational instability of the plane waves.We also find that the increase of the nonlocality of the orientational effect and the opt-thermal nonlinearity tend to suppress the MI by decreasing the gain bandwidth and the maximum gain.More importantly,there is a critical power for the modulation instability of plane waves due to the effect of competing nonlocal nonlinearities.Interestingly,the critical power is independent on the nonlocality of thermal nonlinear effect and molecular orientation effect,but only related to thermal nonlinear coefficient under the condition of equal nonlocality of the thermal effect and the reorientational effect.Secondly,we study the interaction between bright solitons in media with inhomogeneous thermal nonlocality,molecular orientation,and thermal nonlinearity.We can control the width of bright soliton and the interaction between soliton pairs by changing the nonlocality degree corresponding to the molecular orientation and thermal effect,and the thermal nonlinear coefficient.For a single soliton,when the degree of thermal nonlocality and the coefficient of thermal nonlinearity are fixed,the beam width will be compressed as the soliton moves from the region with larger nonlocality to the smaller,and vice versa.The width and shape of bright soliton are not influenced by the changing thermal nonlocality when the molecular orientation nonlocality and thermal nonlinearity are fixed.It is found by numerical simulations that the change of thermal nonlinear coefficient has little influence on the transmission of bright solitons.For out-of-phase soliton pairs,the interaction is the most sensitive to the change of thermal nonlinearity,followed by the degree of nonlocality of molecular orientation and the least effect of thermal nonlocality.More importantly,out-of-phase soliton pairs will split into two independent solitons from the small thermal nonlinear coefficient region into the larger region.And the splitting angle of this soliton pair is not only related to the changing thermal nonlinear coefficient but also related to the initial spacing of the beam.For in-phase soliton pairs,the degree of molecular orientation nonlocality,thermal nonlocality,and thermal effect coefficient has little effect on the interaction.This discovery may provide a new way for all-optical interconnection based on soliton interaction.