Dynamic Manipulation Of Third-hamrmonic Generations Of Nanostructures

Nonlinear nanooptics mainly investigate the nonlinear optical effects of nanostructures or nanomaterials,e.g.,two-photon absorption,second-harmonic generation?SHG?,third-harmonic generation?THG?.Specifically,THG finds wide applications in frequency conversion,optical information processing,biomedical imaging,and other nanophotonic devices due to their unique properties.However,the low THG excitation efficiency prevent their direct applications in industry products.Moreover,most of the current nonlinear nanophotonic devices are static devices.Dynamic THG devices are desirable for their potential applications.Phase change materials have two switchable states,i.e.,crystalline and amorphous,and demonstrate significantly different linear and nonlinear optical propertiess in these two states,e.g.,refractive indexes.They can be used to tune the THG signals of nanostructures and are ideal materials for dynamic THG devices.Therefore,this paper attempts to use the classic phase-change material Ge₂Sb₂Te₅?GST?to manipulate the nonlinear optical effects?THG?of metal nanostructures to achieve dynamic THG nanophotonic devices.Firstly,a gold-GST-gold sandwiched nanostructure consisting of gold-nanodisks/GST-disk/gold-film is designed to realize dynamic THG nanodevice.The presented sandwich nanostructure has greatly enhanced electric field within the cavity between gold disks and gold film,where the GST material sitting.Therefore,the nonlinear absorption coefficient of GST can be greatly improved,which in turn leading to an increased THG signal of GST.The calculation results show that the THG signal of GST in the cavity is greatly increased up to 400,000 times with respect to GST disks without putting in the cavity.More importantly,we use the reversable nonlinear coefficients of crystalline and amorphous GST to achieve dynamic THG nanodevices.When GST is in amorphous state,the designed structure can generate the maximum THG signal at 450 nm.With switching the GST into crystalline state,the maximum THG signal is shifted to 665 nm,and the THG at 450 nm is decreased to zero.Hence,the THG signal can be actively tuned by switching the structural states of GST.Secondly,we also designed two active THG nanodevices using bright-bright and bright-dark coupling modes.The first one is a H-shaped nanostructure which consists of GST nanorods,where a base nanorod and two lateral nano rods osciallate as bright dipole modes.The coupling and hybridization between the dipole modes lead to a plasmonic-induced transparency?PIT?of the structure.Through analyzing the electric-field distributions at different resonance frequencies in the PIT transmission spectrum,we demonstrate the nature of bright-bright mode coupling contribute to the enhanced THG signal.Then,by changing the structural parameters of the structural unit,the reversable THG was demonstrated.Finally,a hybrid THG nanostructure composed of nanorods and triangles is proposed,in which the nanorods and triangles act as dark-bright mode coupling structure.Here,we artificially set the value of the nonlinear coefficient of the materials in the bright and dark structures to 0,and the theory proves that the THG signal in the structure is mainly contributed by the nanorod structure.In addition,by changing the phase state of the GST material between the amorphous state and the crystalline state,while maintaining the wavelength of the excitation light at the position of the transparent window,the THG intensity has achieved three orders of magnitude changes.In the end,we realized the dynamic regulation of THG signal in the near infrared range.The work in this paper provides solutions for dynamic nonlinear optoelectronic devices.