Mdulation Of Local Surface Plasmon Resonance Performance And Nonlinear Optical Phenomena In Cu_xS Nanoparticles

Localized Surface Plasmon Resonance（LSPR）absorption is a unique optical property of nanoparticles（NPs）,and is also one of the most important research topics in the field of nanooptics.LSPR can enhance the visible-near infrared（NIR）light absorption,luminescence and photothermal conversion of nanoparticles,which has important application value in optoelectronic communication,photothermal therapy and photoacoustic imaging.Traditional materials with LSPR properties are generally noble metal nanomaterials.Because of their intrinsic high free carrier concentration,their LSPR band usually appears in the visible range,which is mismatch to the optical fiber communication window（850nm,1310nm and 1550nm）and biological tissue optical window（650nm～1350nm）.Hence,the current research on the nonlinear self-focusing phenomenon of light beams mainly focuses on the visible light band.However,many biomedical applications require the wavelength of incident light to be within the optical window of biological tissue in order to reduce the damage of incident light to human body,such as photodamage or heating,so it is necessary to achieve the low scattering propagation and deep penetration of infrared light in colloidal materials.Cu_xS NPs with self-doping nature play an important role in this aspect due to their LSPR absorption peak located in the near infrared region,high photothermal conversion efficiency,low-priced and other advantages.In this paper,the effects of size,morphology of nanoparticles,and dielectric constant of solvent on LSPR properties were firstly studied,and the wavelength control to LSPR absorption in a large range was realized.Then Cu_xS NPs with different stoichiometry were selected as the research object,and the tunable nonlinear response of Cu_xS nanoparticles suspension was studied.The main research contents and results are as follows:（1）Synthesis of Cu_xS nanoparticles and regulation of LSPR absorption characteristics.Cu_xS NPs were prepared by hot injection method,and the effects of different factors on LSPR absorption properties were studied.Firstly,the stoichiometric ratio dependent LSPR properties of Cu_xS nanoparticles were studied:a series of Cu_xS NPs with different Cu components（x=1.01,1.35,1.50,1.82 and 1.91）were prepared by controlling the Cu/S ratio in the precursor.The free hole densities obtained by Cu vacancy control ranged from（2.8～4.0）×10²¹cm^-3.The corresponding LSPR absorption peak ranges from 1170 to 1640 nm,which matches the second optical window of biological tissues.Secondly,the effects of the size and morphology of Cu₇S₄ NPs on the properties of LSPR was studied:by adjusting the reaction time of Cu/S precursor,Cu₇S₄ NPs with the same phase structure but different size and morphology（4.2±0.7 nm,6.5±0.9nm,11.5±3.3 nm）were prepared,the corresponding LSPR absorption peak position could be adjusted in the larger spectral range of 1142 nm～1423 nm.Finally,the influence of the dielectric constant of the surrounding environment of Cu S NPs on the properties of LSPR was studied:by dispersing the same amount of Cu S NPs in different organic solvents and testing UV-vis-NIR spectra,it was found that the absorption peak of LSPR gradually shifted to red with the increase of the dielectric constant of the solvent,and the position of the LSPR absorption peak could be adjusted in the small spectral range of 1170 nm～1285 nm.（2）Study on nonlinear optical response of Cu_xS NP suspension.We selected a group of Cu_xS NPs with different stoichiometric ratios as the research objects,and observed the nonlinear self-focusing effect of near-infrared light（1064 nm）for the first time.It is found that under different laser power,the laser beam in Cu_xS NP suspension will go through three stages,which are beam diffraction at low power,beam trapping at high power and beam defocus at high power.After the low-power laser beam passes through the 2.5 cm transmission distance in the Cu_xS NP suspension,the size of the output beam can be reduced by 4 times,and the self-focusing propagation and deep penetration of the laser beam in the Cu_xS NP suspension medium are realized.By changing the concentration and stoichiometric ratio of Cu_xS NP suspensions to adjust the polarizability,the tunable nonlinear self-focusing effect of near-infrared light is realized.It was found that the nonlinearity increases with the increase of the concentration and the decrease of the stoichiometric ratio of Cu_xS NPs.These results not only enrich the candidate materials for LSPR nonlinear optical phenomena,but also mean that Cu_xS NPs have high potential in biomedical applications due to their self-focusing transport with low scattering and high transmission and high photothermal conversion efficiency.