Plasmon-enhanced Nonlinear Spectroscopy Studies Of Noble Metal Nanorods

Precious metal nanostructures will generate surface plasmon resonance effects under the action of external light fields.The local electromagnetic field enhancement effects produced by them can effectively enhance optical signals such as fluorescent radiation and Raman scattering.They are widely used in biological imaging and chemical sensing,high-sensitivity detection and other fields.Recently,researchers have successfully prepared a variety of enhanced substrates,and the design and synthesis of new nanostructures has become a hot topic of research today.The study found that due to the unique shape of nanorods,the surface plasmons produced by them are anisotropic.So nanorod structures are often used as enhanced substrates for fluorescent radiation and Raman scattering,and various complex structures have been derived to further obtain highly sensitive optical signals.In this paper,three types of nanorod structures are designed and the main research purpose is to enhance nonlinear signals.The geometric parameters and excitation methods of the structure were reasonably selected,and a combination of experiments and simulations were used to systematically study the effects of enhanced two-photon coherent anti-Stokes Raman scattering(TPCARS)and upconversion fluorescence radiation.The main tasks as follows:(1)The enhancement of nonlinear optical signals by the multi-surface plasmon resonances(MSPR)of Au@Ag nanorod was studied.The results show that Au@Ag nanorod shows strong MSPR peaks at 800 nm and 400 nm,which can enhance the nonlinear optical signals at the fundamental frequency and frequency doubling,respectively.Theoretical calculations using the finite element method have found that MSPR of Au@Ag nanorod can significantly enhance two-photon excited fluorescence(TPEF)and TPCARS signals,reaching orders of magnitude of 104 and 1016,respectively.This provides new theoretical support for improving the resolution of nonlinear optical imaging.(2)A plasmon dimerstructure composed of two Ag nanorods was designed,and its enhancement effect on TPCRAS signal was studied.The results show that reasonable selection and design of structural parameters can obtain strong MSPR peaks at 1020 nm and 505 nm,which can enhance the TPCARS signal based on the frequency matching of the fundamental frequency and frequency doubling.By changing the angle of incident light,the angle between the two rods,and the angle of space rotation,the enhancement factor of TPCARS can reach as high as 3.66×1028 with significant electric field enhancements.In addition,by changing the aspect ratio and distance of the Ag nanorods,the two-photon process can be controlled at different optical frequencies.(3)A self-assembly method of droplet evaporation was used to prepare a horizontally aligned gold nanorod array,and the enhancement effect of the array on the surface NaYF4:Yb3+/Er3+rare earth nanoparticles was studied.The results show that the introduction of the Al2O3 spacer does not achieve the enhancement effect but weakened.The reason is that CTAB on the surface of the gold nanorods acts as a certain thickness isolation layer to prevent the quenching effect.Therefore,when there is no spacer,the array has the strongest enhancement of the luminescence of the rare earth nanoparticles,with an enhancement factor of about 140 times.In addition,the horizontally aligned gold nanorod arrays also exhibit excellent polarization characteristics.As the polarization angle of incident light gradually increases,the intensity of fluorescent radiation gradually decreases,and it is explained and analyzed by simulating the electric field distribution.The research is expected to have potential applications in highly sensitive areas of biology and chemistry.