Study On The Nonlinear Optical Properties Of Metallic Nanostructures

Compared with traditional dielectric materials,metallic nanostructures have unique advantages on nonlinear optical effects.The metallic nanostructure itself possesses a large nonlinear susceptibility,the light of resonance frequency can be highly confined around the metal to produce a strong local electromagnetic field to enhance nonlinear response.In addition,the resonance frequency can be changed by the variation of the geometric structure,and thus the nonlinear response,that shows good tunable property for metallic nanostructures.In recent years,nonlinear optical effects in diverse metallic nanostructures have been widely studied,such as second harmonic generation(SHG),third harmonic generation(THG),four-wave mixing and multiphoton excited luminescence.Generally,preparations of metallic nanostructures have two ways,which are using the state-of-art nanofabrication technique and chemical growth.When we experimentally prepare samples,the emergence of geometric defects which deviate from the designed ideal structure is inevitable.The previous studies show that a minor difference between actual structure and ideal structure indeed have a great impact on the linear and nonlinear optical effects.The main research are as follows:1.Firstly,we introduce the background of nonlinear optics,the development and application of the second harmonic and three harmonics.And then introduce the fundamental frequency field equation of numerical simulation,the second harmonic and third harmonic generation theory.2.By using the finite element method,we study the linear and second harmonic responses of ideal rectangular gold gratings.Then,the linear reflection and the second harmonic generation of the trapezoidal gold gratings are studied in detail.The reflectivity of trapezoidal grating and the second harmonic generation are discussed in this paper.One is the symmetry change on both sides of the gold grating.The other is the asymmetry of the two sides of the grating.Through the analysis of the results,we found that gold grating of trapezoidal cross-section has important influence on second harmonic response.Compared with the ideal gold grating,the second harmonic generation of trapezoidal gold grating can be increased by double.3.We studied the third harmonic responses of grating microstructure by using the same method.Firstly,we studied the three harmonics of the ideal gold gratings,and then discussed the trapezoidal gratings.Through the analysis of the results,it is found that the third harmonic responses also have a great influence for the deviation of the two sides of the gold grating.Compared with the ideal gold grating,the third harmonic generated of the cross section of the trapezoid and symmetric change has been reduced by an order of magnitude.4.The hot spots of metal nano-structure are at the tip or gap,and there have few hot spots in the single structure.With the progress of nano-processing technology,the complex structures have been widely studied.We use the same method to study the scattering spectra and nonlinear response of the Koch snowflake iteration structure.In the ideal case,the linear and nonlinear response of the Koch snowflake structure is a reference,and we focus on the influence of the defect state of the structure on the linear and nonlinear response.In this paper,we study the scattering spectrum and nonlinear response of iteration structures on the basis of equilateral triangle.