Study On The Numerical Simulation For Near Field Enhancement On Substrate Surface Assisted By Nano-structure

Nanoscale processing with spatial resolution beyond the diffraction limit can be achieved by controlling the interaction between light and nanostructure through localizing the laser energy.Near field enhancement is formed on the contact region of nanostructures and substrate when femtosecond laser irradiates.To this end,near field enhancement was studied for the fabrication of nanohole and regular surface ripple beyond the diffraction limit.We performed numerical analysis for near field enhancement and regular surface ripple formation by femtosecond laser irradiation of the sample.Finite-difference time-domain?FDTD?simulation method has been used in the whole simulation.The fundamental physics of near field enhancement and regular surface ripple formation on the substrate were analyzed.In the study of the near field enhancement used in nanohole processing by femtosecond laser irradiation of the sample,we presented the analysis of the near field properties by femtosecond laser irradiation of Au,Si and TiO₂ nanoparticles.We proposed a method that laser induced near field enhancement assisted by high refractive index TiO₂ arrayed particles on the substrate.The simulation results show that TiO₂ particles can concentrate the laser energy to a region about 100 nm around the particle,which is certainly beyond the diffraction limit.Compared with the single particle,the near field enhancement of the TiO₂ arrayed particles reduces by less than 30%.Nevertheless,it is still 140 times stronger than the incident laser intensity,which is beneficial to the super diffraction limit fabrication of nanostructure.At the same time,the theoretical results show that near field enhancement can be achieved on most metals as well as dielectric substrate surfaces,and the near field is enhanced with the increase of substrate refractive index.These results are attributed to the scattering effect in that the high dielectric constant TiO₂ particle supports forward near field intensity pattern which may lead to the large near field enhancement and an increased interaction of the particle with the near field of substrate.The above results are of great significance to the application of femtosecond laser near field nano-processing with feature size beyond the diffraction limit.At present,although regular ripples formed by femtosecond laser irradiation on the substrate are simple,it is difficult to ensure the structural clarity.Therefore,the study on the periodic distribution of the nanostructure on the substrate surface was carried out.The distribution of the scattering field on the typical nanostructures assisted such as spherical,regular hexahedron and rectangular parallelepiped is calculated.It was found that the nanostructure parameters,including the shape,material and spatial distribution of the nanostructures have a significant effect on the scattering field distribution and intensity of the substrate surface.And ripple distribution can be optimized by changing the nanostructure parameters.The results are important for the fabrication and optimization of nano-periodic ripples beyond diffraction limit.