The Generation Technology Of Nano Optical Needle Based On Vector Light Field

Micro-nano structure with high depth-to-width ratio has been widely applied in many fields.Therefore,it is crucial to realize the processing of high depth-to-width ratio micro-nano structures.For ordinary micro-nano structures,photolithography is the first choice for the preparation of such devices.After years of development,lithography technologies such as nanoimprint lithography,projection lithography,and laser direct writing technology have been widely used in the preparation of micro-nano structures.Among them,the laser direct writing technology without mask has become a good choice for preparing high depth-to-width ratio micro-nano structure due to its flexibility in processing and the variety of processing patterns.However,in laser direct writing technology,the beam quality of direct writing spot directly determines the structure.In focusing system,the focal spot size and focal depth are mutually restricted,so it is difficult to meet the requirements of high resolution and long focal depth simultaneously.Therefore,it is difficult to realize the preparation of high-aspect-ratio micro-nano structures by using traditional focused light spots.How to break through the limitation of traditional theory and obtain a focal spot with an optical needle structure that can combine high resolution and long focal depth has become an urgent problem to be solved in the direct writing processing of high aspect ratio micro-nano structures.Due to the non-uniform distribution of polarization,the vector light field is often accompanied by some new optical phenomena after being focused by the lens.In this paper,by studying the focusing characteristics of the vector light field,a scheme based on the vector light field to generate an ultra-long optical needle was presented and verified by experiments.In this study,the superiority of azimuthally polarized beam in generating focusing spot with long focal depth was proved by comparative analysis.Furthermore,the influence of the incident light field distribution and the numerical aperture of the focusing lens on the focusing spot was analyzed,and the theoretical formula was given as a guide.On this basis,the model of generating nano optical needle based on azimuthally polarized beam was preliminarily established,and the model was designed and optimized in detail.In the optimization,in order to solve the problem that the optical needle structure is difficult to characterize,a combination of a 100 x micro-magnification system and a piezoelectric ceramic stage was used to obtain information on the size and depth of the focused spot,and then the three-dimensional structure parameters of the focused spot were obtained.Aiming at the diffraction phenomenon in the system,a specially designed soft-edge aperture is used to optimize the system.Finally,the theory is verified through experiments.In the experiment,the designed system was used to focus the azimuthally polarized vortex annular beam,which produced a nano-needle with a spot size of 0.416? and a focal depth of 15.6?,proving the feasibility and superiority of using azimuthally polarized beams for optical needle generation.