Investigations Of Diffractive Focusing Effect Based On Metallic Hole Arrays

Diffraction of light field is the basic characteristic of light wave.Diffraction behavior of light field with limited space will occur during propagation,which makes the light field widen gradually and the central energy decrease accordingly.How to effectively control the diffraction behavior of optical field is of great significance to optical long-distance communication,optical superresolution and micro-nano particle manipulation.In the early stage,researchers used the methods of amplitude control,phase control and amplitude-phase joint control to manipulate the diffraction behavior of light field,resulting in novel spatial structured light fields,such as non-diffractive Bessel light field and self-accelerated Airy light field.At the same time,the diffraction focusing effect of light field is realized,and the focal spot can even break through the optical diffraction limit.It is worth mentioning that the current method of optical diffraction focusing depends on the optical sub-wavelength structure,which is complex,and hence greatly increases the difficulty of micro-nano processing.In this paper,a micro-nano structure unit is proposed to realize the diffraction focusing behavior of light field.The size of the micro-structure unit is far beyond the sub-wavelength range,which greatly reduces the cost and difficulty of device fabrication.Specifically,the research contents of this paper include:Firstly,the diffractive focusing behavior induced by periodic microporous arrays.Primarily,we studied the diffraction behavior of plane wave passing through a slit structure with sharp edges in this paper,and remarkable diffraction focusing phenomenon is observed.This phenomenon is caused by the coherent superposition of higher order components induced by the sharp edge slit diffraction,which is different from the traditional diffraction emission behavior.On this basis,the diffraction behavior of light field with periodic slit arrays and two-dimensional aperture arrays is further studied in this paper.It is found that Talbot effect appears in the diffraction light field induced by periodic arrays,which makes the light field unable to form a single diffraction focus spot in the plane;Furthermore,when the incident light field is modulated by a specific phase,the diffraction light field passing through the microporous array will form a minimal focusing spot,which is of sub-wavelength magnitude.Secondly,the diffraction focusing effect induced by aperiodic microporous arrays.In this paper,we will design a microporous array structure with aperiodic spatial distribution based on Vogel helix,and fabricate the structure sample based on 100 nm thick metal film in the experiment.Theoretical and experimental studies show that the Fourier spectrum corresponding to this structure is continuous and has a circular symmetric distribution.This unique property will make the diffracted light field form a single focal spot in the focusing plane.The size of the focal spot depends on the size of the structural sample;It is found that the structure sample has wide-band response characteristics: we carried out experiments with wavelength of 450 nm,632.8 nm and 670 nm.In all these cases,a single diffractive focusing spot was observed;In addition,it is found that the local orbital angular momentum is induced by the diffracted light field due to the modulation of Vogel helix in the propagation process,which makes the energy of the diffracted light field rotate in the propagation process.