Optimal Design And Fabrication Of Super-oscillatory Lenses For Superfocusing

Since there exists an optical diffraction limit,the resolution of the optical microscope system is greatly restricted.It remains a hot and difficult problem to achieve the super-resolution beyond this resolution limit.With the development of micro/nano technologies,optical super-resolution technology is widely used in many fields such as optical nanofabrication,super-resolution imaging of biology and medicine,particle acceleration,and high-density optical data record.Currently,there are two main methods to achieve the super-resolution focusing with the planar metallic nanostructural films.Surface plasmons were exploited to concentrate and channel the evanescent wave to image in the near field.However,the working distance is usually very small and it is hard to achieve the super-resolution imaging in the quasi-far-field and far-field regions.Another new method was also proposed recently that an arbitrarily small focal spot can be created in the far field by precisely tailoring the interference of diffracted beams,which is related to the phenomenon of the optical super-oscillation.In this paper,a further study about the optical super-oscillation was made to achieve a controllable design of super-oscillatory lenses(SOL)for superfocusing with the vectorial diffraction theory,and the performance of the super-oscillatory lens was tested and analyzed by the numerical simulation and experiment.The main research contents are as follows:Firstly,the mechanism between the optical super-oscillation and super-resolution was explored,and a further study on the design methods of the super-oscillatory lenses was made.It is shown that the diffraction limit is just a restriction for practical applications.According to the phenomenon of optical super-oscillation,in principle,there is no physical limitation on resolution,and the key point is to make a tradeoff in transfer of intensity between the sidebands and the hot-spot.As the performance of the reported super-oscillatory lenses was not under good control,a multi-objective and multi-constraint optimization model was proposed to achieve a controllable design of SOL.Secondly,according to the diffraction theory and the structural feature,the vectorial analysis method is more universal for the design of SOL,and the diffractive fields of the SOLs with different polarized lights were derived accordingly.Finite-difference time-domain(FDTD)numerical simulation technique was used to analyze the diffraction patterns of different diameter of circular aperture,which affirmed the validity of the vectorial diffraction theory.Thirdly,the optimization models of SOLs for the sub-diffraction-limit spot and optical needle were established based on the vectorial angular spectrum theory,and meanwhile,the optimizing procedure is self-designed using the Matlab programming language based on the genetic algorithm(GA)and fast Hankel transform algorithm.Numerical simulation was done by FDTD and the results show good agreement with the designed ones,which demonstrated that the models were effective to well control the performance of the SOLs.Fourthly,according to the capability of micro/nanofabrication technologies and the feature size of SOLs,the focused ion beam(FIB)was used to fabricate the devices.The optical test platform was established based on the NIKON inverted microscope,and the key considerations in the experiment were analyzed.At last,the experimental results were compared with the theoretical results,and the error was analyzed.The main innovation of this research is: a multi-objective and multi-constraint optimization model was proposed to achieve the controllable design of SOLs.