| Recently,it is difficult to meet the needs of the rapid development of life science,medicine and environment with the resolution of conventional microscopy.There has been a growing interest in breaking the diffraction limit and achieving super-resolution microscopy.However,the existing near-field super-resolution technology and far-field fluorescence-labeled super-resolution microscopy have some shortages in practical application,where the former can only observe the sample surface because its working distance is much less than one wavelength,and the latter is limited by the fluorescence bleaching and photo-toxicity.Super-oscillation is a phenomenon that the optical field components with lower spatial frequency form a local rapid oscillation in far field,which can be used to realize deep compression of the point spread function of the optical microscopy,and provide a new way to realize far-field label-free super-resolution microscopy.To date,optical super-oscillation lens has been successfully demonstrated in far-field super-resolution microscopic systems.However,the previously reported super-oscillation lenses suffer from serious off-axis aberration,and the corresponding optical system can only achieve point-scan super-resolution imaging by moving the samples at normal incident plane wave,which greatly limits the speed of imaging.Flat-field lens is an optical element which can focus the off-axis beam and improve the imaging speed of the system by means of beam scanning.However,the conventional flat-field lens is restricted by the diffraction limit and cannot be applied to far-field label-free super-resolution microscopic imaging.Therefore,it is significant to investigate flat-field super-resolution focusing devices to improve the imaging speed of super-resolution microscopy.In order to solve the problems of flat-field super-resolution focusing device,such as the contradiction between compressing super-resolution focal spot and suppressing sidelobe on the one hand,and correcting off-axis aberration one the other,etc.An approach is proposed using the vector angular spectrum diffraction and particle swarm optimization algorithm to correct super-resolution off-axis aberration,and an optimal design of flat-field super-resolution focusing device is realized by using subwavelength geometric phase meta-atom to modulate the optical field by continuous phase.For the wavelength λ=632.8 nm,a flat-field super-resolution focusing device is designed,with the radius of 240 λ(151.87 μm),the focal length of 60 λ(37.97 μm),the numerical aperture NA of 0.97 and the corresponding diffraction limit of 0.515 λ(0.5 λ /NA).The simulation results show that the full-width-at-half-maximum of focal spots on the focal plane are less than 0.45 λ(0.285 μm),the size is less than the diffraction limit,the sidelobe ratio of focal spots on the focal plane are less than 25%,the linear correlation coefficient between focal spot offset and incident angle is 0.9997,and the maximum offset of focal spot is 4 λ(2.53 μm)within the incident angle range of 0°～2°.According to the design requirements,the device was fabricated by using chemical vapor deposition,electron beam lithography and plasma etching.Then,the testing system of flat-field super-resolution focusing device was built,and tested by a wide field of view and large numerical aperture microscope.The experimental results show that the full-width-at-half-maximum of focal spots on the focal plane are less than 0.47 λ(0.297μm),the sidelobe ratio of focal spots on the focal plane are less than 40%,the linear correlation coefficient between focal spot offset and incident angle is 0.9966,and the maximum offset of focal spot is 4.1 λ(2.59 μm)within the incident angle range of0°～2°.The device can compress the super-resolution focal spot and suppress the sidelobe,and correct the off-axis aberration of the focal spot.Thus,the flat-field super-resolution focusing device offers a new way toward fast-scanning far-field label-free super-resolution microscopy. |
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