Super-resolution Focusing Devices Based On Sub-wavelength Optical Fields Modulation

Breaking through the diffraction limit to achieve higher resolution and smaller focal spot is a major challenge in the field of optics.Diffraction limit effect has seriously restricted the further development of deep sub-wavelength lithography,ultra-high density storage,super-resolution optical microscopy,micro-nano optical processing,medical imaging and other fields.In recent years,researchers have achieved super-resolution imaging to some extent by using fluorescence labeling,image reconstruction and other techniques,but they cannot reflect the optical nature of the object under test due to their indirect imaging.How to break through the restriction of diffraction limit,improve the precision of light field regulation,study the diffraction spot with smaller size and the super-resolution focusing device with higher efficiency are the urgent problems to be solved.In order to overcome the shortcomings of existing super-oscillatory optical focusing devices that can only focus transverse polarization component on the focal plane,a super-oscillatory focusing device based on binary amplitude and binary phase modulation is proposed in this paper.The super-oscillatory focusing of the total optical field consisting of transverse polarization component and longitudinal polarization component on the focal plane is realized.For the circularly polarized light with wavelength of 632.8nm,vector angular spectrum diffraction method as well as PSO algorithm are combined to optimize the far-field super-oscillatory focusing device.The radius of the device is 500?,the focal length is 400?,and the numerical aperture is 0.78.The theoretical design results of the focal spot are as follows:the full width at half maximum?FWHM?is 0.44?;the ratio of side lobe to peak intensity?Rsidelobe?is 25%;there is no significant sideband within the designed light field range of[-500?,500?];the metal concentric ring and the medium concentric ring are used as the basic unitcell of the device to realize binary amplitude and binary phase regulation respectively;chemical vapor deposition,magnetron sputtering,electron beam lithography,inductively coupled plasma etching and other processes were used to fabricate the focusing devices based on binary amplitude and phase modulation,which consisted of Al-Si₃N₄ concentric ring structures;the far-field super-resolution optical field test system based on nano-fiber probe is used to characterize the total field of the far-field focal spot generated by the super-oscillator.The experimental results show that the focal length of the device is 252.8?m?399.5??,the total full width of half maximum of the focal spot is 0.454?,the ratio of side lobe to peak intensity is 26%,and no edge band within the tested range of[-142?,142?].The super-oscillatory focusing of the incident circularly polarized light is realized,and the size of focal spot of the total optical field is smaller than 0.487??0.38?/NA?,which solves the problem of realizing the super-oscillatory focusing of the total field?including transverse polarization component and longitudinal polarization component of light field?.A set of reflective birefringence metasurface structures with phase regulation function are proposed to solve the problem of multi-dimensional?phase and polarization?regulation of light field.Adopting metal-dielectric-metal structure as the unitcell and utilize the GSP resonance characteristic of the antenna,a 90-degree fixed phase difference of the orthogonal polarization state is formed to achieve the subwavelength quarter-wave plate.At the same time,by optimizing the geometrical and material parameters of Au-MgF₂-Au metasurface structure of sub-wavelength quarter-wave plate,the quasi-continuous regulation of the phase of the emitted light field can be realized.Under the circumstance of circularly polarized light incident,the continuous regulation of the polarization direction of the emitted light field can be realized by rotating a quarter-wave plate.Combined with the phase control function of the quarter-wave plate group,the phase and polarization of the emitted light field can be independently controlled,and the problem of multi-dimensional?phase and polarization?control of the light field can be solved.The coaxial alignment error between lens and beam seriously affects the quality of super-resolution focusing in the process of realizing super-resolution focusing of column-vector beam with traditional optical lens.On this basis,a reflection type super-resolution focusing device with integrated polarization and phase control function is proposed.For circularly polarized incident light with a wavelength of 1550nm,a radially polarized focusing device and an azimuthally polarized focusing device are designed by using hyperbolic phase distribution.The two devices have the same radius and focal length,with values of 7?and 2?respectively,and the corresponding numerical aperture is 0.962.CST STUDIO SUITE is adopted to calculate the performing of the two devices.The results show that under the condition of circularly polarized light with?=1550nm incident:?1?The focal length of the radially polarized focusing device is 2?,forming a solid focal spot dominated by a longitudinal electric field component,with a FWHM of 0.41?and the ratio of sidelobe to peak intensity of18.4%.The focal spot size is less than the diffraction limit of 0.52??0.5?/NA?.The monolithic integration of radial polarization conversion and super-resolution focusing is realized.?2?The focal length of the azimuthally polarized focusing device is 2?,forming a hollow focal spot dominated by transverse electric field component,with a FWHM of 0.34?and the ratio of side lobe to peak intensity of 31.5%.The focal spot size is less than the diffraction limit of 0.52??0.5?/NA?.The monolithic integration of azimuthal polarization conversion and super-resolution focusing is realized.This device realizes the self-alignment between the center of the cylindrical vector beam and the center of the focusing device,and solves the difficulty of alignment between the center of the beam and the center of the device in realizing the super-resolution focusing of the cylindrical vector beam with discrete elements.