Research On Far-field Subwavelength Focusing

The capacity to focus light beams with subwavlength resolution is essential in various applications, such as optical data storage, nanolithography, biomedical imaging and optical microscopy. The research of life science and technology has been speeding up in the 21th century, which makes the subwavelength resolution microscopic imaging with more important applications. This thesis will find new methods to achieve optical far-field subwavelength focusing from three aspects: micro-nano structures,focal field distribution under high numerical aperture objective condition and the research about stimulated emission of depletion (STED) fluorescence microscopy.In the first chapter, the significance of the far-field subwavelength focusing is introduced. In addition, we list the research results of different research groups at home and abroad about subwavelength focusing.And then in the second chapter, according to the principle of the constructive interference of the output light field in the diffraction region and the study of the micro-nano structure, we designed the micro-nano germanium rings and the microsphere array to achieve the far-field subwavelength focusing. The sample of the micro-nano germanium rings was manufactured by focused ion beam milling (FIB) and the subwavelength focal spot was detected by the imaging system. We chose the 3D finite-difference time-domain (FDTD) computational solutions to take the calculation of the subwavelength focusing effect of the dielectric microsphere array.In the third chapter, we mainly investigated the influence of the polarization and phase of the incidence beam on the focused light field under high numerical aperture. Based on the vector diffraction theory, the modulation effect of the vortex phase plate on the focal spot on the radial polarization beam and the azimuthally polarized beam was studied. In addition, when the azimuthally polarized beam was phase-encoded by the 0/πphase plate, with the annular pupil aperture employed, the sharper focal spot with full width at half maximum (FWHM) belowλ/4 can be achieved. In the fourth chapter, we built the experimental system of STED fluorescence microscopy. After preparing the sample of fluorescent microspheres, we now have taken the resolution measurement of the confocal microscopic imaging part of the system and achieved subwavelength focusing.At the end of the thesis, the research of the subject is summarized and the following research direction is indicated.