Research On Vectorial Diffraction Far-field Super-resolution Focusing Related Theory And Confocal Microscopic Imaging

Since Abbe’s diffraction resolution limit was proposed in1873, super-resolutionfocusing and imaging has persistedly been one of the hottest and most tough problems.Currently, conventional microscopy is undergoing a revival towards nanoscopy, whichhas a wide and profound influence on many disiplines including physics, biology,medicine, life sciences, materials science, nanotechnology, precision engineering, etc. Inthe realm of traditional far-field optics, using ordinary refraction, reflection or (binary)diffraction optical elements, to beat Abbe’s diffraction resolution limit of0/(2NA)and realize super-resolution diffraction focusing in the order of0/4~0/3withoutevanescent waves has important scientific research values and practical meanings. Ifthese super-resolution focusing techniques are further applied in confocal microscopy,super-resolution scanning optical imaging can be attained.The aims of this project “Research on vectorial diffraction far-field super-resoltionfocusing related theory and confocal microscopic imaging” are twofold: first to exploreapproaches and techniques in producing super-resolution focusing spots as well assubwavelength purely longitudinally polarized light needles, which can approach orreach the far-field optical diffraction limit of0.360/NA; second to explore potentialsuper-resolution confocal optical imaging techniques when combining with the abovesuper-resolution focusing approaches. The study of the project provides importanttheoretical and practical reference values in microlithography, particle manipulation,laser direct writing, high density optical storage, biological/medical nanoscopy,materials science, industrial/engineering inspection, etc.The main research contents of this dissertation are as follows:Firstly, for the key problems in super-resolution focusing with pupil filtering andconfocal microscopic imaging, spoke wheel filtering optimization design method isproposed, in which spoke wheel filters are constructed to obtain focusing beams witharbitrarily extended flattop focal depth and transverse super-resolution. By introducingan amplitude apodization function and balance factors, consine synthesized filter isconstrcucted to modulate arbitrarily extended, subwavelength, longitudinally polarized,uniform light needles, e.g., with transverse FWHM of0.420and axial uniform focaldepth as long as4.70(among the distance, peak-valley intensity fluctuation less than2.8%). Analytical coherent transfer function for confocal microscopy with arbitraryToraldo filter is derived to reveal the fundamental mechanism of super-resolutionimaging in the frequency domain.Secondly, to overcome the theoretical limitation in super-oscillatory diffractionsubwavelength focusing, a universal, efficient optimization design method is proposed. Integral representations for describing the light field propagation after asuper-oscillatory diffraction element (called super-oscillatory lens, SOL) of arbitraryfinite size and illuminated under typical vectorial light beams, are derived. Based onscalar and vectorial angular spectrum theories, scalar and vectorial optimization modelsfor designing subwavelength focusing SOLs are established, respectively. Configuredgenetic algorithm and fast Hankel transform algorithm are programmed to solve theabove optimization models. In the scalar optimization, super-resolution focusing spotswith transverse FWHM of0.360(in air) and0.260(oil immersion) are realized,with working distances both larger than600. While in the vectorial optimization withthe radially polarized light illumination, super-resolution focusing spots with0.390(in air) and~0.250(oil immersion) are reached and theoretically validated by thevectorial Rayleigh-Sommerfeld diffraction integral. Theoretical calculations show thatbinary phase SOLs are much more light efficient, e.g., the maximum intensity of thecentral hotspot focused by a binary phase SOL can be five times as high as that for thebinary amplitude counterpart. A binary amplitude SOL is fabricated using electron beamlithography (EBL) and inspected using the integrated industrial confocal microscope;for a longitudinal-mode laser with wavelength of532.4nm, the subwavelength focusingproperty of the designed SOL is experimentally studied in air; key techniques of focalplane nano-positioning by confocal mechanism and focusing pattern detection bywide-field coherent imaging are adopted in the experiment, and the validity of theproposed optimization method as well as the design result are verified by theexperimental result.Thirdly, in order to solve the contradiction among transversely suppressing thebeam width of longitudinally polarized light needle, with axially extending the focaldepth and homogenizing the intensity fluctuation, ultra-high NA paraboloid mirror andFresnel zone plate (FZP) systems are studied for subwavelength focusing. The strongaxial dispersion effect of a high-NA FZP is used to modulate subwavelength lightneedles, by separating the design wavelength from the illumination wavelength, andintroducing a slidable narrow comb window function to an annular FZP; underillumination of the radially polarized light, purely longitudinally polarized light needlewith transverse FWHM of0.3660and as long as470(within the distance,peak-valley intensity fluctuation is8.5%) is created by optimization. Alternatively,under an annular paraboloid mirror system of NA=1, consine synthesized filter is usedto modulate the incident radially polarized light; purely longitudinally light needles withtransverse FWHM of0.360can be produced with uniform focal depth up to100(among the distance, peak-valley intensity fluctuation is0.7%). Based on vectorialStratton-Chu diffraction integral, a quasi-rigorous vectorial diffraction integral is given for arbitrarily opening annular paraboloid mirror system under illumination of theradially polarized light; under the far-field approximation condition, the derived integralrepresentation reduces to the result based on the classic vectorial Debye-Wolfdiffraction integral; axial focal shift effect is verified by comparing the quasi-rigorousresult with the approximate result; theoretical calculation further indicates that thetransverse FWHM of the focal spot reaches its minimum to be0.390when themaximum focusing semi-angle is about115.Lastly, integration of confocal microscopic imaging and measurement instrument isimplemented in order to provide a3D imaging and profile inspection tool formicro-/nanostructures, and also provide technical references for developing asuper-resolution confocal microscope. Key techniques for integrating a confocalmicroscope are expounded. A visualized fast alignment and real-time monitoringmethod of confocal detection pinhole is used. An industrial confocal microscopeinstrument platform is integrated, incorporating conventional wide-field imaging,confocal scanning imaging and3D sectioning measurement, differential confocalmeasurement modules, etc. The measured FWHM of confocal axial sectioning responsecurve is564nm for wavelength of532nm and NA=0.9. Typical microstructurespecimens, including the designed and fabricated binary amplitude SOL, are scannedfor imaging and profile tomography.