Tightly Focused Spot Shaping and its Applications in Optical Imaging and Trapping

Abstract

The Rayleigh criterion explains the diffraction limit and provides guidance for improving the performance of an imaging system namely by decreasing the wavelength of the illumination and/or increasing the aperture (NA) of the objective lens. If the wavelength and NA are set, is it possible to improve the spatial resolution further? This question motivates the research work of this thesis. Polarization is an important property of light and it can not be ignored in a tightly focusing system. It is demonstrated both theoretically and experimentally that radially polarized light can produce a sharper focal spot in a high NA focusing system because of the tight longitudinal field component. Based on this, in this thesis, we start our investigation on the unique focusing properties of the radially polarized beam with the vectorial diffraction theory. We show that the amplitude of the focal field can be shaped by engineering the pupil field of the radially polarized beam. The shaped focal spot is smaller than the unmodulated one, which can be used to improve the resolution of optical systems. Here, we consider a confocal scanning imaging system, offering several advantages over conventional widefield microscopy. In the simulation, longitudinal electric dipoles are regarded as the objects to make the full use of the optimized longitudinal component. An experimental proof is also given, showing that higher spatial resolution can be achieved when the modulated radially polarized light is applied in the confocal imaging set-up as compared to the non-modulated case. Radially polarized light can be obtained with a liquid crystal based polarization convertor, starting with a linearly polarized beam. Amplitude modulation of the pupil such as the annular pupil field and the designed pupil field where the amplitude increases gradually with the radius can be realized with a spatial light modulator (SLM). The substrate is essential for supporting the sample to be imaged. Usually, the material of the substrate is glass. In the near field, when the object interacts with the light field, it may produce evanescent waves which decays very quickly and has little influence on the imaging. However, the evanescent wave carries higher spatial frequency than the propagating wave. A well designed substrate with a thin TiOኼ layer on top can enhance the evanescent wave in the near field. The enhanced field transfers to a propagating wave with the help of the object deposited on the substrate and it can be detected in the far field. The principle can be explained with a dipole model, and simulated using nanospheres. It is demonstrated that the designed structure helps to improve the imaging quality including contrast and resolution. In addition, such sample model can be combined with other imaging techniques, e.g. confocal scanning microscopy, widefield imaging system, etc. Besides amplitude and polarization, focal fields can also be shaped in phase. Unlike the specific radially or azimuthally polarized vector beam, the cylindrical vector beam is a more general form. The focusing properties and the spin-orbit interacitions of cylindrical vector vortex beams in high NA focusing systems are theoretically studied. An absorptive nanosphere can be trapped at the hot-spot of the focused field, even when the field has its axial symmetry broken. The analysis on the influence of parameters such as the initial phase of the vortex beam, the topological charge, or the size and the material of the trapping sphere on the interplay between spin and angular momentum may be helpful for optical trapping, particle transport and super-resolution.