GENERATION OF SUB-WAVELENGTH FAR AND NEAR-FIELD FOCUSING PATTERNS USING OPTICAL VECTOR BEAMS

Abstract

Optical fields with complex spatial distribution are of great importance in various applications such as super-resolution imaging, laser nanofabrication, molecule or nanoparticle trapping and manipulation. This dissertation focuses mainly on both far-field and near-field focusing and shaping using optical vector fields. The focusing characteristics of cylindrical vector beams such as doughnut Gaussian beam and Bessel-Gaussian beam in a high numerical aperture focusing system were theoretically investigated based on the Richards-Wolf diffraction integral theory. Cosine function based complex amplitude filters were introduced to the high numerical aperture system to achieve a focusing field with a long depth of focus. Results show that a focusing spot with sub-wavelength lateral size and short depth of focus can be obtained when the radially polarized doughnut Gaussian incident beam is properly designed. A cosine function based complex amplitude filter was designed to increase the depth of focus of the focusing field. Using this complex amplitude filter, hollow beams with a long focal depth were successfully generated in a high numerical aperture focusing system. A radially polarized Bessel-Gaussian beam was used as the incident beam and a second-order vortex phase filter was used to create the null intensity on the optical axis. In addition, the radially polarized Bessel-Gaussian beam and the complex amplitude filter were used in a high numerical aperture 4π focusing system. A long longitudinally polarized optical chain was generated successfully. For the near-field case, the impact of the illumination polarization on the obtained photonic nanojets was numerically investigated for the model of a microsphere illuminated by plane waves and Gaussian beams with different polarizations. Both linearly and circularly polarized plane waves and linearly, circularly, radially, and azimuthally polarized Gaussian beams were used to generate photonic nanojets. Results show that one can precisely engineer the overall shape, intensity, location, and transverse and longitudinal size of the generated photonic nanojet at will for different applications by controlling the polarization and the amplitude profile of the illumination beam.