Investigation of Image Characteristics in Super-resolution Microsphere Nanoscope

  • Wenxuan Shi

Student thesis: Master of Philosophy


Super-resolution optical microscopes have emerged as powerful and enabling microscopic imaging tools available today. Microsphere-based microscopy is one of the methods which provide the resolution below the diffraction limit.In this thesis, the analytical separation-of-variables method is used to determine the internal, near and far field distributions of the light fields with microsphere lenses based on Mie theory. The microspheres can focus light and enhance the electromagnetic field along the incidence axis for sub-wavelength illumination. They also enable the collection of near field evanescent diffraction-free lights and transfer them to far field propagating waves. The influence of the refractive index, wavelength and the microsphere size on the location of light focal spot is studied. It has been found that, the distance between focal spot and microsphere center increases with decreasing refractive index contrast, increasing diameter of the microsphere, and decreasing wavelength of the illumination. The properties of the intensity enhancement capability of photonic nanojets are investigated. The maximum intensity of nanojet is sensitive to the size of microsphere, refractive index, and wavelength. If refractive index contrast is low, the photonic jet can be elongated up to 30 times the wavelength. The dependence of the FWHM on the refractive index, size of microspheres and wavelength of incident light is analyzed.The virtual imaging plane position and image magnification factor of microspheres are discussed. The characteristics of magnification with different wavelengths, refractive index, and microsphere diameter are analyzed by combination of geometrical optics and Mie theory. The comparison of the focal lengths of microsphere, which is predicted by Mie theory and geometrical optics, is given. The corresponding relative errors are discussed. The images formed by microsphere are mainly affected by radial distortion. This occurs because the magnification of microsphere is different with distance from the optical axis. Several experimental images of microspheres, which produce pincushion distortion, were carried out. A radial distortion model is presented. The distortion of images formed by microsphere are reduced using Matlab applying the model.
Date of Award31 Dec 2016
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorLin Li (Supervisor) & Wei Guo (Supervisor)

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