Investigation of nanophotonic structures for imaging and sensing
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The ability to image micro/nano scale objectives with miniaturized optical components has always been of great interest due to its great potential in applications such as microscopy, nanofabrication, and biomedical monitoring. However, in traditional practice using dielectric lenses, the focal size is inevitably limited by the Abbe’s diffraction limit (0.51fλ/ρ). Here, λ is the wavelength in vacuum, and f and ρ are the focal length and the radius of the lens, respectively. Moreover, the performance of conventional spherical lenses deteriorates as their sizes approach the wavelength. On the other hand, owing to the recent advances in micro/nano fabrication techniques, miniature sensors have received much attention, which are highly desirable in many sensing applications for physical, chemical, and biomedical parameter measurements. However, the performance of miniature sensors usually suffers from the similar difficulty as miniaturized imaging systems. Recently nanophotonic structures have been explored for the development of miniaturizing imaging and sensing systems due to their capability of confining and manipulating light at a subwavelength scale. In this dissertation work, several different mechanisms that nanophotonic structures can be used to help enhance the performance of imaging and sensing in miniaturized systems are investigated. First, plasmonic lens utilizing the nanophotonic structure to achieve the subwavelength focusing ability is studied. Three different regions in the plasmonic lens design are defined. Furthermore, a plasmonic lens in the Fresnel’s region is designed and k.ed to achieve a sub-diffraction limit focus. Second, radially polarized light generated by the TEM mode in the annular aperture in metal is investigated, which can further enhance the focusing ability. Third, in terms of sensing, an ultra-thin plasmonic interferometer constructed with a nano-hole array is fabricated on a fiber facet. By using this structure, the multi-parameter sensing capability of this interferometer is demonstrated; high sensitivity refractive index and temperature sensing are achieved. Finally, a novel sensor design based on the cladding modes and buffer modes generated by the planar grating on the fiber facet is proposed. Experimental studies of this sensor demonstrate its superior temperature sensitivity and the potential of multi-parameter sensing.