WAVEGUIDE, PHOTODETECTOR, AND IMAGING APPLICATIONS OF MICROSPHERICAL PHOTONICS
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Abstract
Dielectric microspheres with diameters (D) on the order of several wavelengths of light have attracted increasing attention from the photonics community due to their ability to produce extraordinarily tightly focused beams termed "photonic nanojets," to be used as microlenses for achieving optical super-resolution or to develop sensors based on whispering gallery mode resonances. In this dissertation, we study the optical properties of more complicated structures formed by multiple spheres which can be assembled as linear chains, clusters or arrays, integrated with waveguides or embedded inside other materials to achieve new optical properties or device functionalities.For linear chains of polystyrene microspheres (n=1.59), we observed a transition from the regime of geometrical optics (at D>20 times the wavelength ) to the regime of wave optics (at D<20 times the wavelength ). We showed that this transition is accompanied by a dramatic change of focusing and optical transport properties of microsphere-chain waveguides. The results are found to be in qualitative agreement with numerical modeling.We developed, designed, and tested a single-mode microprobe device based on spheres integrated with a waveguide for ultraprecise laser surgery. Our design is optimized using a hollow-core microstructured fiber as a delivery system with a single-mode Er:YAG laser operating at an illuminating wavelength of 2.94 micron. Using a high-index (n~1.7-1.9) microsphere as the focusing element we demonstrate experimentally a beam waist of ~4 times the wavelength, which is sufficiently small for achieving ultraprecise surgery.For embedded microspherical arrays, we developed a technology to incorporate high-index (n~1.9-2.1) spheres inside thin-films made from polydimethylsiloxane (PDMS). We showed that by using liquid lubrication, such thin-films can be translated along the surface to investigate structures and align different spheres with various objects. Rigorous resolution treatment was implemented and we demonstrated a resolution of ~1/7 of the wavlength of illumination, which can be obtained by such thin-films.We experimentally demonstrated that microspheres integrated with mid-IR photodetectors produce up to 100 times photocurrent enhancement over a broad range of wavelengths from 2 to 5 microns. This effect is explained by an increased power density produced by the photonic jet coupled to the active device layers through the photodetector mesas. The photocurrent gain provided by photonic jets is found to be in good agreement with the numerical modeling.