Microspherical photonics emerged in recent years in the context of fundamentally new optical properties of structures formed by coupled dielectric microspheres. These include coupling between whispering gallery modes (WGMs), photonic nanojets, nanojet-induced modes, resonant light pressure and optical super-resolution effects. The bottleneck problem in this area is connected with size disorder of individual microspheres which leads to optical losses and degraded performance of coupled devices.
In this dissertation novel resonant propulsion of dielectric microspheres is studied with the goal of sorting spheres with identical resonances, which are critical for developing microspherical photonics. First, evanescent field couplers were developed by fixing tapered microfibers in mechanically robust platforms. The tapers with ~1 µm waist diameter were obtained by chemical etching techniques. Using these platforms, WGMs modal numbers, coupling regimes and quality factors were determined for various spheres and compared with theory. Second, the spectroscopic properties of photonic molecules formed by spheres sorted by individual characterization with better than 0.05% uniformity of WGM resonances were studied. It was shown that various spatial configurations of coupled-cavities present relatively stable mode splitting patterns in the fiber transmission spectra which can be used as spectral signatures to distinguish such photonic molecules. The third part of the dissertation is devoted to the observation and study of giant resonant propulsion forces exerted on microspheres in the evanescent microfiber couplers. This effect was observed in suspensions of polystyrene spheres with sufficiently large diameters (D > 10 µm). By integrating optical tweezers for individual sphere manipulation, the wavelength detuning between a tunable laser and WGMs in each of the spheres was precisely controlled. Resonant enhancement of optical forces was directly demonstrated in experiments. The spectral shape, position and magnitude of the observed propulsion force peaks were explained by efficient transfer of light momentum to microspheres under resonant conditions. The peak magnitude of the resonant force is shown to approach total absorption limit imposed by the conservation of momentum. The transverse movement of the spheres during the propulsion process was studied and the existence of a stable radial trap was demonstrated. Giant resonant propulsion forces can be used for large-scale sorting of microspheres with ultrahigh uniform resonant properties.
|Advisor:||Astratov, Vasily N.|
|Commitee:||Davies, Angela D., Fiddy, Michael A., Smith, Stuart T., Zhang, Yong|
|School:||The University of North Carolina at Charlotte|
|Department:||Optical Science & Engineering|
|School Location:||United States -- North Carolina|
|Source:||DAI-B 76/11(E), Dissertation Abstracts International|
|Keywords:||Evanescent coupling, Microspheres, Optical forces, Photonic molecules, Resonant enhancement, Tapered microfiber|
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