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Title: Novel microbottle resonators for advanced sensing applications
Author: Mohd Nasir, Mohd Narizee
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2016
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Optical microresonators supporting whispering-gallery modes (WGMs) have shown great potential towards miniaturization and applications in advanced optical microsystem by confining light to small volumes along with high-quality (Q). A new class of WGM microresonator, known as microbottle resonators (MBRs), have gained much interest in recent years because of their ability to confine light in a true 3D manner and survival of WGMs through the smooth deformation of spheroids. Due to MBRs’ broken mode degeneracies, strong overlap of different WGM families result to very dense and complex transmission spectra. A systematic study on the effects of microtaper fibre diameters on the spectral characteristics of WGM MBR is presented. Progressively cleaner and simpler spectra of the MBR were observed when the utilized microtaper waist diameter (퐷푡) was increased from 2 to 10 μm. The maximum transmission depth at resonance varies with different microtaper fibre utilized from ~20 dB (퐷푡=2 휇푚) to ~4 dB (퐷푡=10 휇푚). The loaded Q-factors were observed to be unaffected by the increase of 퐷푡 with values of > 106 being measured in all cases. Mode transformation of MBR was also experimentally investigated and compared to a microdisc FDTD simulation by studying near-field images of the output beam on the waist of the microtaper fibres. For the first time, experimental observation of mode transformation from LP01 to LP11 across scanned WGM resonances is being reported. Spectral simplification of MBRs’ WGMs is also demonstrated by introducing scatteres on the surface of the resonators. Through focused ion-beam (FIB) milling, periodic micro-scars were inscribed along two MBRs’ axial and azimuthal paths. High Q-factor value of > 105 are retained by the MBRs before and after the inscription process. Utilizing a highly discriminating polarization-resolved set-up, MBRs’ excited TE- and TM-WGMs are also accurately distinguished with clear free-spectral ranges (FSRs) designation. Microtaper excitation arrangement near the MBRs’ bottle-neck with TE-polarized input light yields a single sharp and distinguishable WGM resonance (~ 18 dB and ~ 8 dB) over a broad wavelength range. FDTD simulation performed on a microdisc indicates that two or more WGMs are able to beat together to circumvent the defects introduced by the micro-scars and maintain strong WGM confinement near the resonator’s surface. Since excitation of WGMs microresonators (MRs) is polarization sensitive, the polarization state of the evanescently launched lights affects the number, type and strength of each resonance. To understand MRs performance under TE/TM launching conditions by means of microtaper fibre evanescent coupling, a polarimetric set-up was designed to study the transmission, polarization-cross coupling (PXC) and scattering of a WGM microsphere, milled-MR, microcylinder and scarred-MBR. Experimental results show that the transmission dip and PXC minima are shifted with respect to the ideally expected input polarization angel. A theoretical model developed based on Jones matrix calculus have attributed the observed shift to residual birefringence of the output part of the microtaper fibre. The model demonstrated that the scattered light is not affected by this residual birefringence and therefore is a better matric to ensure precise TE or TM polarization launching. It is also observed that MRs with tighter axial power confinement (milled-MR and scarred-MBR) show larger cross-polarization coupling efficiencies and are MRs of choice to be utilized as polarization converters. In addition, it is shown that utilizing the TE WGM resonances results in superior cross-polarization performance. Demonstration of a hybrid plasmonics-MBR with high-Q is presented for surface plasmon sensing applications. To excite surface plasmon polaritons (SPPs), thin gold-metal-film was deposited on the surface of the MBR. Surface curvature of the MBR result in deposition of the thin metal-film to half of the resonator’s surface with a meniscus profile. The meniscus thin metal-film profile creates appropriately tapered edges on the two sides of the MBR that would facilitate to a low-loss adiabatic transformation of the MBR’s WGMs to SPPs, and vice versa. Calculated Q-factor value of the plasmonics TM transmission dips are in the range of 850. Accurate Lorentzian fitting performed on the measured data reveal that each dip is actually composite resonances comprising of three partially overlapping WGMs with higher Q-factor value in access of 2500. Excitation strength of the three overlapping resonances varies with different microtaper coupling arrangements relative to the position of the meniscus thin metal-film. In some cases, generation of a single plasmonics resonance with high excitation strength is demonstrated while supressing the others of the same family group-mode. FDTD simulation performed on a WGM microdisc with thin metal-gold-film of meniscus profile shows good agreement with the plasmonics-MBR experimental observations. Simulated steady state distribution of the magnetic H-field also reveals that plasmonics radiation into air is strongest near the starting-edge of the meniscus thin-metal film, making it the particular region of interest to be explored for surface plasmon sensing. Performance of MBRs as a highly tuneable WGM source and a strain-sensing device is presented. The solid MBRs under study were fabricated by slightly overlapping two SMFs during an arc-fusion-splicing process. Simplification of the dense MBR-WGMs spectral feature is achieved by evanescently coupling light on the up-taper region of an optical tapered fibre while retaining its Q-factor value of > 105. TE- and TM-WGM resonance shift corresponding to applied mechanical strain were investigated with the utilization of a highly discriminating polarization-resolved set-up. TM-resonance shift of the MBR shows record tunability of an entire FSR (~4.1 nm), the highest ever demonstrated by a solid WGM optical microresonator, with TM-/TE-WGM resonance shift ratio of 1.67.
Supervisor: Zervas, Michael Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available