Slow Cooking of Photonic Microresonators

Abstract

New techniques to improve the fabrication precision of microphotonic devices is required to develop their applications in optical signal processing, telecommunication and quantum computing. In addition, advances in microfluidic sensing using WGM (whispering gallery mode) spectroscopy, has shown to be a worthwhile endeavour to achieve ultrahigh sensing of biological, chemical and mechanical processes, reaching single molecule detection. This thesis simultaneously advances these fields via the incorporation of microfluidics on the SNAP (surface nanoscale axial photonics) platform. This work has culminated in the discovery of the slow cooking phenomenon which originates from the silica-water interaction at a water-filled microcapillary fibre. Chapter 1 presents a background to WGM microfluidic sensing and a review of the known silica-water interaction processes. In Chapter 2, we offer the theoretical background to SNAP technology [1,2], which uses nanoscale modifications of the effective radius variation (ERV) of optical fibres to develop photonic microresonator devices such as frequency comb generators [3,4], delay lines [5], optical buffers [6], and, tunable [7] and transient [8] microresonators. We characterize the discovered slow-cooking phenomenon in Chapter 3, measuring the temporal and spatial variations of the cut-off resonant wavelength (CWV). Our experimentally simple setup uses two microfibres (MF) to couple into a water-filled capillary fibre. The first MF excites WGMs in the fibre to detect the CWV of the resonant wavelength. The second MF is used for optical heating by broadband light of optical power 56-100 mW, which evanescently penetrates into and becomes absorbed by water to induce heating [9] and water motion [10]. We demonstrate the fabrication of SNAP microresonators over hour-long optical heating durations, which displays linear growth for sufficient heating power and time. However, for higher slow-cooking powers and durations, the growth becomes nonlinear and nonmonotonic. We advance our fabrication precision by reducing the slow-cooking duration in Chapter 4, to achieve precision in CWV of 1.3 pm/10mins limited by the OSA resolution. We propose that further reduction of the slow-cooking duration can achieve 0.02 pm/10secs in CWV corresponding to precision of just 0.6 pm in ERV. This estimated fabrication precision improves the developed laser post-processing techniques [11,12] on the SNAP platform by two orders of magnitude. Throughout the thesis we attempt to relate the observed CWVs to known silica-water interaction processes. Most prominently, we suggest the observations of the de- and re-hydroxylation at ambient temperatures [13] in Chapter 3 and the structural relaxation of silica after heating ceases in Chapter 4. To our knowledge, these are the first experimental demonstration of these processes using WGM spectroscopy. Overall, the demonstrated integration of microfluidics with SNAP technology produces a multitude of avenues worthy of pursuit, summarized in Chapter 5. These include advances in the fabrication of the aforementioned photonic devices and enhancement of microfluidic sensing capabilities which can be applied to a variety of fields of interest.