Slow Cooking of Photonic Microresonators

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

Discovery of parabolic SNAP microresonators produced in fibre tapering

We present a novel method based on optical fibre tapering for fabrication of Surface Nanoscale Axial Photonics (SNAP) devices with parabolic profiles with an unprecedentedly large number of axial eigenmodes. Tapering of a commercial 125 μm single-mode optical fibre to a 30 μm diameter waist by laser brushing creates a SNAP bottle microresonator with parabolic radius variation in the centre of the tapered region. Ideal parabolic resonators should demonstrate equal spacing between resonances. Our spectral measurement of the parabolic profile shows spacing of ∼6 GHz with 10% deviation over a bandwidth of 2.5 THz containing up to 400 axial eigenfrequencies. This new discovery for the creation of SNAP parabolic microresonator devices is important for fabrication of miniature delay lines, buffers and frequency comb generators. Characterisation of our exemplar microresonators is briefly explored, particularly for broadband frequency comb generators which require equidistant frequency spacing. Further investigations include scaling of the parabolic feature with tapering process parameters, repeatability testing, and the fabrication of more complex shapes

Photonic Microresonators Created by Slow Optical Cooking

Silica and water are known as exceptionally inert chemical materials whose interaction is not completely understood. Here we show that the effect of this interaction can be significantly enhanced by optical whispering gallery modes (WGMs) propagating in a silica microcapillary filled with water. Our experiments demonstrate that WGMs, which evanescently heat liquid water over several hours, induce permanent alterations in silica material characterized by the subnanometer variation of the WGM spectrum. We use the discovered effect to fabricate optical WGM microresonators having potential applications in optical signal processing and microfluidic sensing. Our results pave the way for the ultraprecise fabrication of resonant optical microdevices and the ultra-accurate characterization of physical and chemical processes at solid-liquid interfaces

Tunable SNAP Microresonators via Internal Ohmic Heating

We demonstrate a thermally tunable Surface Nanoscale Axial Photonics (SNAP) platform. Stable tuning is achieved by heating a SNAP structure fabricated on the surface of a silica capillary with a metal wire positioned inside. Heating a SNAP microresonator with a uniform wire introduces uniform variation of its effective radius which results in constant shift of its resonance wavelengths. Heating with a nonuniform wire allows local nanoscale variation of the capillary effective radius, which enables differential tuning of the spectrum of SNAP structures as well as creation of temporary SNAP microresonators that exist only when current is applied. As an example, we fabricate two bottle microresonators coupled to each other and demonstrate differential tuning of their resonance wavelengths into and out of degeneracy with precision better than 0.2 pm. The developed approach is beneficial for ultraprecise fabrication of tunable ultralow loss parity-time symmetric, optomechanical, and cavity QED devices