G. Senthil Murugan

Chalcogenide glass microsphere laser

Laser action has been demonstrated in chalcogenide glass microsphere. A sub millimeter neodymium-doped gallium lanthanum sulphide glass sphere was pumped at 808 nm with a laser diode and single and multimode laser action demonstrated at wavelengths between 1075 and 1086 nm. The gallium lanthanum sulphide family of glass offer higher thermal stability compared to other chalcogenide glasses, and this, along with an optimized Q-factor for the microcavity allowed laser action to be achieved. When varying the pump power, changes in the output spectrum suggest nonlinear and/or thermal effects have a strong effect on laser action.

Spectral cleaning and output modal transformations in whispering-gallery-mode microresonators

A systematic study on the effects of microtaper fiber diameters on the spectral characteristics of a whispering-gallery-mode (WGM) microbottle resonator (MBR) is presented. Progressively cleaner and simpler spectra of the MBR were observed when the utilized microtaper fiber waist diameter (Dt) was increased from 2 to 10 μm. The maximum transmission depth at resonance varies with different microtaper fiber utilized from ∼20  dB (Dt=2  μm) to ∼4  dB (Dt=10  μm). The loaded Q-factors were observed to be unaffected by the increase of Dt with values of >106 being measured in all cases. Mode transformation of MBR was also experimentally investigated and compared to a microdisc finite-difference time-domain simulation by studying near-field images of the output beam on the waist of the microtaper fibers. For the first time, experimental observation of mode transformation from LP01 to LP11 across scanned WGM resonances is being reported.

Microtaper Fiber Excitation Effects in Bottle Microresonators

We have carried out a systematic study of the effect of microtaper diameter on the spectral characteristics of bottle microresonators. By increasing the microtaper-diameter (Dt) from 2μm to 10μm results in progressively cleaner spectra. The transmission depth at resonance varies from ~15dB (@Dt=2μm) to >3dB (@Dt=10μm). The loaded Q factors were measured to be >10+6 in all cases. However, with microtaper Dt=10μm clearly-resolved single resonance peaks could be observed and free-spectral ranges could be easily identified. At some transmission resonances, we have observed LP01→LP11 mode transformation at the excitation microtaper waist, for the first time, as the resonance is scanned.

Integrated optical waveguides and inertial focussing microfluidics in silica for microflow cytometry applications

A key challenge in the development of a microflow cytometry platform is the integration of the optical components with the fluidics as this requires compatible micro-optical and microfluidic technologies. In this work a microflow cytometry platform is presented comprising monolithically integrated waveguides and deep microfluidics in a rugged silica chip. Integrated waveguides are used to deliver excitation light to an etched microfluidic channel and also collect transmitted light. The fluidics are designed to employ inertial focussing, a particle positioning technique, to reduce signal variation by bringing the flowing particles onto the same plane as the excitation light beam. A fabrication process is described which exploits microelectronics mass production techniques including: sputtering, ICP etching and PECVD. Example devices were fabricated and the effectiveness of inertial focussing of 5.6 µm fluorescent beads was studied showing lateral and vertical confinement of flowing beads within the microfluidic channel. The fluorescence signals from flowing calibration beads were quantified demonstrating a CV of 26%. Finally the potential of this type of device for measuring the variation in optical transmission from input to output waveguide as beads flowed through the beam was evaluated.

High-Q plasmonic bottle microresonator

In this paper, we demonstrate a hybrid plasmonic bottle microresonator (PBMR) which supports whispering gallery modes (WGMs) along with surface plasmon waves (SPWs) for high performance optical sensor applications. The BMR was fabricated through “soften-and-compress” technique with a thin gold layer deposited on top of the resonator. A polarization-resolved measurement was set-up in order to fully characterize the fabricated PBMR. Initially, the uncoated BMR with waist diameter of 181 μm, stem diameter of 125 μm and length of 400 μm was fabricated and then gold film was deposited on the surface. Due to surface curvature, the gold film covering half of the BMR had a characteristic meniscus shape and maximum thickness of 30 nm. The meniscus provides appropriately tapered edges which facilitate the adiabatic transformation of BMR WGMs to SPWs and vice versa. This results in low transition losses, which combined with partially-metal-coated resonator, can result in high hybrid-PBMR Q’s. The transmission spectra of the hybrid PBMR are dramatically different to the original uncoated BMR. Under TE(TM) excitation, the PBMR showed composite resonances with Q of ~2100(850) and almost identical ~ 3 nm FSR. We have accurately fitted the observed transmission resonances with Lorentzian-shaped curves and showed that the TE and TM excitations are actually composite resonances comprise of two and three partially overlapping resonances with Q’s in excess of 2900 and 2500, respectively. To the best of our knowledge these are the highest Qs observed in plasmonic microcavities.

Optical microdiscus resonators

We present a new “soften-and-squash” method for turning microspheres into high quality, stand-alone microdiscus resonators. Due to the annealing involved in the fabrication process, their measured Q factors are one order of magnitude better than the starting microspheres.

Robust mode-selection in optical bottle microresonators

We have demonstrated a robust and accurate method of selecting whispering gallery modes in optical bottle microresonators (BMR) by inscribing scars on BMRs surface by focused ion beam milling. A “cleaned-up” transmission spectrum was obtained after microstructuring.

Whispering gallery modes in semiconductor optical fibres and optical bottle microresonators

We explore the optical properties of whispering gallery modes (WGMs) in silicon optical fibres and optical bottle microresonators. In particular, a pump-probe technique is used to experimentally demonstrate thermally induced all-optical modulation. High quality (Q) factors and small mode volumes are utilized to demonstrate ultrafast Kerr effect based modulation and switching. Q factors exceeding ~107 are demonstrated for novel microbottle resonators (MBRs), fabricated from standard telecommunications fibres.

Optical Excitation and Probing of Bottle Microresonators

Fiber bottle microresonators supporting helical whispering gallery modes and exhibiting field maxima symmetrically located on either side of the neck of the bottle have been demonstrated. Channel dropping characteristics have been studied experimentally for the first time in this type of microresonator, using tapered excitation and probe fibers symmetrically placed on both sides of the bottle microresonator. Selective excitation on one side of the bottle microresonator leads to symmetrically located turning points and power localization on both sides of the bottle, leading to the potential to construct add‐drop filters.

Whispering gallery modes in bottle microresonators

Selective excitation of an extended range of whispering gallery modes (WGM) supported by bottle microresonators fabricated on a short section of optical fiber is demonstrated experimentally and discussed theoretically. Decrease of the observed Q factors, as the excitation point moves away from the bottle centre, are associated with increased mode leakage. Compared to standard-fiber cylindrical resonators, the bottle microresonator results in a 35x increase of the observed Q factor.