Ramgopal Madugani

Similar to microspheres, thin-walled microbubble resonators support whispering gallery modes (WGMs) that combine ultrahigh Q -factors and small effective mode volumes. In contrast, their hollow nature enables enhanced interactions with encapsulated materials and lower spectral mode density due to the tight radial confinement of the optical modes. However, the existence of a high axial-mode density still leads to significant mode mixing and modal interference that can complicate spectral shift measurements, thereby limiting sensing applications. To address this limitation, we have fabricated geometric filters directly on the surface of microbubbles using focussed ion beam milling. Based on numerical calculations, we first designed and then fabricated large tapered patterns, such as circular surface dips or holes, that could effectively filter modes while minimising optical scattering losses. Local lateral mode confinement and partial recovery of the loaded Q -factors of up to 58% were experimentally achieved by adding shallow slit patterns. Using few-mode engineered microbubble resonators, we subsequently demonstrated pressure sensing and spectral tuning of WGMs free from mode-mixing artefacts. This precision engineering approach may provide improved mode isolation, tunable directional emission, and high sensitivity measurements in microbubble resonator devices.

In this work, we theoretically and experimentally investigate the optomechanical transduction mechanism in a system of two coupled microresonators under different coupling scenarios, categorized into electromagnetically induced transparency, intermediate, and Autler–Townes splitting regimes. We show that interference between the dispersion and dissipation channels gives rise to an asymmetric line shape in the optomechanical spectrum. These effects are experimentally demonstrated using a fiber taper-coupled whispering gallery resonator system consisting of a microbubble resonator and a micropendulum. Finally, we conduct a comprehensive analysis of the optical forces between the microbubble and the micropendulum and propose that an experimentally feasible trapping scheme is achievable, even in the presence of strong optomechanical interference.

Acceleration measurement is widely used in commercial, scientific, and defense applications, but the resolution and accuracy achievable for demanding applications is limited by the current technology used to build and calibrate accelerometers. We report an optomechanical accelerometer based on a Fabry–Perot microcavity in a silicon chip that is extremely precise, field deployable, and can self-calibrate. The measured acceleration resolution is the highest reported to date for a microfabricated optomechanical accelerometer and is achieved over a wide frequency range ( 314 n m ⋅ s − 2 / √ H z over 6.8 kHz). The combination of a single vibrational mode in the mechanical spectrum and the broadband thermally limited resolution enables accurate conversion from sensor displacement to acceleration. This also allows measurement of acceleration directly in terms of the laser wavelength, making it possible for sensors to calibrate internally and serve as intrinsic standards. This sensing platform is applicable to a range of measurements from industrial accelerometry and inertial navigation to gravimetry and fundamental physics.

The tunability of an optical cavity is an essential requirement for many areas of research. Here, we use the Pound–Drever–Hall technique to lock a laser to a whispering gallery mode (WGM) of a microbubble resonator, to show that linear tuning of the WGM, and the corresponding locked laser, display almost zero hysteresis. By applying aerostatic pressure to the interior surface of the microbubble resonator, optical mode shift rates of around 58 GHz/MPa are achieved. The microbubble can measure pressure with a detection limit of $2\times 10^{-4}$ MPa, which is an improvement made on pressure sensing using this device. The long-term frequency stability of this tuning method for different input pressures is measured. The frequency noise of the WGM measured over 10 min for an input pressure of 0.5 MPa has a maximum standard deviation of 36 MHz.

Dissipative optomechanics has some advantages in cooling compared to the conventional dispersion dominated systems. Here, we study the optical response of a cantilever-like, silica, microsphere pendulum, evanescently coupled to a fiber taper. In a whispering gallery mode resonator, the cavity mode and motion of the pendulum result in both dispersive and dissipative optomechanical interactions. This unique mechanism leads to an experimentally observable, asymmetric response function of the transduction spectrum, which can be explained using coupled-mode theory. The optomechanical transduction and its relationship to the external coupling gap are investigated, and we show that the experimental behavior is in good agreement with the theoretical predictions. A deep understanding of this mechanism is necessary to explore trapping and cooling in dissipative optomechanical systems.

We study the noise spectrum arising from absorptive and dispersive modulations to the motion of a micropendulum via transmission through the coupling tapered fiber. The observed asymmetrical line shape matches theoretical predictions.

Microbubbles are thin spherical shells that can support whispering gallery modes, with evanescent fields inside and outside the resonator. We report on using microbubble resonators for temperature sensing via the large temperature dependent refractive index of fluid contained in the microbubble and for optomechanics.

2 THz tuning of optical WGMs is observed in a stretchable PDMS microresonator, demonstrating sensitivities as high as 0.13 nm/μN. The structure also provides the possibility of observing mechanical modes with frequencies in kHz range.