In this project, we explore optomechanical interactions using glass fibre "nanospikes" fabricated by thermally tapering a step-index single-mode fibre (SMF) and chemically etching the final part to a sub-µm tip diameter. A smooth tapering process guarantees the adiabatic evolution of the guided optical modes along the nanospike and at the same time, the resulting nanospike forms a high-Q mechanical resonator at low gas pressures, ensuring perfect confinement of the acoustic phonons inside the tip. These two aspects, i.e. low-loss optical guidance and weak acoustic dissipation, enable a rich platform for optomechanical experiments on such a simple device.
In this work a nanospike (subwavelength in diameter over its insertion length with a final tip diameter of ~150 nm) is inserted into the core of a HC-PCF where it becomes trapped at core-centre due to interaction with the guided mode in the HC-PCF and subsequent optomechanical back-action [Xie (2016)]. Fed by light launched into the untapered SMF, the guided fundamental mode spreads out into the surrounding space as it travels along the nanospike, adiabatically evolving into an eigenmode of "nanospike plus hollow core" structure. The presence of the hollow core strongly affects the local field profiles, leading to the appearance of an optical restoring force when the nanospike is moved away from the axis. The system offers a novel way to couple light from SMF to HC-PCF, with a demonstrated launching efficiency of 87.8% (the near-field mode image at the output end of the HC-PCF is shown in the inset of figure (a)) and a Fresnel reflection as low as ~0.05%. More importantly, it uniquely allows self-alignment and self-stabilization via the optomechanical trapping force. The coupling scheme uses no electronics, and the self-alignment/stabilization effect improves for higher laser powers, with potential applications in safely launching very high power laser light into HC-PCF for laser machining applications. We are also exploring the use of the technique to launch light into liquid-filled hollow core PCFs, with applications in microfluidic circuits.
Appropriately designed optomechanical devices are ideal for making ultra-sensitive measurements. The nanospike in the figure below supports a flexural resonance with a quality factor greater than 100,000 at room temperature in vacuum [Pennetta (2016)]. It is designed so that the core-guided optical mode in SMF evolves adiabatically into the fundamental mode of the air-glass waveguide at the tip. The mechanical resonance has an ultra-narrow linewidth of ~20 milli-Hz (figure b), limited by the intrinsic material dissipation of fused-silica. These features make it possible to measure extremely small changes in the resonant frequency of the nanospike, making to possible to detect very small forces when operating at low pressure in the Knudsen regime, when the mean-free-path of the gas molecules is of the same order as the dimensions of the vacuum chamber.