Photothermal effects, Optomechanics and Optical Levitation

Abstract

The invention of lasers enabled strong and controllable interactions between light and small objects. Radiation pressure provides a unique tool for optomechanical control and manipulation of the quantum state of a mechanical oscillator. The high power density and photon flux of laser radiation lead to extremely high heating rates at surfaces, which induce measurement noise and is hence considered harmful in sensing systems. Optomechanical interaction can also be mediated by photothermal effects which, although frequently overlooked, may compete with radiation pressure interaction. A complete understanding of how these phenomena affect the coherent exchange of information between optical and mechanical degrees of freedom is yet undeveloped, particularly in mesoscale high-power systems where photothermal effects can fully dominate the interaction.

Here we investigate the photothermal effects in a unique optomechanical system: a cavity-enhanced setup for macroscopic optical levitation, where a free-standing mirror acts as the optomechanical oscillator. In this system, we observe the photothermally induced instability when the optical cavity is driven by a high-power laser. We show that the possible thermally induced parametric gain can be reduced and even cancelled out with a modification to the photothermal properties of an optomechanical cavity experimentally, by inserting windows into the cavity.

Theocratically, we report an effective theoretical model to predict and successfully reconstruct the dynamics of this system. By decomposing the photothermal interaction into two opposing light-induced effects, photothermal expansion and thermo-optic effects, we reconstruct a heuristic model based on the mutual interaction between the intracavity field and four position degrees of freedom, offering refined predictions that provide a high agreement with the experimental result. We also provided a detailed discussion on the effectiveness of different models, and have concluded that two photothermal effects are necessary in the model for simulating the dynamics of the optomechanical levitation system studied.

Overall, the experimental and theoretical investigations presented in this thesis set essential groundwork for the characterization and stabilisation of existing systems and a deeper understanding of photothermal effects in optomechanical systems. The methods that used in the investigation and cancellation of photothermal effects in this system can be easily applied to other similar optomechanical system where high-intensity laser is used or high precision of the measurement is desired. This work facilitates the development of new and more precise optomechanical systems for integrated photonics and sensing, and paves the way to fundamental studies in quantum mechanics. Show more