Photothermal nonlinearity in optical cavities and optomechanical systems
Creators: name: Ma, Jinyong
Publisher: name: The Australian National University
Year: 2020

description: The nonlinear interaction between photothermal effects and optical cavities may have significant impacts on cavity-based applications. Depending on the situation, the consequences of photothermal effects may be beneficial or detrimental. Despite the fact that the photothermal nonlinearity may impose a fundamental limit to the sensitivity of measurements and the production of squeezed light, it may also be employed to suppress the Brownian noise of a mechanical oscillator and to cool the oscillator close to its quantum ground state. In either case, it becomes essential to establish control of how this type of interaction influences the cavities. In this thesis, we uncover two novel beneficial effects in a monolithic optical cavity strongly coupled to photothermal effects. We also investigate the nonlinear dynamics caused by photothermal interactions in an optomechanical system designed for optical levitation. We first propose and experimentally demonstrate an optically controllable transparency effect in an optical cavity strongly coupled to photothermal effects. We refer to this novel phenomenon as ''photothermally induced transparency'' (PTIT), in analogy to electromagnetically induced transparency (EIT) and optomechanically induced transparency (OMIT). Similar to the established mechanisms for EIT and OMIT, PTIT can suppress the coupling between an optical resonator and a traveling optical field. We further show that the dispersion of the resonator can be modified to exhibit slow or fast light. We observe the bandwidth of PTIT to be 15.9 Hz, which theoretically suggests a group velocity of as low as 5 m/s. The second effect identified in a cavity ruled by photothermal coupling is the hyper-thermorelaxation by optical back action; that is, the intracavity optical field alters the natural photothermal relaxation rate as a function of cavity detuning. This optical correction is crucial when exploring the dynamics of an optical system with apparent photothermal effects. With the help of this effect, we evidence a precise estimation of the photothermal relaxation rate with the relative precision being an order of magnitude higher than previous works. The other unique feature of this scheme is that it is compact and versatile, enabling a quick and precise characterization of photothermal effects. The next part of the thesis focuses on the investigations of the dynamics of a small free-standing mirror. The mirror acts as the top reflector of a vertical optical cavity, designed as a testbed for a cavity tripod optical levitation setup. At high laser power, we identify three prominent effects driving the mirror: excitation of acoustic vibrations, expansion due to photothermal absorption, and pick-up by radiation pressure force. These effects are intercoupled via the intracavity optical field and induce rich system dynamics inclusive of high-order sideband generation, optical bistability, parametric amplification, and optical spring effect. To counteract the system instability produced by the photothermal expansion, we insert a laser window at Brewster's angle inside the levitation cavity. The intracavity optical field heats the window and can cause a refractive index change of the window. We show that the presence of photothermal refraction balances the change of optical path length induced by the photothermal expansion and can flip the coefficient sign of net photothermal effects. We also show that the nonlinear interaction between the photothermal refraction and cavity mode provides effective damping to the system and thus can be employed to suppress the excitation of the acoustic modes and to stabilize the optical levitation. Additionally we propose, in the extension part of this thesis, a novel approach using optomechanics to extend the concept of the traditional carrier-envelope phase in the few-cycle regime to mechanical pulses. We further develop a two-step model to give physical insight into this effect.