Ultrastrong coupling in a cavity-electromechanical device

Abstract

Cavity optomechanics is a field that explores the interaction between light and motion of matter via radiation pressure force. Such systems mainly consist of an optical/microwave cavity coupled to a mechanical resonator. Cavity optomechanical systems provide a convenient way to realize a wide range of experiments down to the quantum regime. Various quantum effects, such as cooling to the quantum ground state of the mechanical resonator, coherent state transfer, and quantum entanglement between microwave and mechanical modes, have been demonstrated experimentally. In addition, such systems have shown excellent performance in sensing the smallest of displacements, mass, and forces. The coupling strength between the cavity and the mechanical resonator is a crucial parameter in such systems. A coupling strength that exceeds the dissipations of the system opens the possibility for various interesting experiments mentioned above. Ultra-strong coupling is a regime in which two coupled oscillators (cavity and mechanical resonator) exchange energy in a time shorter than their time periods of oscillations. In this thesis, I will describe our experimental results from microwave optomechanical devices operating in the ultra-strong coupling limit, including the engineering aspects that enable such large coupling rates.

In the first part of the thesis, I present results from a microwave cavity-optomechanical system using a rectangular 3-dimensional (3D) superconducting waveguide cavity to couple mechanical vibrations of a nanomechanical resonator. The mechanical resonator is a thin circular plate of aluminum suspended over another aluminum plate on a substrate via clamps, forming a parallel plate capacitor. The oscillation of the mechanical membrane modulates the capacitance of the parallel plate capacitor, which in turn modulates the dressed resonance frequency of the microwave cavity. It enables the interaction between the microwave photons in the 3D cavity and the motion of the mechanical resonator. The single photon coupling strength is a crucial parameter that determines how strongly the mechanical motion is coupled to the electromagnetic field inside the 3D cavity. To maximize the single photon coupling strength, the device must be engineered to minimize parasitic capacitance. To facilitate it, we perform numerous simulations of various design parameters and analysis strategies to engineer such devices. After the design is optimized, we fabricate our cavity optomechanical device based on the optimized parameters and demonstrate that our experimental results are in line with the simulation results. We also include experimental results showing effects such as optomechanically-induced Absorption (OMIA).

In the second part of the thesis, I demonstrate the highest level of parametric coupling that is possible between the microwave mode and the mechanical resonator. It is achieved with the help of a parametric drive, which brings these two oscillators into resonance. We achieve the ultra-strong coupling where the coupling strength is comparable to the frequency of the mechanical resonator itself, exceeding all the system's dissipation rates. We also perform time-domain measurements of the OMIA effect, from which we can infer the energy swap rate between the microwave mode and the mechanical resonator. We found that the shortest swap time is on par with the time period of the mechanical oscillation. We also demonstrate that by increasing the drive signal strength beyond a certain threshold in the red sideband of the cavity frequency, the system shows parametric instabilities. I discuss various nonlinear phenomena, such as period doubling, tripling, and chaotic mechanical vibrations that lie in the parametric instability phase space.