A study of optomechanics

Abstract

In recent years, cavity optomechanics has attracted lots of attention. In this system, photons inside a cavity interact with a macroscopic harmonic oscillator through the radiation pressure force. Depending on the type of structure, opto-mechanical systems can be used in, for example, precision sensing, including force and acceleration measurements, testing fundamental physics such as quantum mechanics at a macroscopic scale and the link between quantum physics and gravity. Also, it potentially plays an important role in quantum information, where optical photons can be used to transmit quantum information in a long range and the mechanical oscillation is used to couple to other types of local qubits where quantum information is processed.

Previously, many of the aforementioned applications have been demonstrated and are shown it to be a promising system. The mechanical oscillator has been cooled down to close to its ground state. Furthermore, for the opto-mechanical system with high quality oscillator, the macroscopic harmonic oscillator is possible to be cooled down to its quantum ground state from room temperature, opening up a new regime where quantum experiments on massive objects can be done at room temperature. Moreover, non-classical states between photons and the harmonic oscillator have been achieved, making it close to real applications in quantum information and approaching the testing and realizing quantum entanglement of large objects. In an opto-mechanical system, there are different structures. Two examples, which are both of great interested, are the Fabry-Pérot cavity with photonic crystal and the nanobeam. In a Fabry-Pérot cavity, which is the prototype of most of the cavity opto-mechanics system, there are two reflectors. One is fixed, and another one acts as an harmonic oscillator. To enlarge the interaction, the oscillating reflector is made of thin photonic crystal slab. For the nanobeam cavity, the mechanical resonator is the nanobeam itself, and it also forms a cavity where the optical field is trapped inside. This two systems have their own strengths and drawbacks, which are suitable for different applications.

In this work, both types are explored. In the Fabry-Pérot cavity, the set-up is usually bulky. Is it possible to make the footprint of the whole set-up smaller such that it can be integrated into a chip? If the answer is yes, it would be a great boost to its sensing application where easy-to-use devices are usually needed. Moreover, for the photonic crystal, is there a limitation on its thickness? Previously, in our group, we have noticed that the reflectivity of a photonic crystal drops sharply if the thickness is reduced, though the simulation still yields a reflectivity close to unity. The results of these two problems are presented in chapter 3 and chapter 4, respectively. The results for the nanobeam cavity are shown in chapter 5. Designs are proposed, aiming at increasing the photon-phonon coupling while keeping the photon decay rate unchanged.