Abstract
With recent advances in the field of optomechanics, researchers are always looking for new candidate materials with superior optical and mechanical characteristics. Diamond is a promising material due to its outstanding optical and mechanical properties, and holds great promise for a wide range of applications, in particular within the field of cavity optomechanics that couples optical and mechanical resonators. With a large transparency window that spans visible and infrared wavelengths, and a high optical refractive index, it allows light to be strongly confined in wavelength scale nanophotonic structures. Diamond has the highest thermal conductivity of any material at room temperature which allows thermal energy to be dissipated quickly when operating under high optical intensity. Owing to its high Young’s modulus, diamond is the hardest known material and diamond nanostructures support mechanical resonances with high frequencies compared to similar geometries in other materials. Moreover, diamond has been found to host more than a hundred colour centers, including the nitrogen vacancy (〖NV〗^-) and silicon vacancy (SiV) that hold a great promise for quantum information processing applications. The spin states of the 〖NV〗^- can be controlled both with photons and phonons, and fabricating optomechanical cavities in diamond makes it a viable platform for the realization of photon–phonon–spin interactions.
Although these properties make diamond a great material for applications in cavity optomechanics and quantum information processing applications, fabrication of devices from bulk singlecrystal diamond (SCD) has proven to be challenging, as a sacrificial heterolayer does not exist for this material. While previous studies have shown that patterns with vertical profiles can be transferred to the SCD substrate using standard nanofabrication techniques, a major challenge in the fabrication of free standing optomechanical nanostructures is to etch away the layer underneath. Mechanical structures such as cantilevers and nanobeams require release from the substrate, enabling the study of mechanical vibrations. Similarly, optical cavities such as microdisk and photonic crystal cavities require undercut to prevent optical loss to the substrate.
In this thesis, we demonstrate a new fabrication technique that utilizes oxygen plasma etching
to fabricate optomechanical structures such as nanobeams and microdisks. The fabrication process is characterized, and found to demonstrate an undercut along diamond crystal planes. This technique uses standard nanofabrication equipment and can be extended to other bulk materials. The optical and mechanical characteristics of fabricated nanobeams and microdisks are analyzed using a dimpled fiber taper that evanescently couples light in and out of these devices. Nanobeams support mechanical modes with quality factors greater than 700,000 at cryogenic temperatures. Microdisks optomechanical cavities support whispering gallery optical modes with quality factors greater than 100,000 and room–temperature Q_m.f_m = 1.9×〖10〗^13, where Q_m and f_m are the mechanical quality factor and resonance frequencies of the microdisk mechanical breathing mode. Strong phonon–spin coupling is expected to be observed in future experiments by self–oscillating the mechanical devices, through photothermal or optical radiation pressure forces, providing a new scheme for phonon mediated optical control of 〖NV〗^- spin states.
