Totally confined gigahertz mechanical mode with a complete phononic bandgap for a nano-optomechanic cavity

INTRODUCTION One-dimensional optomechanical crystals are commonly defined as quasi-periodic structures in which propagation and coupling of optical and mechanical waves can be engineered. Here, we present a... [ view full abstract ]

INTRODUCTION

One-dimensional optomechanical crystals are commonly defined as quasi-periodic structures in which propagation and coupling of optical and mechanical waves can be engineered. Here, we present a nano-optomechanical cavity that incorporates two different unit cells based on high optical quality factor cavities, which have been modified to have a complete phononic bandgap. An optomechanical coupling rate of 600 kHz, an optical quality factor of 3.8·10⁴ and a mechanical frequency of 5.3 GHz has been found.

METHODS

The design of the nano-optomechanic cavity has been performed through an analysis of the phononic and photonic diagram bands evaluated using finite element methods (FEM) as COMSOL and RSoft Bandsolve, respectively. On the one hand, solving these eigenvalue problems, it was possible to design the corrugated unit cell to have the mechanical cavity frequency of 5.3 GHz inside the total bandgap, as can be seen in Fig. 1. On the other hand, the proper middle cavity was built according to Appl. Phys. Lett. 101, 081115 (2012), width a waveguide width of 500 nm and a quadratic variation of the lattice parameter along the structure in order to fit with the corrugated optomechanical crystal in the surroundings. This corrugated one-dimensional crystal was used to shield the acoustic mode of the cavity and it is based on Nat. Commun. 5, 4452 (2014).

RESULTS

The fundamental optical and mechanical defect modes are presented in Fig. 2 besides the total structure. Both modes show an optomechanical coupling rate of 600 kHz where its main contribution is given by the photoelastic effect (PE), instead of the moving boundary effect (MB), as can be seen in Fig 1c. As both contributions are additives, the optimization process being performed works in order to be able to maximize both simultaneously.

DISCUSSION

The main advantage of this structure is that the optical properties can be tuned with the width beam and hole spacing and size of the center of the structure and, separetly, the mechanical properties can be tuned with the corrugated unit cell in the surroundings. Furthermore, its design enables coupling via lateral bend waveguides.