Engineering multiple GHz mechanical modes in optomechanical crystal cavities

TL;DR

This paper demonstrates a method to engineer optomechanical crystal cavities that support multiple GHz mechanical modes with high coupling rates within a phononic bandgap, enabling advanced multimode optomechanical interactions.

Contribution

The authors present a novel design approach to produce multiple GHz mechanical modes with large coupling rates in OMCCs using standard silicon nanofabrication techniques.

Findings

Multiple mechanical modes with up to 600 kHz coupling rate achieved

Mechanical modes confined within a full phononic bandgap

Design is compatible with standard silicon nanotechnology

Abstract

Optomechanical crystal cavities (OMCCs) are fundamental nanostructures for a wide range of phenomena and applications. Usually, optomechanical interaction in such OMCCs is limited to a single optical mode and a unique mechanical mode. In this sense, eliminating the single mode constraint - for instance, by adding more mechanical modes - should enable more complex physical phenomena, giving rise to a context of multimode optomechanical interaction. However, a general method to produce in a controlled way multiple mechanical modes with large coupling rates in OMCCs is still missing. In this work, we present a route to confine multiple GHz mechanical modes coupled to the same optical field with similar optomechanical coupling rates - up to 600 kHz - by OMCC engineering. In essence, we increase the number of unit cells (consisting of a silicon nanobrick perforated by a circular holes with corrugations at its both sides) in the adiabatic transition between the cavity center and the mirror region. Remarkably, the mechanical modes in our cavities are located within a full phononic bandgap, which is a key requirement to achieve ultra high mechanical Q factors at cryogenic temperatures. The multimode bevavior in a full phononic bandgap and the easiness of realization using standard silicon nanotechnology make our OMCCs highly appealing for applications in the classical and quantum realms.