Strong optomechanical coupling in a slotted photonic crystal nanobeam cavity with an ultrahigh quality factor-to-mode volume ratio

TL;DR

This paper reports the design and fabrication of a silicon photonic crystal cavity with a tiny mode volume and high quality factor, achieving strong optomechanical coupling dominated by the moving-boundary effect.

Contribution

The work introduces a novel slotted photonic crystal nanobeam cavity with ultrahigh Q/V ratio and demonstrates a dominant moving-boundary optomechanical coupling mechanism.

Findings

Optical mode volume of 0.017 $(rac{\u03bb_{vac}}{n})^3$

Optomechanical vacuum coupling rate of 310 kHz

Optical quality factor up to 1.2 x 10^5

Abstract

We describe the design, fabrication, and characterization of a one-dimensional silicon photonic crystal cavity in which a central slot is used to enhance the overlap between highly localized optical and mechanical modes. The optical mode has an extremely small mode volume of 0.017 $(\lambda_{vac}/n)^3$, and an optomechanical vacuum coupling rate of 310 kHz is measured. With optical quality factors up to $1.2 \cdot 10^5$, fabricated devices are in the resolved-sideband regime. The electric field has its maximum at the slot wall and couples to the in-plane breathing motion of the slot. The optomechanical coupling is thus dominated by the moving-boundary effect, which we simulate to be six times greater than the photoelastic effect, in contrast to most structures, where the photoelastic effect is often the primary coupling mechanism.