[Paper Review] High-Efficiency Low-Noise Optomechanical Crystal Photon-Phonon Transducers

Sameer Sonar, Utku Hatipoğlu+4 more|arXiv (Cornell University)|Jun 22, 2024

Mechanical and Optical Resonators4 citations

TL;DR

This paper presents a side-coupled two-dimensional optomechanical crystal (2D-OMC) resonator that achieves a six-fold reduction in acoustic heating at millikelvin temperatures, enabling phonon-to-photon conversion with 93.1 ± 0.8% efficiency and only 0.25 ± 0.01 added quanta of noise. The design decouples mechanical and optical waveguides while maintaining evanescent coupling, significantly reducing thermal noise from parasitic absorption.

ABSTRACT

Optomechanical crystals (OMCs) enable coherent interactions between optical photons and microwave acoustic phonons, and represent a platform for implementing quantum transduction between microwave and optical signals. Optical absorption-induced thermal noise at cryogenic (millikelvin) temperatures is one of the primary limitations of performance for OMC-based quantum transducers. Here, we address this challenge with a two-dimensional silicon OMC resonator that is side-coupled to a mechanically detached optical waveguide, realizing a six-fold reduction in the heating rate of the acoustic resonator compared to prior state-of-the-art, while operating in a regime of high optomechanical-backaction and millikelvin base temperature. This reduced heating translates into a demonstrated phonon-to-photon conversion efficiency of 93.1 $\pm$ 0.8% at an added noise of 0.25 $\pm$ 0.01 quanta, representing a significant advance toward quantum-limited microwave-optical frequency conversion and optically-controlled quantum acoustic memories.

Motivation & Objective

• To overcome optical absorption-induced thermal noise in optomechanical crystals (OMCs) operating at millikelvin temperatures.
• To reduce heating in acoustic resonators without compromising optomechanical coupling or optical quality factors.
• To enable high-fidelity quantum transduction and optically controlled quantum acoustic memories by minimizing added noise.
• To engineer a mechanically detached, side-coupled 2D OMC structure that maintains evanescent optical coupling while isolating the cavity from thermal influx.

Proposed method

• Designing a 2D silicon optomechanical crystal (OMC) on a silicon-on-insulator (SOI) substrate with a snowflake lattice and a central fish-bone waveguide to confine optical and acoustic modes.
• Implementing a side-coupled optical waveguide that is mechanically detached from the OMC cavity but maintains evanescent optical coupling via a half-snowflake unit cell.
• Using finite element method (FEM) simulations to optimize the optical and acoustic mode profiles, achieving strong confinement at 1550 nm and 10.3 GHz, respectively.
• Employing optomechanical sideband thermometry to measure the effective phonon bath temperature and heating rate under continuous and pulsed optical excitation.
• Applying a power-law scaling analysis of the heating rate to extract phonon bath dimensionality and identify dominant damping mechanisms.
• Measuring transduction efficiency and added noise via phonon-to-photon conversion under varying optical powers and base temperatures.

Figure 2: Characterization of optical absorption-induced hot bath. a, Schematic showing interactions of the acoustic resonator with various baths considered in our heating model. b, Schematic of measurement setup for time-resolved measurements of the hot bath using single-photon counting on the opti

Experimental results

Research questions

• RQ1 Can mechanical decoupling of the optical waveguide from the OMC cavity reduce parasitic heating without degrading optical coupling?
• RQ2 What is the impact of reduced heating on the phonon occupation and added noise in optomechanical transduction at millikelvin temperatures?
• RQ3 How does the side-coupled 2D OMC geometry compare to prior 1D and butt-coupled OMCs in terms of heating rate and transduction efficiency?
• RQ4 What is the phonon bath dimensionality and dominant damping mechanism in the high-power regime of the 2D OMC?
• RQ5 Can the device achieve near-quantum-limited performance in continuous-wave operation with low added noise and high efficiency?

Key findings

• The side-coupled 2D OMC design reduces the acoustic resonator heating rate by a factor of six compared to prior state-of-the-art OMCs.
• Phonon-to-photon conversion efficiency reaches 93.1 ± 0.8% under pulsed excitation with only 0.25 ± 0.01 added quanta of noise.
• In continuous-wave operation, the device achieves 97% transduction efficiency with a thermal occupation of only 0.42, an order of magnitude lower than in 1D-OMCs.
• The single-photon heralding rate is improved to 465 Hz (vs. 20 Hz for 1D-OMC), and the photon coincidence rate reaches 21 Hz, a ~500-fold improvement.
• The power-law scaling of the heating rate indicates a phonon bath dimensionality of approximately 2, consistent with 2D thermal transport, with a 0.29 power law in the high-power regime.
• The device demonstrates a high optomechanical backaction rate of 1.1 MHz, enabling strong coupling and efficient quantum transduction at millikelvin temperatures.

Figure 3: Phonon-to-photon transduction under continuous-wave excitation. a, Schematic of measurement setup showing single-photon counting of up-converted photons at the optical resonance frequency with the OMC pumped continuously on the red-detuned sideband ( $\Delta=-\Omega_{\text{m}}$ ) of the op