[Paper Review] Optomechanical dark matter instrument for direct detection

Christopher G. Baker, Warwick P. Bowen+4 more|arXiv (Cornell University)|Jun 16, 2023

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper proposes ODIN, an optomechanical dark matter instrument that detects keV-scale dark matter via phonon excitation in superfluid helium, using laser-driven optomechanical transduction to convert low-energy phonons into detectable photons. The system achieves projected sensitivity to dark matter–nucleon cross-sections as low as $\mathcal{O}(10^{-32})\,\text{cm}^2$, with background suppression to $\sim 10^{-5}$ Hz via cryogenic filtering and phonon lasing for enhanced signal-to-noise.

ABSTRACT

We propose the Optomechanical Dark-matter INstrument (ODIN), based on a new method for the direct detection of low-mass dark matter. We consider dark matter interacting with superfluid helium in an optomechanical cavity. Using an effective field theory, we calculate the rate at which dark matter scatters off phonons in a highly populated, driven acoustic mode of the cavity. This scattering process deposits a phonon into a second acoustic mode in its ground state. The deposited phonon ($μ$eV range) is then converted to a photon (eV range) via an optomechanical interaction with a pump laser. This photon can be efficiently detected, providing a means to sensitively probe keV scale dark matter. We provide realistic estimates of the backgrounds and discuss the technical challenges associated with such an experiment. We calculate projected limits on dark matter-nucleon interactions for dark matter masses ranging from 0.5 to 300 keV and estimate that a future device could probe cross-sections as low as $\mathcal{O}(10^{-32})$ cm$^2$.

Motivation & Objective

• To address the unexplored parameter space of low-mass dark matter (keV scale) beyond the reach of existing direct detection experiments.
• To overcome the challenge of detecting low-energy phonon signals from dark matter scattering in superfluid helium.
• To develop a novel detection mechanism that converts undetectable phonons into measurable photons using optomechanical transduction.
• To suppress background noise through cryogenic filtering, phonon lasing, and lock-in detection via controlled phonon mode occupation.
• To project sensitivity limits for dark matter–nucleon interactions across 0.5–300 keV dark matter masses.

Proposed method

• Utilizes an optomechanical cavity filled with superfluid helium to coherently couple optical and acoustic modes with high spatial overlap.
• Employs a 1064 nm laser to drive a highly populated phonon mode (scattering mode) via phonon lasing, enhancing scattering cross-sections through Bose enhancement.
• Uses a 564 nm pump laser to coherently convert dark matter-induced phonons into anti-Stokes photons via resolved-sideband optomechanical interaction.
• Deploys cascaded Fabry-Perot filters and cryogenic band-pass filters to suppress optical pump leakage by >200 dB, ensuring single-photon detection fidelity.
• Leverages cryogenic dilution refrigeration at ~4 mK to minimize thermal phonon occupation in the readout mode, reducing background to $\sim 10^{-5}$ Hz.
• Applies spectral asymmetry via a SiO₂ slab to suppress unwanted optomechanical cooling while enhancing amplification by a factor of $10^6$.

Experimental results

Research questions

• RQ1 Can optomechanical transduction enable detection of keV-scale dark matter via phonon-to-photon conversion in superfluid helium?
• RQ2 How can phonon lasing in a driven acoustic mode enhance the signal rate for low-mass dark matter scattering?
• RQ3 What level of background suppression is achievable with cryogenic filtering and temperature control in a superfluid optomechanical system?
• RQ4 What are the projected limits on dark matter–nucleon cross-sections for masses between 0.5 and 300 keV?
• RQ5 Can the system enable lock-in detection by modulating the phonon occupation in the scattering mode to reject DC noise?

Key findings

• The ODIN instrument achieves projected sensitivity to dark matter–nucleon cross-sections as low as $\mathcal{O}(10^{-32})\,\text{cm}^2$ for dark matter masses between 0.5 and 300 keV.
• Background rates are estimated at $\sim 10^{-5}$ Hz, primarily limited by single-photon detector dark counts and optical leakage, both suppressible via cryogenic filtering.
• Phonon lasing in the scattering mode provides a Bose enhancement factor proportional to the phonon occupation number, significantly boosting the effective scattering rate.
• Optical filtering with cascaded Fabry-Perot cavities and band-pass filters suppresses pump leakage by over 200 dB, enabling single-photon detection sensitivity.
• The system achieves a $10^6$ suppression of unwanted optomechanical cooling relative to amplification through engineered spectral asymmetry in the cavity FSR.
• Thermal phonon occupation in the readout mode is reduced to $\sim 10^{-5}$ at 4 mK, minimizing thermal background and enabling high-fidelity phonon detection.