[Paper Review] Emitter-Optomechanical Interaction in Ultra-High-Q hBN Nanocavities

Chenjiang Qian, Viviana Villafañe+7 more|arXiv (Cornell University)|Oct 1, 2022

Mechanical and Optical Resonators4 citations

TL;DR

This study demonstrates phonon-induced emitter-optomechanical coupling in ultra-high-Q hexagonal boron nitride (hBN) nanocavities hosting charged boron vacancy (V_B⁻) centers. Using spatially resolved photoluminescence and Raman spectroscopy, it reveals that cavity mechanical modes induce asymmetric spectral lineshapes and phonon polaritons via intrinsic electron-phonon coupling, with key asymmetry emerging only when Q > 10⁴, confirming a novel hybrid quantum interface between spin, photons, and phonons beyond the Jaynes-Cummings model.

ABSTRACT

Integrating quantum emitters into nanocavities which simultaneously couple to the photonic and mechanical modes is critical for interfacing electron spins, photons and phonons in the cavity QED system. Here, we investigate the interaction between the charged boron vacancy $V_B^-$, ultra-high-Q ($\sim10^5$) cavity photonic modes and local phonon modes. A pronounced asymmetry is observed in the emission spectrum for cavities with Q-factor above a threshold of 10$^4$. Similar asymmetries are not observed for cavities without $V_B^-$ centers. To explain our findings, we model the system with phonon-induced light-matter coupling based on $V_B^-$ centers, and compare to the Jaynes-Cummings model for usual emitters. Our results reveal that the multipartite interplay arises during the light-matter coupling of $V_B^-$ centers, illustrating that it is phonon-induced, rather than being caused by thermal population of phonon modes. Such emitter-optomechanical interaction between different photon ($V_B^-$ emission, cavity photonic) and phonon ($V_B^-$ phonon, cavity mechanical) modes provides a novel system to interface spin defects, photons and phonons in condensed matters.

Motivation & Objective

• To explore the interaction between quantum emitters, photonic modes, and mechanical vibrations in nanocavities.
• To investigate whether phonon-induced light-matter coupling in V_B⁻ centers leads to optomechanical effects beyond the standard Jaynes-Cummings model.
• To establish a hybrid quantum system where spin, photons, and phonons are coherently coupled via intrinsic electron-phonon interactions.
• To demonstrate control of V_B⁻ emission and phonon modes through cavity optomechanics using resonant excitation.

Proposed method

• Fabricated ultra-high-Q (Q ~ 10⁵) hBN/Si₃N₄ nanobeam cavities with localized V_B⁻ centers via 30 keV N⁺ ion irradiation.
• Performed spatially resolved photoluminescence (PL) and Raman spectroscopy to map emission and phonon modes across the cavity.
• Used resonant cw-laser excitation (532 nm, ~1 MHz linewidth) tuned across the cavity photonic mode (731 nm) to probe optomechanical coupling.
• Constructed a numerical model based on phonon-induced light-matter coupling for V_B⁻ centers, contrasting it with the standard Jaynes-Cummings model.
• Correlated position-dependent shifts in PL and Raman peaks to identify anticrossings and phonon polaritons arising from mechanical mode coupling.
• Analyzed spectral asymmetry in PL lineshapes as a function of cavity Q-factor, identifying a threshold at Q ≈ 10⁴.

Experimental results

Research questions

• RQ1 Does the presence of V_B⁻ centers in ultra-high-Q hBN nanocavities lead to observable optomechanical coupling between photonic and mechanical modes?
• RQ2 Is the observed spectral asymmetry in the emission lineshape due to phonon-induced light-matter coupling rather than thermal phonon population?
• RQ3 Can cavity mechanical modes mediate coupling between cavity photons and V_B⁻ phonons, as evidenced by asymmetric Raman enhancement?
• RQ4 How does the Q-factor of the cavity influence the emergence of optomechanical effects in this system?
• RQ5 To what extent do anticrossing features in PL and Raman spectra confirm the formation of phonon polaritons?

Key findings

• A pronounced spectral asymmetry in the photoluminescence lineshape emerges only when the cavity Q-factor exceeds ~10⁴, indicating a threshold for optomechanical coupling.
• The asymmetry is absent in cavities without V_B⁻ centers, confirming its origin in the emitter-optomechanical interaction.
• The observed spectral features and anticrossings in PL and Raman spectra are strongly correlated, indicating the formation of phonon polaritons from coupled V_B⁻ and mechanical modes.
• Resonant excitation reveals asymmetric enhancement of V_B⁻ phonon Raman peaks: >10× stronger at blue detuning (heating) than at red detuning (cooling), indicating mechanical mode-mediated coupling.
• The enhancement of Si₃N₄ phonon peaks is symmetric and weak (~10%), confirming that cavity photons couple directly to Si₃N₄ phonons without mechanical mode mediation.
• The experimental data are well described by a phonon-induced light-matter coupling model but fail to match predictions from the standard Jaynes-Cummings model, confirming the role of intrinsic electron-phonon coupling.