Chiral phonons driven by chiral cavities

Chiral phonons—lattice vibrations that carry angular momentum—are typically generated using external magnetic fields or short, intense light pulses. While effective, pulse-based methods are transient, meaning the phonon population decays rapidly once the pulse ends. This paper explores an alternative: using a driven electromagnetic cavity to create a steady-state population of chiral phonons. The authors model a system where a material hosting phonons is placed inside a chiral cavity—a cavity engineered to support electromagnetic modes with a specific handedness. By driving this cavity with a continuous monochromatic laser, they derive the conditions under which the cavity-phonon interaction leads to a stable, non-transient imbalance between left- and right-circularly polarized phonons.

The authors demonstrate that a coherently driven chiral cavity can effectively generate a steady-state population of chiral phonons. By calculating the cavity-phonon Hamiltonian, they show that the system reaches a steady state where the phonon population is determined by the drive power and the coupling strength. For realistic state-of-the-art cavity parameters, the authors estimate that a drive power of approximately 10 nW can generate a steady-state population of about 6 phonons. This corresponds to an effective phonon-induced magnetic field of roughly 4 mT. This magnitude is comparable to fields achieved in pulsed THz experiments, but with the distinct advantage of being a continuous, steady-state effect rather than a transient one. The study also explores an alternative setup using a linear cavity with two degenerate, orthogonally polarized modes, showing that chiral phonons can still be generated if these modes are driven with a specific phase relationship.

This research provides a new framework for manipulating angular momentum in condensed-matter systems without relying on external magnetic biases or high-power, short-duration laser pulses. By enabling steady-state control over chiral phonons, this approach opens doors for long-term investigations into the phono-magnetic effect and potential applications in phononics and quantum information processing, where persistent control over lattice excitations is highly desirable.