Controlling Radiative Phonons in the van der Waals Ferroelectric NbOI₂

Area of Science:

• Quantum optics
• Condensed matter physics
• Materials science

Background:

• The optical properties of quantum systems are influenced by their electromagnetic environment via the local density of optical states (LDOS).
• Environmental engineering has successfully controlled atomic, excitonic, and electronic transitions, but phonon radiative properties remain underexplored.
• Phonons, as lattice excitations, are crucial for heat transport and optoelectronic phenomena.

Purpose of the Study:

• To demonstrate the control of terahertz (THz) phonon emission by engineering the photonic environment.
• To investigate the influence of the local density of optical states (LDOS) on phonon radiative properties.
• To establish radiative phonons as tunable quasiparticles controllable by photonic boundary conditions.

Main Methods:

• Utilizing a van der Waals ferroelectric material, NbOI₂, for phonon emission studies.
• Employing a simple mirror geometry to modify the photonic environment and thus the LDOS.
• Measuring and analyzing the terahertz (THz) phonon emission characteristics, including Q-factor, frequency, linewidth, and intensity.

Main Results:

• Achieved pronounced and reversible tuning of phonon emission properties (Q-factor, frequency, linewidth, intensity) by modifying the photonic environment.
• Observed modulation of phonon emission approaching an order of magnitude.
• Demonstrated Purcell-like enhancement and suppression of the phonon radiative rate, indicating near-unity radiative quantum efficiency in NbOI₂.

Conclusions:

• Radiative phonons can be controlled by engineering their photonic environment, similar to electronic and optical properties.
• Phonon emission properties are governed by photonic boundary conditions, establishing phonons as tunable quasiparticles.
• This work extends LDOS engineering to lattice excitations, opening avenues for tunable THz sources and phonon-based quantum technologies.