Observation of coherent ferron emission and propagation.

Ordered phases give rise to collective modes and quasiparticles, such as spin waves and magnons emerging from magnetic order. Extending this paradigm to ferroelectrics suggests the existence of polarization waves and their fundamental quanta, ferrons. A coherent ferron-that is, a polarization wave-modulates the magnitude of the electric polarization and is thus an amplitude (Higgs) mode of the ferroelectric order. Here we observe coherent ferrons from the pulsed laser excitation of van der Waals ferroelectrics, NbOI2 and WO2Br2. We demonstrate two complementary manifestations of coherent ferrons: intense narrow-band terahertz emission at the ferroelectric transverse optical phonon frequency, and uniaxial propagation along the polar axis as hyperbolic phonon polaritons with exceptionally long coherence times. These long-lived, uniaxial and dipole-carrying polarization waves may find applications in narrow-band terahertz emission, ferronic information processing and coherent electric control.