Cherenkov Excitation of Bulk and Nonlinear Surface Phonon Polaritons by Strong-Field Terahertz Surface Waves

However, conventional excitation of Phonon polaritons (PhPs) is constrained by frequency and momentum matching, limiting access to high-wavevector and nonlinear modes. Here, we demonstrate a fundamentally different approach: Cherenkov effect excitation by strong-field THz surface wave.

A GV/m-level THz-SW pulse guided on a metal wire propagates faster than the phase velocity of the TGG phonon polaritons and therefore acts as a moving polarization source, exciting PhPs through broadband generalized Cherenkov phase matching rather than through a narrowband resonant process. These high-amplitude modes further engage anharmonic lattice coupling, generating a distinct third-order nonlinear surface PhP mode at ~10 THz, associated with a transient effective magnetic field of ~60 T. Using ultrafast pump-probe magneto-optic imaging, we visualize the formation, excitation and decay of both linear and nonlinear PhPs in both real time and space.