Nonlinear Response and Strong Coupling of Surface Phonon Polaritons

Analogous to surface plasmon polaritons in metals, polar dielectrics support surface phonon polaritons (SPhP) in the mid infrared (MIR) spectral region. In contrast to their plasmonic counterpart, however, SPhPs exhibit much longer lifetimes, and hence constitute a promising alternative in the development of nanophotonics. Employing an MIR free-electron laser, we here experimentally reveal resonant second harmonic generation from SPhPs, excited in the Otto geometry for prism coupling. In a second system featuring an ultrathin layer on top of the SPhP active material, we observe strong coupling of the bulk SPhP to the epsilon near zero mode supported by the thin layer. Our experimental findings are corroborated using a specifically developed matrix formalism for anisotropic multilayer wave propagation