Strong Coupling in a Self-Coupled Terahertz Photonic Crystal

Vibrational strong coupling is a phenomenon in which a vibrational transition in a material placed inside a photonic structure is hybridized with its optical modes to form composite light–matter excitations known as vibro-polaritons. Here we demonstrate a new concept of vibrational strong coupling: we show that a monolithic photonic crystal, made of a resonant material, can exhibit strong coupling between the optical modes confined in the structure and the terahertz vibrational excitations of the same material. We study this system both experimentally and numerically to characterize the dispersion of the photonic modes for various sample thicknesses and reveal their coupling with the vibrational resonances. Finally, our time-domain THz measurements allow us to isolate the free induction decay signal from the grating modes as well as from the vibro-polaritons.