Femtosecond Formation of Ultrastrong Light-Matter Interaction

Intersubband cavity polaritons in a GaAs/AIGaAs quantum well waveguide structure are photogenerated by a 12-fs near-infrared pulse. Multi-THz field transients directly trace the non-adiabatic switch-on of light-matter coupling with a vacuum polariton splitting of up to 44% of the bare photon frequency. We study the decay of a coherent photon state during quasi-instantaneous switching with sub-cycle resolution.