Ultrafast triggering of strong coupling in a semiconductor terahertz Fabry-Pérot cavity.

Strong light-matter coupling - the hybridization of material excitations with the electromagnetic modes of a photonic resonator, has led to striking modifications in the physical and chemical properties of materials, including their chemical reactivity. To date, experimental studies of these effects have been limited to steady-state conditions, namely long after the formation of polaritons. Here, we overcome this constraint by introducing an ultrafast, optically switchable Fabry-Pérot cavity that enables on-demand triggering of strong coupling with a switch-on time of just a few picoseconds. We demonstrate dynamic activation of strong coupling in the terahertz domain between a cavity mode and the collective vibration of crystalline α-lactose powder pellet embedded in the cavity. By photoexciting silicon-based cavity mirrors, we induce an abrupt spectral shift and linewidth narrowing of the photonic cavity mode alone - leading to a substantial increase in the Q-factor and tuning the cavity mode into resonance with the vibrational transition of the cavity-embedded material. This, in turn, enables ultrafast light-matter hybridization through the formation of vibrational polaritons. The ability to switch the cavity 'on demand' at picosecond timescales provides means to actively trigger strong light-matter coupling and to optically manipulate the coupling strength, offering new opportunities to study its dynamics and guide chemical reactivity.