A terahertz-driven non-equilibrium phase transition in a room temperature atomic vapour

There are few demonstrated examples of phase transitions that may be driven directly by terahertz frequency electric fields, and those that are known require field strengths exceeding 1 MV cm^-1. Here we report a non-equilibrium phase transition driven by a weak (≪1 V cm^-1), continuous-wave terahertz electric field. The system consists of room temperature caesium vapour under continuous optical excitation to a high-lying Rydberg state, which is resonantly coupled to a nearby level by the terahertz electric field. We use a simple model to understand the underlying physical behaviour, and we demonstrate two protocols to exploit the phase transition as a narrowband terahertz detector: the first with a fast (20 μs) non-linear response to nano-Watts of incident radiation, and the second with a linearised response and effective noise equivalent power ≤1 pW Hz^-1/2. The work opens the door to a class of terahertz devices controlled with low-field intensities and operating in a room temperature environment.