This thesis reports the investigation of cooperative non-equilibrium dynamics in a thermal Rydberg ensemble. Cooperative behaviour arises due to resonant dipole-dipole interactions between highly excited Rydberg atoms. In order to transfer atomic population from the ground state to the Rydberg state in a caesium vapour, a three-photon excitation scheme is developed. This scheme has a number of benefits over traditional two-photon Rydberg excitation as each transition utilises in- expensive high-power diode lasers. The process of developing the excitation scheme produces a number of important results, including an excited-state polarisation spectroscopy technique and the observation of coherent three-photon electromagnetically induced transparency.
The optical response and atomic dynamics of the interacting ensemble can be separated into two distinct phases. When the Rydberg number density is low and the interactions are negligible, the system can be described by the behaviour of a single atom. However, when the Rydberg number density is high, resonant dipole-dipole interactions result in a significant modification of the ensemble properties. This cooperative many-body phase cannot be described by the behaviour of a single atom. In the frequency domain, the interactions produce an excitation-dependent cooperative energy shift that is observed using probe transmission spectroscopy. In the time domain, the interactions result in a cooperative enhancement of the atomic decay rate that is analysed using fluorescence spectroscopy.
At the transition between the single-body and many-body phase, a first-order non-equilibrium phase transition occurs. This is observed spatially along the length of the excitation region as a sharp switch in the emitted fluorescence. The first-order phase transition is also observed in the temporal response of the ensemble through critical slowing down. The divergence of the switching time to steady state follows a universal scaling law for phase transitions and the determined critical exponent is in excellent agreement with previous work on non-equilibrium phase transitions
