Terahertz Wave Detection and Imaging with a Hot Rydberg Vapour

Abstract

This thesis investigates the resonant interaction between Rydberg atoms in a hot caesium vapour and terahertz frequency electromagnetic fields, and explores hyperfine quantum beats modified by driving an excited state transition in an inverted ladder scheme. The 21P caesium Rydberg atoms are excited using a three-step ladder scheme and we use a terahertz field resonant with the 21P to 21S transition (0.634 THz), to measure Autler-Townes splitting of a 3-photon Rydberg electromagnetically induced transparency (EIT) feature. The Autler-Townes splitting allows us to infer the terahertz electric field amplitude, and we show a worked example measurement of a low-amplitude electric field, yielding .

By driving an off-resonant Raman transition which combines the laser and terahertz fields, we restrict the Rydberg excitation to areas of the caesium vapour where the laser and terahertz fields spatially overlap. We show that the terahertz field intensity is proportional to the pixel intensity of a camera image of the atomic fluorescence, and demonstrate an image of a terahertz standing wave. The camera image is used to fit a model for a corresponding Autler-Townes spectrum, giving the scale of the electric field amplitude, and we use a video camera to record real-time images of the terahertz wave.

In the regime of intrinsic optical bistability we study a Rydberg atom phase transition and critical slowing down, and we find that the terahertz field drives the collective Rydberg atom phase transition at low terahertz intensity ( ). We measure a linear shift of the phase transition laser detuning with coefficient , and we use the frequency shift to detect incident terahertz radiation with sensitivity, . When the system is initialised in one of two bistable states a single 1 ms terahertz pulse with energy of order 10 fJ can permanently flip the system to the twin state.