A High-speed THz Imaging System based on THz-to-optical Conversion in Atomic Vapour

Abstract

This thesis investigates the interaction between Rydberg atoms and THz radiation and details development of an atom-based imaging system for THz frequency fields. Room temperature caesium atoms are excited to a Rydberg state using resonant infrared lasers and the response of these atoms to THz radiation is studied in multiple ways. We look at the absorption of the initial excitation laser and show how the observed lineshape changes in response to an applied THz field. We use this to measure the power of a THz field across a range of 350 GHz. These measurements also lead us to determine values for the quantum defects in caesium without reliance on data from any other sources. These values agree within error with those previously reported in literature. We study the optical fluorescence emitted by the laser-excited
vapour and develop a model to predict this emission. We highlight deviations from this model and suggest possible mechanisms for this discrepancy. We show how this model can be used to determine the optimum transition for atom-based THz imaging within our experimental parameters. We demonstrate a high-speed THz imaging system using Rydberg atoms and show that it has near diffraction-limited
spatial resolution and is able to capture images at kHz framerates. We measure the minimum THz power that the system can detect and show how it compares to other technologies. This system is then used to perform some proof-of-principle experiments to highlight its usefulness in commercial settings. Using optical elements
designed and produced in-house we create and characterise THz vortex beams with both radial and azimuthal phase. We show how these beams are similar to Laguerre-Gauss modes and highlight some important differences.