Materials Analysis Using a THz
Imaging System Based on
Atomic Vapour

Abstract

This thesis studies the response of the interaction between Rydberg atomic vapour and a THz frequency field. When Caesium atoms at room temperature
are excited to a Rydberg state using three infrared lasers and a 0.55 THz field
resonant with the 14P3/2 → 13D5/2 transition is applied, the atoms respond
by emitting a green optical fluorescence corresponding to the 13D5/2 → 6P3/2
decay. This response is exploited to investigate the absorption coefficient for
different polymer materials that transmit well in the THz frequency range
using the Beer–Lambert law. We calibrate the system to obtain a measure
of THz intensity. As the THz imaging system is highly sensitive to environmental changes, and to show that our results are consistent, we provide a
comparison of results between our atomic detection method and a commercial
thermal power meter. Additionally, we measure the absorption coefficient of
the same materials at a frequency of 1.1 THz, and the results are compared
with those measured at 0.55 THz. The THz imaging system is also used to
perform some experiments in order to demonstrate its effectiveness in real-world applications. The system provides an interesting image contrast in the
case of a sample containing two different polymer materials measured at two
THz frequencies. The result is a proof-of-concept that multispectral THz imaging can provide additional information and is motivation to improve our THz
imaging system by introducing a dual-species THz imager. We also investigate
the polarisation spectroscopy of an excited-state transition of rubidium vapour
at room temperature as a step towards a rubidium THz imaging system. The
narrow dispersive signal produced by this spectroscopy technique is ideal for
laser frequency stabilisation of excited-state transitions.