A practical guide to Terahertz imaging using thermal atomic vapour

Abstract

This tutorial aims to provide details on the underlying principles and methodologies of atom-based terahertz imaging techniques. Terahertz imaging is a growing field of research which can provide complementary information to techniques using other regions of the electromagnetic spectrum. Unlike infrared, visible and ultraviolet radiation, terahertz passes through many everyday materials, such as plastics, cloth and card. Compared with images formed using lower frequencies, terahertz images have superior spatial resolution due to the shorter wavelength, while compared to x-rays and gamma rays, terahertz radiation is non-ionising and safe to use. The tutorial begins with the basic principles of terahertz to optical conversion in alkali atoms before discussing how to construct a model to predict the fluorescent spectra of the atoms, on which the imaging method depends. We discuss the practical aspects of constructing an imaging system, including the subsystem specifications. We then review the typical characteristics of the imaging system including spatial resolution, sensitivity and bandwidth. We conclude with a brief discussion of some potential applications. Introduction. Using Atoms as SensorsAtoms in dilute vapour can make very effective sensors [1,2]. Atoms have no moving parts to wear out, they are relatively unperturbed by inter-atomic interactions, their energy levels are sensitive to applied fields, and crucially, each atom of the same isotope is identical. This final point means that atomic sensors are in effect pre-calibrated and measurements made using them are reproducible and should be, at least in principle, traceable to the SI system of measurements. Atomic systems already provide a platform for precision clocks [3,4], gyroscopes [5,6], magnetometers [7,8], gravimeters [9,10] and gradiometers [11]. Many atom-based sensors achieve optimal performance by using laser-cooling techniques to create very cold atomic samples [12], and although efforts are ongoing to simplify and miniaturise such apparatus [13,14], laser cooling inevitably introduces significant experimental complexity and cost to the setup. In contrast, for atomic sensors where inhomogeneous Doppler broadening is not a problem, setups using