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arXiv:2209.12609 · New Journal of Physics

A practical guide to terahertz imaging using thermal atomic vapour

Lucy A. Downes, Lara Torralbo-Campo, Kevin J. Weatherill

Published Feb 1, 2023• 23 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1088/1367-2630/acb80c

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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.
Terahertz imaging has become increasingly important across various fields and practical production applications. Achieving both high sensitivity and high speed remains a primary challenge for most current THz imaging technologies. In this work, we present an imaging system based on Rydberg atomic vapor operating at frequencies exceeding 1 THz. By utilizing Cesium 133 atoms coupled with THz waves, de-excitation radiation generated after atomic excitation to Rydberg states converts invisible THz waves into visible light, enabling the acquisition of spatial information and intensity distributions of THz fields. The system achieves a minimum detectable power of 330 fW /s 1/2 per (2.4×2.4) μm 2 , an imaging resolution of approximately 1.7 mm, and an imaging bandwidth of 8.6 MHz. Additionally, the theoretical imaging rate can exceed 1 MHz. The Rydberg-atom-based THz imaging system exhibits a simple structure and is amenable to miniaturization. This THz imaging method holds promise for advancing applications in THz nondestructive testing, biomedical imaging, and concealed object detection.
Abstract Rydberg-atom–based Terahertz (THz) reflection imaging provides ultra-high sensitivity and, in principle, the potential for MHz-level imaging speed, overcoming the limitations of THz transmission imaging for strongly absorbing samples. Here, we experimentally demonstrate a Rydberg-atom THz reflection imaging system, converting reflected THz fields into visible fluorescence using a cesium vapor cell along the 14 P 3/2 → 13 D 5/2 → 6 P 3/2 pathway, and sequentially imaging the fluorescence with a sCMOS camera. The system achieves clear reflection images with a minimum resolvable line-pair period of approximately 1.26 mm, and a minimum detectable power of 2.41 fW·s -1/2 per (6.5×6.5) μ m 2 pixel. Representative applications include reflection imaging of metallic samples, discrimination of Polar Liquids, and non-contact visualization of superficial human vasculature and finger motion. This system offers a promising route toward high-frame-rate, room-temperature, full-field THz imaging for both industrial and scientific applications.
Modern terahertz detectors enable direct measurement of the electric field, providing simultaneous access to amplitude and phase information of the terahertz radiation. However, conventional systems are often based on single-pixel detectors, limiting most imaging applications to slow raster-scanning techniques. This work presents the demonstration of a novel approach for amplitude- and phase-sensitive imaging in the terahertz range using quantum imaging with undetected photons. By decoupling the sample measurement in the terahertz range from the detection of the information in the visible, this technique enables the use of cost-effective, highly developed multi-pixel optical components. Central achievements are the observation of quantum interference with terahertz photons and its application to material characterization and molecular concentration analysis. Furthermore, image information was transferred from the terahertz to the visible spectral range, achieving a spatial resolution close to the terahertz wavelength. This approach thus provides direct access to amplitude, phase, and spectral information for imaging applications in the terahertz range using a multi-pixel camera.
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 \nvapour 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 \nspatial 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 \ndesigned 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.
Abstract Quantum imaging offers a powerful method for spatially transferring amplitude and phase information between spectral regions of interest, even when separated by several orders of magnitude in photon energy. However, so far demonstrations have been limited to regions of similar orders of magnitude in frequency, where commercial cameras are available. Here, amplitude‐ and phase‐sensitive imaging in the terahertz frequency region (1.5 THz) is demonstrated by detecting only visible photons (662.2 nm, 452.7 THz), separated by over two orders of magnitude. This allows reliable detection of terahertz information using a standard CMOS camera without cooling while still achieving spatial resolution close to the terahertz wavelength. The impact of strong noise contributions is reduced through quantum distillation.The experimental results are in good agreement with numerical simulations of the imaging process and demonstrate the huge potential of this method for exploring challenging spectral regions where conventional cameras are not available.
We demonstrate simultaneous imaging at 0.549 THz and 1.012 THz with an optical overlay using a two-species atomic-vapour-based technique. The atomic vapour, comprising laser-pumped rubidium and caesium atoms contained within the same cell, is used to convert two narrowband terahertz signals to optical frequencies which can then be detected using standard CMOS sensors. We use the system to image and perform spectral analysis of material samples. As atomic vapour is optically transparent, by using optically-transparent terahertz lenses, we can achieve simultaneous optical imaging, allowing for potential integration of terahertz sensitivity into a range of optical imaging devices.