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arXiv:2512.01825

Simultaneous optical and multi-band terahertz imaging using an atomic quantum sensor

Andrew R. MacKellar, Charles S. Adams, Kevin J. Weatherill

Published Dec 1, 2025• 0 citations
arXivHTMLPDF

Abstract

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.

DOI
10.48550/arxiv.2512.01825

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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.
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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.
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.
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.
We experimentally demonstrate digital communications in the terahertz (THz) band using a rubidium vapor cell as a quantum receiver. We utilize amplitude modulation to encode digital information in THz photons, which are coherently upconverted to optical photons via a Rydberg six-wave-mixing process. We achieve a data transmission rate of up to 1.16 Mbit/s and a tunable bandwidth of up to 142 MHz near a 0.11 THz carrier. With reduced data rate and increased integration time per bit, we demonstrate weak-field THz transmission with a receiver sensitivity in the −130 dBm range. As a proof of principle, we perform an end-to-end transmission of digital color images in the THz band. Our work provides the possibility of THz communication at the single-photon level.
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.