High-speed Terahertz Imaging using Rydberg Atoms & Quantum Cascade Lasers

The terahertz (THz) region of the electromagnetic spectrum (radiation with frequencies around 10^12 Hertz) has traditionally been considered a difficult region to work in because it falls into a technology gap, with electronic, microwave sources at lower frequencies and photonic, infrared devices at higher frequencies. In recent decades, considerable efforts have been made to develop technologies that operate in the THz range in order to take advantage of the unique combination of properties exhibited by terahertz waves. For example, many everyday materials, such as plastics, paper, cloth etc. are transparent to THz waves, meaning that we can penetrate deeply into samples. However, unlike the more familiar X-rays, THz waves are safe to use because they are low energy and non-ionising. For this reason, terahertz imaging techniques are proposed for applications as broad as medical scanning, non-destructive testing, security, production line testing and medicine quality scanning. However, despite considerable efforts, terahertz cameras are still far slower and less sensitive than their optical counterparts and THz imaging applications are limited as a result.

At Durham we have recently developed a novel approach to THz imaging that uses atomic vapour to convert difficult to detect terahertz waves into easy to detect optical frequencies. The atomic vapour is excited to high-lying (Rydberg) states using laser beams and once in these Rydberg states the atoms are very sensitive to perturbation by terahertz waves and emit optical light. This efficient THz to optical conversion process allows us to effectively capture terahertz images using standard optical cameras and observe frames rates exceeding 3000 frames per second, far exceeding the capabilities of other THz imaging techniques.

This proposal intends to develop further our atom-based THz camera by using Quantum Cascade Lasers (QCLs) to provide the illumination. QCLs are semiconductor lasers capable of emitting high power in the terahertz frequency band - using QCLs will result in sharper spatial resolution and the ability to image larger areas and/or probe thicker samples in our imaging applications.

In order to improve the image quality of our technique, we will also develop adaptive optics technology for the terahertz range. OA technologies are used extensively in the optical and infrared range to correct for aberrations in an imaging system. Previous attempts to perform AO in the THz range have been limited by the small range of movement of deformable mirrors and the slow image acquisition rates of THz cameras. We will develop large-stroke deformable mirrors to allow effective AO correction in the THz range. This will enable depth-selection in our THz imaging process and the removal of imaging artefacts and aberrations.

Furthermore, we will add spectral (frequency dependent) functionality to out imager by adding a second atomic species (Rb87 + Cs133) thereby offering spectral sensitivity analogous to colour photography, expanding the capability of our THz imager to include material distinction by spectral response.

Once we have constructed and characterised our QCL illuminated, AO corrected, 2-colour THz imager, we will apply it to a range of industrially relevant applications inspired and guided by our industrial project partners.