Ghost Imaging for Terahertz Microscopy

A team of physicists at the University of Sussex has success­fully developed the first nonlinear camera capable of capturing high-reso­lution images of the interior of solid objects using terahertz radiation. Led by Marco Peccianti of the Emergent Photonics (EPic) Lab, Luana Olivieri, Dr Juan S. Totero Gongora and a team of research students built a new type of THz camera capable of detecting THz electro­magnetic waves with unpre­cedented accuracy.

Artistic illustration: The time-resolved nonlinear ghost imaging camera uses a nonlinear crystal to convert standard laser light to terahertz patterns, allowing the reconstruction of complex samples using a single terahertz pixel. (Source: U. Sussex)

Images produced using THz radiation are called hyper­spectral because the image consists of pixels, each one containing the electro­magnetic signature of the object in that point. Lying between microwaves and infrared in the electro­magnetic spectrum, THz radiation easily penetrates materials like paper, clothes and plastic in the same way X-rays do, but without being harmful. It is safe to use with even the most delicate bio­logical samples. THz imaging makes it possible to see the molecular composition of objects and distinguish between different materials – such as sugar and cocaine, for example.

Explaining the signi­ficance of their achieve­ment, Peccianti said: “The core challenge in THz cameras is not about collecting an image, but it is about preserving the objects spectral finger­print that can be easily corrupted by your technique. This is where the impor­tance of our achieve­ment lies. The finger­print of all the details of the image is preserved in such a way that we can investi­gate the nature of the object in full detail.”

Until now, cameras capable of capturing a hyper­spectral image preserving all the fine details revealed by THz radiation had not been considered possible. The EPic Lab team used a single-pixel camera to image sample objects with patterns of THz light. The proto­type they built can detect how the object alters different patterns of THz light. By combining this infor­mation with the shape of each original pattern, the camera reveals the image of an object as well as its chemical compo­sition.

Sources of THz radiation are very faint and hyper­spectral imaging had, until now, limited fidelity. To overcome this, The Sussex team shone a standard laser onto a unique non-linear material capable of converting visible light to THz. The prototype camera creates THz electro­magnetic waves very close to the sample, similar to how a micro­scope works. As THz waves can travel right through an object without affecting it, the resulting images reveal the shape and composition of objects in three dimensions.

Totero Gongora said: “This is a major step forward because we have demonstrated that all the possi­bilities explored in our previous theoretical research are not only feasible, but our camera works even better than we expected. While building our device, we disco­vered several ways to optimise the imaging process and now the tech­nology is stable and works well. The next phase of our research will be in speeding up the image recon­struction process and taking us closer to applying THz cameras to real-world appli­cations; like airport security, intelligent car sensors, quality control in manu­facturing and even scanners to detect health problems like skin cancer.” (Source: U. Sussex)

Reference: L. Olivieri et al.: Hyperspectral terahertz microscopy via nonlinear ghost imaging, Optica 7, 186 (2020); DOI: 10.1364/OPTICA.381035

Link: Emergent Photonics (EPic) Laboratory, Dept. of Physics and Astronomy, University of Sussex, Brighton, UK

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