New Device for Terahertz Imaging

A new imaging technology rapidly measures the chemical compositions of solids. A conventional image of a sample pill is shown at left; at right, looking at the same surface with terahertz frequencies reveals various ingredients as different colors. (Source: Sterczewski et al.)

In a major step toward developing portable scanners that can rapidly measure molecules in pharma­ceuticals or classify tissue in patients’ skin, researchers have created an imaging system that uses lasers small and efficient enough to fit on a microchip. The system emits and detects electro­magnetic radiation at terahertz frequencies. Imaging using terahertz radiation has long been a goal for engineers, but the difficulty of creating practical systems that work in this frequency range has stymied most appli­cations and resulted in what engineers call the terahertz gap.

“Here, we have a revo­lutionary tech­nology that doesn’t have any moving parts and uses direct emission of terahertz radiation from semi­conductor chips,” said Gerard Wysocki, an associate professor of electrical engineering at Princeton Uni­versity and one of the leaders of the research team. Terahertz radiation can penetrate substances such as fabrics and plastics, is non-ionizing and therefore safe for medical use, and can be used to view materials difficult to image at other frequencies. The new system can quickly probe the identity and arrange­ment of molecules or expose structural damage to materials.

The device uses stable beams of radiation at precise frequencies. A frequency comb contains multiple teeth that each emit a different, well-defined frequency of radiation. The radiation interacts with molecules in the sample material. A dual-comb structure allows the instrument to effi­ciently measure the reflected radiation. Unique patterns, or spectral signatures, in the reflected radiation allow researchers to identify the molecular makeup of the sample.

While current terahertz imaging technologies are expensive to produce and cumbersome to operate, the new system is based on a semi­conductor design that costs less and can generate many images per second. This speed could make it useful for real-time quality control of pharma­ceutical tablets on a production line and other fast-paced uses. “Imagine that every 100 microseconds a tablet is passing by, and you can check if it has a consistent structure and there’s enough of every ingredient you expect,” said Wysocki.

As a proof of concept, the researchers created a tablet with three zones containing common inert ingre­dients in pharma­ceuticals – forms of glucose, lactose and histidine. The terahertz imaging system identified each ingredient and revealed the boundaries between them, as well as a few spots where one chemical had spilled over into a different zone. This type of hot spot represents a frequent problem in pharma­ceutical production that occurs when the active ingredient is not properly mixed into a tablet. The team also demons­trated the system’s resolution by using it to image a U.S. quarter. Fine details like the eagle’s wing feathers, as small as one-fifth of a millimeter wide, were clearly visible.

While the tech­nology makes the industrial and medical use of terahertz imaging more feasible than before, it still requires cooling to a low temperature, a major hurdle for practical appli­cations. Many researchers are now working on lasers that will poten­tially operate at room tempera­ture. The Princeton team said its dual-comb hyper­spectral imaging technique will work well with these new room-temperature laser sources, which could then open many more uses.

Because it is non-ionizing, terahertz radiation is safe for patients and could potentially be used as a diagnostic tool for skin cancer. In addition, the tech­nology’s ability to image metal could be applied to test airplane wings for damage after being struck by an object in flight. (Source: Princeton U.)

Reference: L. A. Sterczewski et al.: Terahertz hyperspectral imaging with dual chip-scale combs, Optica 6, 766 (2019); DOI: 10.1364/OPTICA.6.000766

Link: Applied Physics, Dept. of Electrical Engineering, Princeton University, Princeton, USA

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