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Terahertz imager based on lasers that fit on microchip

30 Jul 2019

Princeton develops frequency comb-based technology with promising applications in pharma discovery, medical diagnoses.

Researchers at Princeton University, NJ, have developed a terahertz imaging system based on lasers small and efficient enough to fit on a microchip. They describe this as “a major step toward developing portable scanners that can rapidly measure molecules in pharmaceuticals or classify tissue in patients’ skin.”

The system emits and detects radiation at terahertz frequencies. Straightforward maging using terahertz radiation has long been a goal for engineers, says the Princeton group, but the difficulty of creating practical systems that work in this frequency range has limited most applications and resulted in what engineers call the “terahertz gap.”

“Here, we have a revolutionary technology without any moving parts that 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.

The new system, described in a paper published in Optica, can quickly probe the identity and arrangement of molecules or expose structural damage to materials.

Updating existing technology

While current terahertz imaging technologies are typically expensive to produce and difficult to operate, the Princeton 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 are able to check if it has a consistent structure and if there is enough of every ingredient you expect,” said Wysocki. As a proof of concept, the researchers created a test tablet with three zones containing common inert ingredients in pharmaceuticals — forms of glucose, lactose and histidine.

The terahertz imaging system identified each ingredient [see above images] and revealed the boundaries, as well as certain 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.

While this 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.

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