Engineering 101

New Lasers Fit on a Microchip

By Dawn Allcot

Engineers have tried to create imaging technology using terahertz radiation for some time, only to be stymied by what they call the “terahertz gap.” Terahertz radiation is useful to penetrate substances such as fabrics and plastics, and to view materials difficult to image at other frequencies. Because the radiation is non-ionizing it’s safe for medical use. But practical systems simply won’t work in that frequency range – higher than radio waves but lower than long-wave infrared light currently used in thermal imaging systems.

Engineers at Princeton University recently discovered a way to harness terahertz energy to probe the identity and arrangement of molecules or expose structural damage to materials. The new imaging system uses lasers small and efficient enough to fit on a microchip.

Ultimately, the discovery could lead to the creation of portable scanners that can rapidly measure molecules in pharmaceuticals or classify tissue in patients’ skin.

The device uses stable beams of radiation at precise frequencies. The setup is called a frequency comb because it 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.

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.

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.


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