An imaging system was developed that emits and detects electromagnetic radiation at terahertz frequencies — higher than radio waves but lower than the long-wave infrared light used for thermal imaging. 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 resulted in the “terahertz gap.”
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. 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.
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 on a production line and other fast-paced uses.
As a proof of concept, 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 among 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. They 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 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.