Terahertz Imaging System on a Chip Offers Speed and Portability

An imaging system developed at Princeton University uses lasers small enough to fit on a microchip, and emits and detects electromagnetic radiation at terahertz (THz) frequencies. The new system, which is based on a semiconductor design, uses a dual-comb structure to efficiently measure the radiation that is reflected from the sample. Spectral signatures in the reflected radiation enable researchers to identify the sample’s molecular makeup.

The device performs hyperspectral imaging with chip-scale frequency combs based on THz quantum cascade lasers. The dual combs are free-running and emit coherent THz radiation that covers a bandwidth of 220 GHz at 3.4 THz with ∼10 μW per line.

The speed of the new system could make it useful for real-time quality control of pharmaceutical tablets on a production line and other fast-paced processes. “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," professor Gerard Wysocki said.

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. An 81- × 53-pixel hyperspectral image was acquired through a raster scan of a solid pressed disk containing the three differently absorbing compounds. The THz 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 problem in pharmaceutical production that can occur when the active ingredient is not properly mixed into a tablet.

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. Such images could aid quality control and development in pharmaceutical manufacturing, as well as medical diagnosis and treatment. Courtesy of Sterczewski et al.

To demonstrate the quality of the image resolution, the team imaged a U.S. quarter. Details on the coin as fine as the eagle’s wing feathers and as small as one-fifth of a millimeter-wide were clearly visible.

Gerard Wysocki (left), an associate professor of electrical engineering, and Jonas Westberg, an associate research scholar, helped create a new terahertz imaging system that represents a major step toward developing portable scanners that could rapidly measure molecules in pharmaceuticals or classify tissue in patients’ skin. Courtesy of David Kelly Crow.

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

Because it is nonionizing, THz radiation is safe for patients and could potentially be used as a diagnostic tool for skin cancer. The technology’s ability to image metal could be applied to test airplane wings for damage after being struck by an object in flight.

The research was published in Optica, a publication of OSA, The Optical Society (https://doi.org/10.1364/OPTICA.6.000766).