New developments in terahertz generators push envelope for nondestructive inspection

Inspecting objects by hand, like searching mail for illicit drugs, can be time-consuming and imperfect. Terahertz (THz) radiation, a relatively underexplored nonionizing band of the electromagnetic spectrum, sitting between radio waves and infrared radiation, has shown promise for its nondestructive transmission through a wide variety of materials. But creating a reliable device that uses THz radiation with a suitable dynamic range has proven difficult.

Murate and Kawase report advances that provide a path forward to nondestructive terahertz inspection of a variety of materials related to mailing envelopes and packages. The injection-seeded THz parametric generator serves as a powerful enough wave source for single-longitudinal mode THz-wave radiation that can be used to inspect everyday objects — one of the first applications of the technology to do so.

Other THz systems use time-domain spectroscopy to inspect objects, but suffer from rapidly deteriorating dynamic range with increasing frequencies, as well as diminished performance when targets generate multiple reflections, refractions and scattering.

Murate and Kawase’s system enjoys a high dynamic range from 1 to 2.5 THz, a large parametric detector and a narrow-linewidth source that is less influenced by scattering, refraction and multiple reflections.

“In this work, we succeeded in nondestructive inspection of reagents inside thick envelopes and three-dimensional computed tomography of plastics, which attenuate THz-waves by more than 60 dB using our system,” said Murate. “Moreover, we showed the possibility of real-time measurement using a multiwavelength injection-seeded terahertz parametric generator.”

Murate said he hopes to expand his group’s current work into real-time inspection using multiwavelength generation and detection with the intent to produce real-world devices for nondestructive, high-throughput inspecting.

Source: “Perspective: Terahertz wave parametric generator and its applications,” by K. Murate and K. Kawase, Journal of Applied Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5050079 .