Palm-sized high-intensity-terahertz-wave light source device created by RIKEN

A research group comprising Team Leader Hiroaki Minamide and Researcher Yuma Takida of the Tera-Photonics Research Team at the RIKEN Center for Advanced Photonics announced the successful development of a palm-sized, high-intensity, frequency-tunable terahertz-wave light source with a peak output power of greater than 10 Wand a weight of 500 g or smaller. It is expected that this will allow implementation of portable, high-resolution terahertz-wave nondestructive testing. The results were presented at "IRMMW-THz 2024," the largest international scientific conference in the field of infrared, millimeter, and terahertz waves, held on September 1-6 in Perth, Australia.

Because terahertz waves have moderate material permeability and good spatial resolution, they are attracting attention as a sensing technology that enables nondestructive, noncontact measurement of internal structures such as paint films. The existing generation methods in this regard that use pulsed lasers as excitation light sources satisfy both high-intensity characteristics and frequency tunability. However, the pulsed-laser systems are large and require precise optical adjustments, making them difficult to miniaturize.

In response, the research group has considered application of the principle of backward terahertz-wave parametric oscillation, a method of optical wavelength conversion discovered in 2017 that does not require optical elements such as resonator mirrors. They have developed a palm-sized terahertz-wave light source that contains the minimum necessary optical system in a housing. The optical system includes a compact microchip pulsed laser as an excitation source and a periodic polarization inversion crystal as a nonlinear optical crystal.

The developed terahertz-wave light source exhibits the same size as a smartphone (13.9 cm in length, 5.5 cm in width, and 3.7 cm in height) and weighs 453 g, making it easily portable and easy to operate by connecting cables, including optical fibers. The light source is equipped with an automatic control mechanism that rotates the periodic polarization inversion crystal with respect to the optical axis of the excitation light. Therefore, the phase-matching condition of the backward terahertz-wave parametric oscillation resonator can be continuously changed externally, allowing tuning of the terahertz-wave frequency. The generated terahertz waves can be extracted through a lens placed on the side of the housing.

When the resulting terahertz waves were measured using a fast-response detector, the pulse width was 0.60 ns and the peak output reached a maximum of 15 W, more than one order of magnitude higher than previously possible. As an example of an application to nondestructive testing, the developed light source and reflective imaging optical system were constructed on a small breadboard (30 × 15 cm), mounted on a two-axis automatic moving stage, and performed measurement with the assumption that it would be mounted on a robot. As a result, they were able to clearly visualize the reflected image of the test target pattern hidden behind a fiber cloth towel. High-intensity characteristics, output stability, and practicality were confirmed.

In the future, the team aims to commercialize a portable nondestructive-inspection terahertz-wave device that can be mounted on a drone or self-propelling robot.

Minamide said, "We paved the way for the generation of high-power terahertz waves by converting laser light 13 years ago and succeeded in miniaturization research approximately 5 years ago. Since that time, I have been strongly motivated to create a light source that can be used outside the laboratory and have been conducting vigorous research into this subject. Even if the terahertz-wave-generating part could be miniaturized, to miniaturize the entire device including the laser, overcoming issues such as instability of laser output that accompanies minimization was necessary. Also, it was necessary to develop special optical elements that have never existed before, such as filters that separate laser light and terahertz waves. The developed terahertz-wave light source is very bright, and we expect it to brightly illuminate the future of various nondestructive testing applications."