Japan’s RIKEN simplifies and enhances THz time-domain spectroscopy...

16 Apr 2025

...and Hamamatsu mass-produces “world’s first” terahertz wave detection modules.

A simple tweak to the usual setup is all that is needed to enhance a spectroscopy technique that uses terahertz wavelengths to probe samples, physicists at RIKEN University, Wakō, Japan, have discovered.

Developing techniques that can obtain spectra from tiny regions extremely rapidly is the ultimate goal of a team that Norihiko Hayazawa leads in the RIKEN Center for Advanced Photonics.

Until recently, the scientists had been focusing on obtaining spectra from nanoscale regions on samples. But now they are concentrating on acquiring spectra rapidly — in the order of nanoseconds — to minimize fluctuations induced by the ambient environment.

To achieve this, Hayazawa turned to terahertz time-domain spectroscopy, which uses short pulses of electromagnetic waves that lie between microwaves and infrared radiation on the electromagnetic spectrum.

Because the signal in terahertz time-domain spectroscopy is weak, most experimental setups add external modulation to the signal for lock-in detection. This allows the signal to be easily distinguished from noise. Hayazawa wondered whether this external modulation was necessary since the train of short laser pulses used to create the terahertz pulses could provide faster, instrinsic modulation.

“I’m not really a terahertz spectroscopy person,” he said. “I wondered why don’t we remove the external moderator? That would simplify the system a lot, plus it would make it much faster to acquire spectra.” The idea worked—provided there was no movement in the lab. But the measurement was extremely sensitive to disturbances, so that even the slightest movement of the operator would disrupt the signal.

Higher harmonics

“It was useless from a practical perspective,” says Hayazawa. “If you stayed very still far away from the system, it worked fine. But the signal would fluctuate wildly as soon as you stood up or moved around.” Then it occurred to Hayazawa that he could check to see what was happening to the higher harmonics demodulations of the lock-in signal. Because the terahertz pulses were not perfectly smooth sinusoidal pulses, they created signals at higher frequencies.

When he checked the higher harmonics, he found they were virtually insensitive to movement. “I had this hunch that the higher harmonics might behave differently,” he said. “But I was still surprised when we checked the data and saw they were so stable.”

The new scheme offers multiple advantages over the conventional one. “It’s fast and stable,” he said. “And because we don’t need an external modulator anymore, the system is much simpler.” Hayazawa is keen to spread the news about its benefits to the research community. A lock-in manufacturer has expressed interest in developing instruments based on the work.

Hamamatsu mass-produces ‘world’s first’ THz wave detection module

Hamamatsu Photonics has mass-produced what it calls the world’s first terahertz wave detection module by utilizing its newly-developed metasurface technology and established photomultiplier tube and image intensifier technologies.

The Japan-based company is offering two products: THz photomultiplier tube (THz PMT) modules, and THz image intensifiers (THz I.I.), which can detect THz wave pulse signals at high speed and with high sensitivity at room temperature.

The company’s announcement states, “Both products are the world’s first modules to apply field electron emission technology to THz wave detection. They have unique properties that are not found anywhere else. These products are expected to further accelerate basic and applied research on THz waves for applications in drug discovery, analysis, semiconductors, non-destructive testing, and more.

How the devices work

Both the THz PMT module and the THz I.I. are THz wave detection modules that utilize metasurfaces for THz wave-to-electron conversion and have sensitivity in the THz wave range. By replacing the photocathode, which converts light into electrons, with a metasurface based on the principle of field electron emission, Hamamatsu says it has succeeded in detecting THz waves. Conventional photocathodes use the principle of the external photoelectric effect, so the wavelength range of light that can be detected is limited to ultraviolet to near-infrared light. This makes it difficult to detect THz waves, as they have longer wavelengths and lower photon energy.

In collaboration with the Technical University of Denmark, Hamamatsu conducted joint research on metasurfaces as a field electron emission technology that utilizes tiny antennas to resonate with THz waves. This enabled the company to establish deposition technology for the metasurface functional layer, combining its core alkali metal deposition technology with the new metasurface technology, specifically the THz wave-electron conversion section.

The development and mass production of a new type of THz wave detection module is expected to accelerate basic and applied research into THz waves for applications in drug discovery, analysis, semiconductors, and nondestructive testing. The company is now accepting orders for both products.