Scientists discover better when it comes to time domain spectroscopy of terahertz

A simple adjustment to a normal setup is all that is needed to enhance spectroscopic techniques using waves in the terahertz region for samples, Riken physicists discovered. Findings can be found in the Journal Applied Physics Letters.

Developing technology that allows for spectra acquisition from small regions is the ultimate goal of Hayazao Yuko’s team at Riken Center at Riken Advanced Photonics.

Until recently, scientists focused on obtaining spectra from nanoscale regions on samples. But now we are focusing on acquiring the spectrum very quickly in the billions of seconds (nanoseconds) to minimize the fluctuations caused by the environment.

To achieve that, hayazawa uses a short pulse of electromagnetic waves between the electron waves and infrared radiation on the electromagnetic spectrum.

Because Terahertz time-domain spectroscopy signals are weak, most experimental setups add external modulation to the signal for lock-in detection. This makes it easy to distinguish the signal from noise.

Unfamiliar with this technique, Hayazawa wondered whether this external modulation was necessary, as the very short train of laser pulses used to create terahertz pulses can provide much faster and intrinsic modulation.

“I’m not really a terahertz spectroscopy,” he says. “As a beginner, I’ll be much faster to get the spectrum because I don’t remove external moderators, why not simplify the system.”

This idea worked. There was no movement in the lab. However, the measurements were very sensitive to turbulence, so even the slightest movement of the operator disrupts the signal.

“It was useless from a practical point of view,” says Hua. “If you keep it very far from the system, it worked. But as soon as you get up or move around, the signal fluctuates wildly.”

At that point, it happened that Hayazawa was able to see what was happening with the revival of the higher harmonics of the lock-in signal. The terahertz pulse was not a completely smooth sine wave pulse, so we created the signal at a higher frequency.

When Yatani checked the higher harmonics, he realized they were practically insensitive to movement.

“I had this vague premonition, so the higher harmonics might behave differently,” recalls Hua. “But when I checked the data and saw them being so stable, I was still really surprised.”

The new scheme offers multiple advantages over the traditional ones. “It’s very fast and stable,” he says. “And the system is much easier because you don’t need an external modulator anymore.”

Hayashi is keen to spread news about its benefits to the research community. Lock-in makers have expressed interest in developing equipment based on it.