Scientists discover simpler is better when it comes to terahertz time-domain spectroscopy

A simple tweak to the usual setup is all that is needed to enhance a spectroscopy technique that uses waves in the terahertz region to probe samples, RIKEN physicists have discovered. The findings are published in the journal Applied Physics Letters.

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

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

To achieve that, 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.

Being new to the technique, Hayazawa wondered whether this external modulation was necessary since the train of very short laser pulses used to create the terahertz pulses could provide much faster, intrinsic modulation.

“I’m not really a terahertz spectroscopy person,” he says. “As a beginner, I naively 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.

“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.”

At that point, 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 Hayazawa checked the higher harmonics, he found they were virtually insensitive to movement.

“I had this vague hunch that the higher harmonics might behave differently,” recalls Hayazawa. “But I was still really surprised when we checked the data and saw they were so stable.”

The new scheme offers multiple advantages over the conventional one. “It’s very fast and stable,” he says. “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 it.