Scientists discover simpler, it is better with regard to spectroscopy of the Terahertz temporal domain

A simple adjustment to the usual configuration is all that is necessary to improve a spectroscopy technique that uses waves in the Terahertz region to probe samples, have discovered Riken physicists. The results are published in the journal Applied physics letters.

Developing techniques that can obtain spectra in tiny regions extremely quickly is the ultimate objective of a team to which Norihiko Hayazawa belongs to Riken Center for Advanced Photonics.

Until recently, scientists focused on obtaining spectra from regions on a nanometric scale on samples. But now, they focus on acquiring specters very quickly – in the order of billions of seconds (nanoseconds) – to minimize the fluctuations induced by the ambient environment.

To achieve this, Hayazawa turned to spectroscopy of the Terahertz temporal domain, which uses short electromagnetic wave pulses which are between microwaves and infrared radiation on the electromagnetic spectrum.

Because the signal in spectroscopy of the Terahertz temporal domain is low, most of the experimental configurations add an external modulation to the signal for locking detection. This allows the signal to be easily distinguished from noise.

Being new in the technique, Hayazawa wondered if this external modulation was necessary because the very short laser pulse train used to create the Terahertz pulses could provide much faster intrinsic modulation.

“I’m not really a Terahertz spectroscopy person,” he said. “As a beginner, I was naively wondering why we do not delete the external moderator? This would simplify the system a lot, in addition it would make spectra much faster.”

The idea worked – provided that there is no movement in the laboratory. But the measure was extremely sensitive to disturbances, so that even the operator’s slightest movement would disrupt the signal.

“It was useless from a practical point of view,” said Hayazawa. “If you have stayed very far from the system, it worked well. But the signal would brush wildly as soon as you get up or have risen.”

At this stage, he came to Hayazawa that he could check to see what was going on to demolish upper harmonics of the locking signal. Because Terahertz pulses were not perfectly smooth sinusoidal impulses, they created signals at higher frequencies.

When Hayazawa checked the upper harmonics, he found that they were practically insensitive to the movement.

“I had this vague intuition according to which the upper harmonics could behave differently,” recalls Hayazawa. “But I have always been really surprised when we checked the data and since they were so stable.”

The new scheme offers multiple advantages compared to that conventional. “It’s very fast and stable,” he said. “And because we no longer need an external modulator, the system is much simpler.”

Hayazawa wishes to disseminate the news of its advantages to the research community. A locking manufacturer expressed interest in developing instruments according to it.