Stretching Light for Faster IR Spectroscopy

By stretching light, researchers in Japan have reportedly developed the world’s fastest infrared (IR) spectrometer—some 100 times faster, they say, than possible with state-of-the-art technology (Commun. Phys., doi: 10.1038/s42005-020-00420-3).

Their proof-of-concept instrument, the researchers report, could lead to advances in a variety of fields that have long relied on IR spectroscopy to investigate molecular systems.

The signal-to-noise dilemma

IR spectroscopy detects and analyzes light bounced off molecules whose inherent vibrations yield chemical fingerprints in the form of spectra. Now, by stretching a short pulse of mid-infrared (MIR) laser light as it is transmitted from a sample, says co-author Takuro Ideguchi at University of Tokyo, Japan, researchers can more rapidly detect and analyze spectra. With this technique, he says, a femtosecond pulse of laser light can be stretched to the nanosecond realm, enabling detection and analysis of as many as 80 million spectra per second.

That rate surpasses the one-million-spectra-per-second rates available today with the technique of dual-comb spectroscopy, team members, including researchers from Hamamatsu Photonics and PRESTO, the Japan Science and Technology Agency, say. There is limited room for improvement in these systems, they add, because the underlying problem is a too-low signal-to-noise ratio (SNR)—not instrument scan rates.

“To improve the measurement speed, one must use another technique that could provide a higher SNR,” the team writes in the paper. Time-stretch spectroscopy—also known as dispersive Fourier-transform spectroscopy—was an option that came to mind, Ideguchi says; however, “it was not demonstrated in the MIR but only in the near IR for high-quality telecommunications optics.”

Laser, stretcher, detector

To achieve time-stretch infrared spectroscopy [TS-IR], says Ideguchi, “we had to prepare three components: an MIR high-repetition-rate femtosecond laser, an MIR time stretcher, and an MIR ultrafast detector.” In fact, he notes, the most difficult aspect of the work was finding and putting together those key components of the system.

For the first component, the team used a femtosecond laser to generate an MIR pulse. The laser light then passed through a sample—phenylacetylene—and was time-stretched in a free-space chirp-enhanced delay (FACED) system. This system consists of a diffraction grating, a pair of concave mirrors and a pair of flat mirrors to do the time stretch.

From there, the light was picked up and analyzed in a quantum cascade detector (QDC), a type of photodetector. The team decided to use a QCD as a detector because of its bandwidth to measure the time stretch, Ideguchi says.

Before QCDs, the bandwidth of the fastest, commercially available MIR detector had been about 1 GHz—which Ideguchi says is “not high enough for time-stretch spectroscopy.” The QCD, on the other hand, “has bandwidth of about 5 GHz and possibly higher—it was perfect for TS-IR.”

Improving the system

For the light source, the team used a bulky, expensive femtosecond optical parametric oscillator pumped with a Ti:Sapphire laser. In the future, says Ideguchi, the team hopes to find a more suitable laser system for TS-IR, especially considering that MIR laser technology is advancing rapidly.

“There are many aspects we want to improve in the system,” Ideguchi says. “For example, we want to broaden the spectral bandwidth so that we can compete with the standard MIR spectrometers such as FTIR. Also, we want to go for applications such as measuring structural change dynamics of proteins. We need to develop a system working at a specific wavelength for such an application.”

Measuring nonrepeating phenomena

While conventional IR spectroscopy, over many decades, has brought tremendous progress in science, the researchers write, it is limited to observations of repetitively reproducible phenomena. They hope their time-stretch system will move the ball toward a new frontier of high-speed continuous measurement of nonrepeating phenomena, for example, gaseous combustion or measuring structural change dynamics of photoreceptive proteins like rhodopsin. Other possibilities could include high-throughput measurements such as liquid biopsy of human blood or flow cytometry for single-cell analysis and sorting.

According to Ideguchi, the team is interested in working with chemists and biologists on these and other potential applications. Because TS-IR has so much potential, he adds, “we will continue to improve the system and demonstrate applications which can be made only with this technique.”