Developing ultrafast THz–fingerprint Raman spectroscopy

Japanese scientists have demonstrated a new Raman spectroscopy technique, dual-detection impulsive vibrational spectroscopy (DIVS), with an ultrafast real-time spectral acquisition rate. According to the team from the University of Tokyo, DIVS could be suitable for applications that demand higher spectral acquisition rates, such as video-rate vibrational imaging or detecting rapid transient phenomena.

Raman spectroscopy is a powerful tool for chemical measurement that obtains chemical specificity by directly probing molecular vibrations without the need for chemical labels. As a result, it has become an indispensable tool in a diverse range of fields, including materials science, biology, pharmaceuticals and food science.

Broadband Raman vibrational spectra are commonly segmented into three distinct spectral regions: terahertz (THz) or low-frequency (<200 cm^-1; <6 THz); fingerprint (200 to 1,800 cm^-1); and high-frequency (2,400 to 4,000 cm^-1). The fingerprint region derives its name from its target-specific intramolecular bond vibrations and the THz region can provide chemical structural information via intermolecular vibrations. The complementary nature of these two regions makes their marriage a powerful tool for chemical analysis.

Despite the rich information provided by broadband THz–fingerprint Raman spectra, existing methods to obtain them typically have low real-time spectral acquisition rates (generally, <10 spectra/sec), which manifests as coarse experimental temporal resolution. Although several research groups have pushed broadband Raman spectral acquisition rates to 10,000 to 100,000 spectra/sec, the methods tend to be limited to detection of either the THz or fingerprint regions alone. These limitations pose a barrier to investigating short-lived irreversible phenomena at the molecular bond and structural level.

In a new paper published in Advanced Photonics, researchers at the University of Tokyo overcame these limits by developing a method for broadband THz–fingerprint Raman spectroscopy at an ultrafast spectral rate of 24,000 spectra/sec. DIVS enables synchronous measurement of two distinct types of vibrational signals, which provide dual-region sensitivity when detected in concert. More technically, DIVS blends optical longpass and shortpass filtering with common-path Sagnac interferometry to simultaneously detect frequency-shifted laser pulses (Fourier-transform coherent Raman spectroscopy [FT-CARS], fingerprint-sensitive) and phase delay-shifted pulses (surface-enhanced impulsive stimulated Raman spectroscopy [SE-ISRS], THz-sensitive). Moreover, according to the team, the DIVS setup is straightforward and requires only a single laser.

The researchers performed DIVS proof-of-concept measurements of several transparent liquid compounds over the Raman spectral range of 66 cm^-1 (2.0 THz) to 1,211-cm^-1. The strongest signature vibrational peaks in single Raman power spectra (acquired in <42μs) were demonstrated with high signal-to-noise ratios of >1000. At the current stage, DIVS is recommended for studying high-concentration transparent samples, though the researchers believe modifications of the design could possibly overcome these boundaries.

The researchers concluded that the work holds the potential for interesting real-time broadband THz–fingerprint Raman measurements at sub-millisecond temporal resolutions. They indicated that a promising DIVS applications could be understanding rapid polymerisation systems at the molecular level in polymer science.