A Raman spectroscopy technique, called dual-detection impulsive vibrational spectroscopy (DIVS) by its developers at the University of Tokyo, allows two types of vibrational signals to be measured concurrently. DIVS enables broadband detection over the low-frequency, or terahertz, region and over the fingerprint region of the Raman spectrum at an ultrafast, real-time spectral rate of 24,000 spectra per second. The terahertz and fingerprint spectral regions provide complementary information. Simultaneous measurement of both regions offers a comprehensive measurement comprising information about molecular structures and interactions. Although other techniques for acquiring broadband Raman spectra from both regions have been demonstrated, the spectral acquisition rate for these methods is typically less than 10 spectra per second. Sagnac interferometry and optical filtering are joined to provide dual-region (terahertz-fingerprint) Raman spectral sensitivity at 24,000 spectra per second. The dual-detection impulsive vibrational spectroscopy (DIVS) technique could be used for temporally resolved measurements of pure or high-concentration samples that have significant vibrational information in both the terahertz and fingerprint regions. Courtesy of Walker Peterson, University of Tokyo. DIVS uses ultrashort laser pulses to excite Raman vibrations, and it collects this vibrational information via Raman probe pulses. The coherent molecular vibrations caused by the pump pulses also cause an oscillating electron cloud, which in turn causes an oscillatory refractive index. After a given delay, the refractive index is detected by a subsequent probe. The probe’s phase is affected by the slope of the refractive index when the probe transmits through the sample. Red shift and blue shift effects are detected by Fourier-transform coherent anti-Stokes Raman scattering (FT-CARS) spectroscopy. A third effect — a phase delay shift — is detected by Sagnac-enhanced impulsive stimulated Raman scattering (SE-ISRS). FT-CARS provides fingerprint region sensitivity in the Raman spectrum because it is sensitive to the time derivative of the refractive index. SE-ISRS provides terahertz region sensitivity because it is sensitive to the refractive index itself. FT-CARS provides fingerprint region sensitivity via optical filtering and detection of the probe frequency shift. SE-ISRS provides terahertz region sensitivity via Sagnac interferometry, enabling detection of the probe phase delay shift. By blending optical filtering with common-path Sagnac interferometry, DIVS simultaneously detects frequency-shifted laser pulses with FT-CARS and phase delay-shifted pulses with SE-ISRS, providing dual-region sensitivity across the terahertz-fingerprint regions. The DIVS setup is straightforward and requires just one laser. The researchers measured four liquid solvents with Raman modes covering a range of 66 cm−1 (2.1 THz) to 1211 cm−1 to demonstrate the ability of DIVS to perform broadband measurements of the THz and fingerprint regions simultaneously and rapidly. The researchers quantitatively compared the two signals by plotting the signal-to-noise ratios (SNRs) as a function of the detected Raman mode to show the comparative strengths of the signals. DIVS enables terahertz-fingerprint Raman spectra (gray trace) at an ultrafast, real-time spectral acquisition rate of 24,000 spectra per second. The key novelty of the method is the simultaneous detection of two complementary Raman signals: SE-ISRS (terahertz region-sensitive, blue trace) and FT-CARS (fingerprint region-sensitive, green trace). Courtesy of Peterson et al. The SE-ISRS signal showed a greater than 500× enhancement of SNR below 200 cm−1 (6.5 THz), compared with the FT-CARS signal. The FT-CARS signal showed a greater than 10× enhancement of fingerprint SNR above 1000 cm−1, compared with the SE-ISRS signal. The strongest signature vibrational peaks in single Raman power spectra, acquired in less than 42 microsiemens (μS), exhibited high SNRs of greater than 1000. DIVS could provide a means to achieve broadband terahertz-fingerprint vibrational spectroscopy in fields that require ultrafast spectral acquisition rates, especially for detecting rapid, transient phenomena. Currently, DIVS could be used for temporally resolved measurements of pure or high-concentration samples that have significant vibrational information in both the terahertz and fingerprint regions. In the future, DIVS could be a useful technique for investigating polymers. In addition to signature molecular bond vibrations in the fingerprint region, polymers are known to exhibit rich structural information in the terahertz region. Ultrafast DIVS could be well suited to understanding rapid polymerization systems at the molecular level. The research was published in Advanced Photonics (www.doi.org/10.1117/1.AP.4.1.016003).
