Ultrafast impulsive Raman spectroscopy across the terahertz–fingerprint region

• Published in: Advanced photonics Vol. 4; no. 1; p. 016003
• Main Authors: Peterson, Walker, de Pablo, Julia Gala, Lindley, Matthew, Hiramatsu, Kotaro, Goda, Keisuke
• Format: Journal Article
• Language: English
• Published: Bellingham Society of Photo-Optical Instrumentation Engineers 01.01.2022; S P I E - International Society for
• Subjects: Bandwidths; Broadband; Coherent scattering; Fingerprints; Fourier transforms; Lasers; Molecular structure; Raman spectra; Raman spectroscopy; Sensitivity enhancement; Spectrum analysis; Terahertz frequencies; optics; Raman spectroscopy; spectroscopy; terahertz; photonics
• ISSN: 2577-5421; 2577-5421
• DOI: 10.1117/1.AP.4.1.016003

Broadband Raman spectroscopy (detection bandwidth >1000  cm  −  1) is a valuable and widely used tool for understanding samples via label-free measurements of their molecular vibrations. Two important Raman spectral regions are the chemically specific “fingerprint” (200 to 1800  cm  −  1) and “low-frequency” or “terahertz” (THz) (<200  cm  −  1; <6  THz) regions, which mostly contain intramolecular and intermolecular vibrations, respectively. These two regions are highly complementary; broadband simultaneous measurement of both regions can provide a big picture comprising information about molecular structures and interactions. Although techniques for acquiring broadband Raman spectra covering both regions have been demonstrated, these methods tend to have spectral acquisition rates <10  spectra  /  s, prohibiting high-speed applications, such as Raman imaging or vibrational detection of transient phenomena. Here, we demonstrate a single-laser method for ultrafast (24,000  spectra  /  s) broadband Raman spectroscopy covering both THz and fingerprint regions. This is achieved by simultaneous detection of Sagnac-enhanced impulsive stimulated Raman scattering (SE-ISRS; THz-sensitive) and Fourier-transform coherent anti-Stokes Raman scattering (FT-CARS; fingerprint-sensitive). With dual-detection impulsive vibrational spectroscopy, the SE-ISRS signal shows a >500  ×   enhancement of <6.5  THz sensitivity compared with that of FT-CARS, and the FT-CARS signal shows a >10  ×   enhancement of fingerprint sensitivity above 1000  cm  −  1 compared with that of SE-ISRS.