Low-Frequency Coherent Raman Spectroscopy Using Spectral-Focusing of Chirped Laser Pulses

Vibrational spectroscopy is generally implemented using two schemes; that is, absorption spectroscopy and Raman spectroscopy. Conventionally, low frequency absorption spectroscopy is carried out using Fourier transform spectrometers equipped with a far infrared radiation source and a thermal detector. On the other hand, low frequency Raman spectroscopy is carried out by way of double or triple monochromaters and high-quality notch filters, whose performance determines the low frequency limit of the Raman spectrometer. In addition, in recent years, terahertz time-domain spectroscopy (THz-TDS) (Hangyo et al., 2005), utilizing femtosecond lasers as the excitation source, has been developed. THz-TDS enabled us to obtain absorption and dispersion spectra with a high signal-to-noise ratio in a frequency region less than 3 THz (100 cm-1) and can be applied for absorption spectroscopy of various substances (Kawase et al., 2009; Korter et al., 2006; Taday et al., 2003;Tani et al., 2004; Tani et al., 2010; Walther et al., 2003; Yamamoto et al., 2005; Yamaguchi et al., 2005) and imaging measurements (Kawase, 2004). There is a keen interest in low frequency vibrational spectroscopy for biomolecules since large amplitude, low frequency modes in macro biomolecules are believed to be associated with their respective function, as in the case of proteins (Chou, 1985, 1988). In order to fully understand the dynamics and function mechanisms of biomolecules, it is necessary to study their large amplitude and anharmonic low-frequency vibrational motions as these govern their thermal and physiochemical properties. A normal mode analysis of protein molecules revealed that large amplitude vibrational modes which are delocalized in the whole molecule lie within the THz region (< 120 cm-1) (Brooks & Karplus, 1985; Go et al., 1983). In addition, the calculations suggested that the entropy of the whole molecule, specifically its thermodynamic characteristics, is governed by the large amplitude vibrational modes in the sub-THz region (<30 cm-1). Consequently, important information related to the functions and dynamics of proteins can be derived by investigating THz vibration spectra.