Sum Frequency Generation (SFG) is widely used as an interface-selective spectroscopic technique. However, conventional SFG technique can give only an absolute square of χ⁽²⁾ because it measures only the intensity of SFG signal (homodyne detection). We developed heterodyne-detected electronic SFG (HD-ESFG) as well as heterodyne-detected vibrational SFG (HD-VSFG), which enable us to measure complex χ⁽²⁾ spectra. Combining these new probing techniques with femtosecond pump-probe method, we have developed time-resolved ESFG, time-resolved HD-VSFG, two-dimensional HD-VSFG and UV-excited time-resolved HD-VSFG spectroscopies. These novel nonlinear spectroscopies enable us to obtain as rich information as we do for a bulk condensed phase. Furthermore, we are now developing next-generation two-dimensional HD-VSFG spectroscopy which employs an interferometric pumping scheme for improving both time- and frequency resolutions.
Nihonyanagi, Sung (Yamaguchi, Singh, Matsuzaki, Inoue)