Development of single-channel heterodyne-detected sum frequency generation spectroscopy and its application to the water/vapor interface.

Single-channel heterodyne-detected sum frequency generation (HD-SFG) spectroscopy for selectively measuring vibrational spectra of liquid interfaces is presented. This new methodology is based on optical interference between sum frequency signal light from a sample interface and phase-controlled local oscillator light. In single-channel HD-SFG, interferometric and spectrometric measurements are simultaneously carried out with an input IR laser scanned in a certain wavenumber range, which results in a less task than existing phase-sensitive sum frequency spectroscopy. The real and imaginary parts of second-order nonlinear optical susceptibility (χ((2))) of interfaces are separately obtained with spectral resolution as high as 4 cm(-1) that is approximately six times better than existing multiplex HD-SFG. In this paper, the experimental procedure and theoretical background of single-channel HD-SFG are explicated, and its application to the water/vapor interface is demonstrated, putting emphasis on the importance of a standard for the complex phase of χ((2)).