Robust Self-Referencing Method for Chiral Sum Frequency Generation Spectroscopy.

Chiral sum frequency generation spectroscopy (SFG) is of great interest for studying biological systems, among others. Whereas the chiral response in circular dichroism is about 0.1% of the achiral response, the chiral SFG response can be the same order of magnitude as the achiral SFG signal. However, chiral SFG is limited by the attainable signal-to-noise of the weak nonlinear signals and therefore extremely sensitive to proper alignment. We present a robust method for chiral SFG and demonstrate the use on solid-air surfaces with achiral and chiral molecules. We simultaneously measure two orthogonal polarizations-either the interference chiral SFG (±45° polarized) or the pure chiral and achiral SFG-using a waveplate and beam displacer. Both optics are placed in the detection arm and can be easily incorporated into any SFG setup. Furthermore, we employ self-referencing to calibrate alignment for each sample individually using a polarizer in the detection arm. These methods greatly increase the reliability and quality of chiral SFG measurements.