Bulk-or-interface assignment of heterodyne-detected chiral vibrational sum frequency generation signal by its polarization dependence

Polarization dependence of heterodyne-detected chiral vibrational sum frequency generation (VSFG) was examined for thin films of polylactic acids and neat limonene liquid far from electronic resonance. The enantiomers of polylactic acid films on silica substrates were successfully distinguished, and their chiral VSFG signals were ascribed not to bulk but to the interfaces by comparing chiral signals observed in reflection in the S-polarized VSFG, P-polarized visible, and P-polarized infrared and P-polarized VSFG, S-polarized visible, and P-polarized infrared polarization combinations with theoretical model calculations. In the same way, the chiral VSFG signal of neat limonene was assigned to bulk, which is consistent with the previous assignment. The method employed for assigning the source of chiral signals to the bulk or the interface may be useful for organic films on substrates with low refractive indices and thick samples.