New chiral vibrational SFG spectroscopic methods using variable retarders

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A spectrometer for chiral vibrational SFG measurements have been constructed with a liquid-crystal retarder plate. A new scheme to modulate the polarization state of the visible probe by the retarder is employed in the system; the visible probe light is modulated between linearly and circularly polarized states. As a result, interference signal between chiral and achiral terms can be detected even when their phase are different by π/2. In such a case, the conventional modulation between two linearly polarization states, which are perpendicular to each other, does not give a detectable chiral signal. It has been confirmed that the new modulation scheme worked properly by measuring the chiral vibrational SFG signals from chiral aggregates of TSPP porphyrin using the constructed apparatus.