Circular Dichroism Spectroscopy by Four-Wave Mixing Using Polarization Grating-Induced Thermal Gratings

A novel four-wave mixing technique for the measurement of circular dichroism in optically active liquid samples is demonstrated. When two cross-polarized continuous-wave laser beams are crossed at a small angle in a circular dichroic liquid, a weak thermal grating is produced with a phase depending on the sign of the circular dichroism. The polarization of one of the beams can be modified to allow coherent interference with an intensity grating-induced thermal grating. A probe beam scattering from the composite grating results in a coherent signal beam that reveals the sign and the magnitude of analyte circular dichroism. The use of this technique to optimize the signal-to-noise ratio in the presence of scattered light and laser noise is discussed.