Spinning Quarter-Wave Plate Polarization Modulator: Test of Feasibility for Vibrational Circular Dichroism Measurements

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Abstract
A novel polarization modulator design based on a rotating quarter-wave plate and preliminary results of its application for vibrational circular dichroism (VCD) are presented. The device permits quarter-wave retardation in the infrared with alternating senses so that the resultant components of circular polarization can be modulated at frequencies on the order of 100 Hz. We have been able to apply this device to measure VCD with a step-scan FT-IR spectrometer by incorporating a stressed ZnSe optical element as the rotating quarter-wave plate. VCD of α-pinene and camphor were obtained. While these test spectra were of low signal-to-noise ratio (S/N), they exhibited the correct VCD spectral features for these chiral molecules. While not yet of competitive, practical utility, this design is potentially adaptable to extension into the far-IR with alternative optical elements, permits variable-frequency polarization modulation, and should be capable of improved S/N with modifications to increase rotation frequency.