High-accuracy circular dichroism measurement using a liquid crystal polarization grating

Abstract

A significant improvement in the accuracy of a circular dichroism (CD) measurement system using a liquid crystal polarization grating (LCPG) for broadband applications can be achieved. The influences of the diffraction efficiency and residue from the opposite diffraction order for an LCPG are studied theoretically and experimentally. The results show that the measurement accuracy is highly dependent on the residue. The diffraction efficiency only affects the measurement resolution. We also find that when an LCPG is operated at a wavelength far from the design wavelength, the LCPG has a high residue, which will generate a significant measurement error. Five lasers with different wavelengths ranging from visible to infrared are applied to investigate the accuracy of our proposed system for broadband applications. The measurement error can be reduced by one order of magnitude for wavelengths far from the design wavelength of an LCPG by subtracting the residue effect. The final measurement accuracy of our CD apparatus is determined to be a few mdeg as shown in a comparison of the measurement results for a vitamin B12 solution with those obtained using a commercial CD spectropolarimeter.