Dual-wavelength polarimeter application in investigations of the optical activity of a langasite crystal.

A method of high-accuracy polarimetry, which includes optical activity measurements' systematic errors, was realized with a dual-wavelength polarimeter for the two wavelengths of 635 and 650 nm. Simultaneous measurements with neighboring wavelengths significantly improved the data processing by increasing the amount of obtained data to eliminate the systematic errors. For a langasite crystal, LaGaSiO, we measured the temperature dependence of the gyration tensor component g. Our acquired value does not exceed 0.47×10 and is much smaller than the previous results obtained by different experimental methods. The results presented in this paper are consistent with the calculated optical rotatory power from the crystal structure data and the polarizabilities of the atoms.