THE 'TILTER': A NOVEL POLARIMETER FOR FAST OPTICAL ACTIVITY MEASUREMENTS IN BIREFRINGENT CRYSTAL SECTIONS. A. M. Glazer, W. Kaminsky, Clarendon Laboratory, Department of Physics, University of Oxford, England

Optical activity and other gyrotropic effects, when masked by birefringence, can be determined reliably with the so called 'HAUP' (High Accuracy Universal Polarimeter) method /1/ if further information is provided from modelling of the phase-retardation /2/. In a previous method (WS-HAUP in the Clarendon Laboratory), the phase-retardation is modelled by changing the wavelength with a high-pressure lamp-monochromater arrangement. In the new 'TILTER'- system, this modelling is established by scanning through sections of the crystal by tilting it with respect to the wave vector to select different values of the birefringence /3/.

Using a laser as a light source and varying the angular parameters of the setup, which are related to the polarizers and the sample, it is possible to separate the gyrotropy, birefringence, orientation of the indicatrix and parasitic contributions. The time required could be minimized to about 2 minutes for a complete scan which has to be compared with about 24 hours in the case of the previous 'HAUP'-system in the Clarendon Laboratory.

By scanning across a crystal section and repeating the tilt-scan procedure, it is now possible to produce maps of optical activity within a day's time. First results show larger variation of optical activity in homogeneous samples than expected. However, twinning and domain-structures are easily analysed by the new technique.

/l/ Kobayashi, J., Uesu, Y., J.Appl.Cryst. 16 (1983) 204-211.

/2/ Devarajan, V., Glazer, A. M., Acta Cryst. A42 (1986) 560-569.

/3/ Kaminsky, W., Glazer, A. M., Ferroelectrics ( 1996) in press.