Measurement of circular dichroism of ferroelectric fresnoite Ba ₂Si₂TiO₈

The urgent goal of the optical polarimetry of solids is simultaneous and accurate measurements of circular dichroism together with circular birefringence. Needless to say, measurements of circular phenomena are extremely difficult compared with those of linear ones. As for circular birefringence, 170 years elapsed since its discovery by Arago until the development of the HAUP method, by which the accurate measurements of the gyration tensor components of a solid became possible for the first time. Subsequent to appearance of the HAUP method, attempts to extend the HAUP theory to being applicable for measurements of circular dichroism of crystals were followed by several authors. However any applications to real crystal were not fully successful. We completed fresh the theory of the extended HAUP. This paper reports the extended HAUP measurement of both circular birefringence and dichroism of fresnoite Ba₂Si₂TiO₈. It is clearly found that the circular dichroism abruptly appears at the transition temperature, where the crystal changes from the high temperature 4mm phase to the low temperature mm2 phase. It increases with decrease of temperature together with circular birefringence. This is the first proof that the HAUP method provides with sufficiently accurate data of circular dichroism of crystals. It is important to note that the circular dichroism affects only θ₀, which is one of the salient parameter characterizing the HAUP method. It means that the HAUP is an exclusive method for measuring circular dichroism of crystals.