Critical behavior of birefringence in the smectic-A phase of chiral smectic liquid crystals

Using a high resolution polarimeter, we have measured the temperature depen dence of birefringence in the vicinity of smectic-A-smectic-C*, smectic-A-s mectic-C-alpha*, and smectic-A-smectic-C-A* phase transitions of chiral sme ctics. We have observed a large pretransitional decrease of the birefringen ce in materials exhibiting smectic-C-alpha* and smectic-C-A* phases, indica ting unusually large pretransitional fluctuations of the tilt angle. These fluctuations show a clear power-law behavior over more than two decades of temperature.