Evidence of a circularly polarized light mode along the optic axis in c-cut NH4H2PO4, induced by circular differential reflection and anomalous birefringence.

The anomalous birefringence and circular differential reflection of NH(4)H(2)PO(4) (point group ̅42m), cut on the optic axis, have been found to cause an additional signal in measurements of the optical rotation employing polarized light technology, with the sample between crossed and slightly modulated linear polarizers (tilting high accuracy universal polarimetry). The azimuthal rotation of the linearly polarized light, up to 100 times larger than expected, is described in terms of a circularly polarized light mode along the optic axis of varying amplitude. Experimental evidence leading to our conclusion is given and a qualitative model for the effect is presented.