Electromodulated absorption in smoky quartz

Abstract

The optical absorption coefficient of “smoky” quartz (containing aluminium) can be modulated by applying an electric field. The effect saturates at high fields and low temperatures and reaches a maximum at 535 nm. The results are discussed in terms of a model consisting of a colour centre, dipolar in character and exhibiting anisotropic absorption. Realignment of the dipoles induces changes in the observed absorption. The experiments suggest that the centre is an XO4 complex with a hole trapped at one of the oxygens, and that X is an ion substituted for a Si4+ ion. X cannot be identified with the well-known aluminium impurity; it is tentatively proposed to be an iron ion (as in amethyst).