No 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 Si 4÷ ion. X cannot be identified with the well-known aluminium impurity; it is tentatively proposed to be an iron ion (as in amethyst).