Technique for measuring refractive index changes in photochromic materials

We describe a new technique for measuring the refractive index change associated with the photoinduced absorption change in photochromic materials. The technique involves writing a thick holographic grating in a sample of the material and observing the interaction of the grating with the beams used to write it. Only three relative intensity measurements are required, and many of the difficulties of previous techniques are avoided. We have demonstrated the technique in experiments on a photodimer material. These experiments also provide an example of a modulated transmission effect related to the Borrmann effect.