Rewritable VPHGs based on photochromic materials

Volume phase holographic gratings (VPHGs) are becoming an interesting alternative to the ruled gratings in modern astronomical instrumentation. Photochromic materials with thermal stability are good candidates for the development of holographic optical elements and in particular for VPHGs. VPHGs based on photochromic materials can be written and erased many times without the degradation of the material; moreover the material does not need any developing process after the exposure, making the writing process very simple. We have already studied photochromic materials for this aim in the framework of the JRA6 (Opticon Project, FP6) and we found that large modulations of the refractive index can be achieved using diarylethene polymers. Going from the materials characterization to technology, substrates with larger thickness and good optical properties are required. Herein we present the development of new technique to tailor the thickness of the photochromic films in two ranges: 5 - 30 micron and 700 - 1000 micron. The former are suitable for the development of broadband VPHGs, whereas the latter are suitable for narrow band tunable filters application. Details on the optical properties of the films are reported. Finally, a holographic set-up based on an Ar+ laser has been optimized in order to write the gratings.