New developments in photochromic materials for volume phase holographic gratings

Volume phase holographic gratings (VPHGs) are becoming widespread dispersing elements in the modern spectrograph. Different materials can be used to make such gratings. We developed photochromic polymers based on a diarylethene unit that make VPHG suitable for observations in the near infrared region (1 - 2 μm) where the materials are highly transparent and show large modulation of the refractive index. This modulation was measured on films of different polymers by using spectral reflectance obtaining values of the order of 10 -2 , These values are large enough to make efficient VPHGs if the thickness of the films are about 25 μm. The sensitivity of the photochromic polyesters was measured at 514 and 633 nm. Prototype of photochromic VPHGs were made by transferring a 600 l/mm pattern of a Ronchi ruling glass slide on a photochromic polyester film (4 μm thick) by using a green laser (543 nm, 30 mW). The grating was successfully transferred as shown by optical microscopy and by the diffraction pattern induced by a white light.