All-optical subtractive convolution in photochromic absorbing media

Optical image recognition represents a promising alterna- tive to traditional digital methods. In this paper, we present a fully optical system that performs subtraction convolu- tion, meaning that common features between two light maps, a reference and an unknown to be recognized, are elimi- nated. This is due to the use of a material with induced absorption at the heart of the beam interaction: a thin film of TII photochromic molecules in epoxy resin. The increased absorption in the illuminated regions of the pho- tochromic material causes the optical convolution between two light beams to eliminate the regions with common fea- tures, leaving only the dissimilar features in transmission. Both theoretical and experimental results demonstrate high selectivity and resolution, highlighting the great potential of photochromic materials for reconfigurable optical proces- sors.