Photochromism induced by infrared two-photon absorption.

An IR laser of 940 nm wavelength induced photochromic reaction in acrylate that contained both spirobenzopyran and rare-earth doped oxide (Gd(2)O(2)S:YbEr). The rare-earth elements were excited by two 940 nm photons and emitted a 550 nm photon, which caused photochromic isomerization of spirobenzopyran. This acrylate turned to its original orange color by either thermal relaxation or ultraviolet irradiation, and was bleached again by IR laser irradiation.