Three-dimensional optical memory

Apparatus dimensional memory for storing information in a volume comprising an active medium, which is capable of switching between a first and a second isomeric form in response to radiation of a light beam having an energy substantially equal to a first energy threshold, wherein the concentration ratio between a first and a second isomeric form in a given volume portion represents a data unit; said memory apparatus being characterized in that said active medium comprises diarylalkene derivatives of the general formula Ar1R1C = CAr2R2, wherein Ar1 and Ar2, which may be the same or different, are aryl substituted or unsubstituted independently, wherein the substituted groups have a strong absorption in the IR region and; R1 and R2, which may be the same or different, are groups having strong absorption in the IR region.