Abstract: The invention concerns a optical data memory, said memory comprising at least one layer of supporting material, said supporting material including molecules having, in a local zone, a collective state of molecules from at least one first collective state of molecules, and a second collective state of molecules. The invention is characterized in that only molecules having the first collective state of molecules in said local zone are capable of generating a second-harmonic signal when they are excited in said local zone by a reading electromagnetic radiation.