The invention relates to materials and devices including these materials having non-erasable optical data capability, as well as to methods of storing optical data and to apparatus for carrying out these functions. In particular there is provided a method of storage of non-erasable optical data comprising exposing data storage material of a three-dimensional optical data storage device to focussed electromagnetic radiation wherein the radiation is of a wavelength and power appropriate to generate micro-cavity formation within the data storage material and wherein the location of micro-cavities encodes for stored data; the data storage material comprising a polymer matrix and a photosensitive agent dispersed through the polymer matrix.
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