Selected domains, normally 2.times.10.sup.3.times.2.times.10.sup.3 such domains arrayed in a plane, within a three-dimensional (3-D) volume of radiation-sensitive medium, typically 1 cm.sup.3 of spirobenzopyran containing 2.times.10.sup.3 such planes, are temporally and spatially simultaneously illuminated by two radiation pulses, normally laser light pulses in various combinations of wavelengths 532 nm and 1024 nm, in order, dependent upon the particular combination of illuminating light, to either write binary data to, or read binary data from, the selected domains by process of two-photon (2-P) interaction/absorption. One laser light pulse is preferably directed to illuminate all domains during its propagation along one directional axis of the volume. The other laser light pulse is first spatially encoded with binary information by 2-D spatial light modulator, and is then (i) directed and (ii) time sequenced to intersection with the other light pulse in a locus of intersection domains. During writing the selected, simultaneously illuminated, intersection domains change their index of refraction, attendant upon a change in isomeric molecular form, by process of 2-P absorption. During reading selected intersection domains selectively refract each of two read light pulses identically--as well as fluoresce--dependent upon their individually pre-established, written, states. The selective refraction of each read pulse in its passage straight through the volume is sensed in a detector array. I/O bandwidth to each cm.sup.3 of radiation-sensitive medium is on the order of 1 Gbit/sec to 1 Tbit/sec.