Transmission and photoluminescence images of three-dimensional memory in vitreous silica

We demonstrate separate readouts of three-dimensional memory by (i) transmission imaging using a conventional optical microscope and (ii) photoluminescence (PL) of the bits created by inducing optical damage within the vitreous silica. Recording was done by tightly focused (objective ×100, numerical aperture 1.3) single shot irradiation of 120 fs duration pulses at a 400 nm wavelength. For the readout, a broadband of PL at 470-600 nm was excited by 400 nm, 120 fs irradiation with pulse energy smaller by a factor of 10⁶ compared with that for the recording. We found an erasing of the PL after 400°C annealing, while the readability of the bits by the transmission was sustained. This shows the potential for two-bit information recording per single bit by means of separate readout procedures.