Two-photon readout of three-dimensional memory in silica

We report the readout of three-dimensional (3D) optical memory in silica by detecting the photoluminescence (PL) of the bits (voxels). A broad defect-related PL band at 400-700 nm was excited by two-photon absorption of femtosecond (pulse duration of 120 fs) illumination at 795 nm. We employed a simple reflection-type scanning readout without the use of a confocal detection scheme to read 3D memory by recording the PL of the bits (the same objective lens was used for the excitation and the collection of the PL). Bit plane separation as small as 3 μm was resolved without cross talk, when the theoretical limit of the axial resolution evaluated as a full-width at half maximum measure of a bit size was 1.4 μm at the fabrication conditions employed.