Crosstalk in Photoluminescence Readout of Three-Dimensional Memory in Vitreous Silica by One- and Two-Photon Excitation

We report the readout of three-dimensional (3-D) memory of bits from their photoluminescence (PL). The bits were recorded inside vitreous silica by optical damage (micro-explosion), which was induced by 150 fs single pulse irradiation at the focal point. The bit can be written by 0.2–0.5 µJ energy pulses at 400 nm or 800 nm (the pulse energy was given at the irradiation point and had an intensity of about 1015 W/cm2 when a high numerical aperture of NA = 1.3 objective lens was used for recording). The readout was obtained by the PL of the 3-D pattern of damaged bits, which were excited by quasi-continuos (repetition rate of 82 MHz) fs-irradiation at much lower pulse intensity (by a factor of 106) in order to exclude secondary damage. The readout was performed by the oil-immersion objective lens of NA = 1.3. The wavelength of PL excitation was 400 nm for one-photon and 800 nm for two-photon excitation of PL with the single pulse energy of less than 1 pJ. We found that the crosstalk of PL readout is the same as that for the transmission imaging and measures 4 µm in the inter-plane distance for the case of one-photon excitation and can be reduced to 3 µm when a reflection-type readout system is used together with two-photon excitation (theoretical axial bit size for fabrication of a given optics was 1.4 µm). Two-photon readout of 3-D memory in silica is demonstrated for the first time.