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 150fs 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 10¹⁵W/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.