Recording and detection (reading) of 3D structures in glass using powerful femtosecond laser pulses has been investigated experimentally and theoretically for 3D optical memory applications. Recording of memory ‘bits’ was found to involve optical breakdown within bulk fused silica leading to plasma formation and strong laser absorption. The measured threshold for in-bulk breakdown of silica of 6 x 1012 W/cm2 was found to be in a close agreement with the theoretical predictions. As a result of these studies the precise control over the size of the memory bits formed by single and multiple pulses has been achieved.
Reading of the recorded structures was carried out by monitoring white continuum generated in the laser-affected sites subsequently irradiated by femtosecond pulses at much lower laser intensity of 1010 W/cm2 - 1011 W/cm2 (figure 1). The recorded memory ‘bit’ formed by 800 nm 150 fs laser pulses was measured to be 0.2 μm, while the distance between the adjacent bits was 0.2 μm. This leads to a memory density of 10 Tbit/cm3. We extended this approach to material modification by cumulative heating during multiple-pulse interaction with a transparent solid. These studies will find application for 3D optical memory devices and for photonic band-gap crystals.