Femtosecond Laser Structuring of As2S3 Glass for Erasable and Permanent Optical Memory
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Femtosecond Laser Structuring of As2S3 Glass for Erasable and Permanent Optical Memory Saulius Juodkazis1, Andrei V. Rode2, Toshiaki Kondo1, Hiroaki Misawa1, Marek Samoc2, and Barry Luther-Davies2 1 Hokkaido University, Sapporo, 001-0021, Japan 2 The Australian National University, Canberra, 0200, Australia
ABSTRACT The nonlinear absorption coefficient of As2S3 glass has been measured to be 2.0 cm/GW for femtosecond pulses at 800 nm. Femtosecond laser structuring via two photon absorption in bulk As2S3 glass by erasable and permanent photo-darkening is demonstrated using both holographic and direct multi-beam laser writing. INTRODUCTION Chalcogenide glasses are nonlinear optical materials with considerable potential for alloptical switching at the IR telecommunications wavelengths [1-4]. A general characteristic of chalcogenides is their susceptibility to photo-darkening which can result in a refractive index change of ∆n ~10-2 that can be used for creating thermally re-writable optical memories and for applications in micro-photonics. This work explores the use of nonlinear absorption of femtosecond laser pulses to achieve permanent/erasable 3D optical data storage [5,6] in a photosensitive chalcogenide glass- a process which has the potential to significantly boost memory storage capacity. EXPERIMENT Ultrafast (sub-ps) 800-nm laser pulses were focused into a block of transparent As2S3 glass to record a three-dimensional (3D) pattern via photo-darkening. The formation of “memory bits” involves two-photon absorption and can be achieved using relatively low energy laser pulses. This provides the opportunity for fast parallel writing of multi-bit patterns using a single laser pulse. Commercial As2S3 glass (Amorphous Materials) with a melting temperature of 310°C was used in this study. Amplified femtosecond laser pulses (wavelength 800nm, pulse duration 150fs) were obtained from a Spectra Physics Hurricane laser operating at 1 kHz repetition rate. Holographic recording was realized by 4- and 5-beam interference using a diffractive optical element (DOE) (see, ref. [7, 8] for details) and focusing with a NA = 0.75 objective lens.
Figure 1. Schematic setup based on diffractive optical element (DOE) for holographic and direct multi-beam recording. For direct laser writing, the holographic setup was modified by simply adding a second lens (Fig. 1). In this geometry a DOE generating 31 beamlets (G1022A) in a single line was utilized and the sample was translated normal to the line there by recording 31 lines in one scan. The scan speed was 0.1 mm/s, which corresponded to 20 nm between successive pulses. The whole sample was then translated laterally by half the pattern width and re-exposed. By this process the line exposed to the most off-center beamlet (the weakest) was subsequently overwritten with the strongest 0th beamlet during the second scan resulting in the formation of a uniform pattern over a comparatively large (∼ 1 cm2) area. The nonlinear absorption of As2S3 was measured between 650 nm and 1200
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