Formation of periodic microstructures on multilayer dielectric gratings prior to total ablation T. Z. Kosc, A. A. Kozlov, and A. W. Schmid Laboratory for Laser Energetics, University of Rochester, 250 East River Road, Rochester, NY 14623-1299
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Abstract: The damage morphology produced by high-power, short-pulse lasers on multilayer dielectric (MLD) gratings has been closely examined. An unusual ripple formation arises under specific laser-fluence conditions and produces a bright diffractive effect. A single irradiation does not produce this morphology, proving that it is a cumulative effect requiring multiple laser shots on a test site. The period of this microstructure is found to be between 2.0 and 2.4 μm. The ripple orientation varies across the test site. Varying several experimental conditions such as pulse length, beam polarization and angle of incidence still produces this periodic microstructure, though not always efficiently. This morphology is not seen on MLD stacks or other homogeneous samples. © 2006 Optical Society of America OCIS Codes: (350.2770) Gratings; (350.2250) Femtosecond phenomena; (350.1820) Damage
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(C) 2006 OSA
Received 20 June 2006; revised 21 September 2006; accepted 22 September 2006
30 October 2006 / Vol. 14, No. 22 / OPTICS EXPRESS 10921
1. Introduction Laser-damage testing is a familiar area for the chirped-pulse-amplification, high-power laser community, particularly those working on inertial confinement fusion. As the community strives to build petawatt-class lasers, new innovations must be made, including the fabrication of large-aperture, high-damage-threshold gratings used for pulse compression. Metallic gratings were initially used [1], but the development of gratings etched into multilayer dielectric (MLD) stacks has improved grating damage thresholds [2]. The damage morphology caused by high-power, short-pulse lasers has been well investigated, often at 800 nm and pulse lengths varying from tens of femtoseconds to nanoseconds, for various homogenous materials [3–5] and even multilayer dielectric stacks [6], but little work has been done with gratings [7]. In this letter, we report the observation of a ripple structure arising on test sites irradiated with short pulses (0.7 to 10 ps) at 1053 nm. The ripples appear as a secondary, overlapping grating and produce a strong dispersion effect with a colorful glint visible to the bare eye when viewed in white light. Ripple formations were first observed in semiconductor materials irradiated with a Q-switched ruby laser [8] and, since then, ripples have been observed in many homogeneous dielectrics [4,9] and semiconductors and metals [10] tested under varying conditions. The new grating-like formation on MLD gratings, however, does not correspond to typical conditions in previous experiments. The theories that describe the ripple formation give a direct relation between the laser wavelength and the ripple period [11], which was not observed in the current morphology. Furthermore, this ripple formation phenomenon is observed to form under a number of conditions that included varying the angle of incidence and changing the beam polarization from s- to p-polarization. Finally similar ripples are found to form, albeit weakly, on gold coated gratings. Here we describe the phenomenology of the formation of periodic microstructures on multilayer dielectric gratings prior to total ablation, with the intention of explaining the underlying mechanism in the future. 2. Experimental setup 2.1 Geometry and laser system The experimental setup for damage testing (Fig. 1) involves a commercial chirped-pulse–amplification (CPA) laser system (Positive Light, Inc.) producing a compressed pulse with up to 50 mJ of energy. The front-end of the laser system consists of a Time Bandwidth Products™ GLX-200 diode-pumped, Nd:glass, master oscillator (~200 fs, 6.5 nm of bandwidth at 1053 nm), a four-pass, single-grating stretcher (1740 1/mm groove density, 61° incident angle, ~700-ps output chirped pulse), and a Spitfire Ti:sapphire regenerative amplifier (
