Laser Damage Resistant Ta2O5/HfO2/SiO2 Multilayer Mirrors by Ion Beam Sputtering

Abstract

We show the threshold fluence for damage of Ta 2 O 5 /SiO 2 multilayer interference coatings measured using 0.19 ns and 4 ns pulses with λ=1.03 µm can be increased by 2x when modifying the coating's top layer design. OCIS codes: (310.1620) Interference coatings; (310.4165) Multilayer design; (310.1860) (350.1820) Damage.Ta 2 O 5 is a prime candidate for a high index material in interference coatings given its low intrinsic stress [1], superior layer smoothness and uniformity [2], and low absorption and scattering losses. Its laser damage resistance at nanosecond pulse durations is, however, lower than similar multilayers that employ HfO 2 or Y 2 O 3 as the high index materialWe have already demonstrated increases in the laser damage threshold fluence of a Ta 2 O 5 /SiO 2 high reflection multilayer through replacement of Ta 2 O 5 with HfO 2 in the top three bilayers when tested using 0.35 ns pulses at λ = 1 μm [3]. We have also demonstrated the role of SiO 2 capping layers that reduce the electric field intensity in the structure to improve the laser induced damage resistance from laser pulses of 0.19 ns and 4 ns duration [4]. In this work, the effect of combining design techniques to increase the damage resistance of Ta 2 O 5 /SiO 2 high reflectors is explored and compared with HfO 2 /SiO 2 and Ta 2 O 5 /SiO 2 quarter-wave designs.Interference coatings based on Ta 2 O 5 /SiO 2 were designed for normal incidence and 99.9% reflectivity at λ = 1μm. The coatings were deposited by ion beam sputtering (IBS) using a metal target for the reactive sputtering of Ta 2 O 5 and a dielectric target for the deposition of SiO 2 . Further information on the deposition process has been presented elsewhere [3]. The interference coatings were deposited on one inch diameter, 0.25 inch thick fused silica substrates with an RMS roughness of 0.6 nm. No post annealing or ion assisting during growth was performed.A quarter-wave stack consisting of 15 pairs of Ta 2 O 5 /SiO 2 layers, (Ta 2 O 5 / SiO 2 ) 15 , was modified to increase laser damage resistance. Each design involves adding an extra λ/4 layer of SiO 2 to the top of the coating, which reduces the overall E-field due to enhanced constructive interference from reflections off the top air-coating interface and successive interfaces. In addition, one of these designs incorporates HfO 2 by replacing Ta 2 O 5 top layers. A final modified design incorporates both of these techniques while reducing the thickness of the top four HfO 2 quarter wave layers to shift the peak of the electric field into thicker SiO 2 layers as proposed by Apfel et al [5].The 100-on-1 laser damage testing was performed using pulses from a Yb:YAG chirped pulse amplification laser system with a 1/e 2 focal spot size of 90 μm [6]. 300 fs pulses from a Yb:KYW oscillator are stretched to .19 ns and sent through a 100 Hz rep rate Yb:YAG regenerative amplifier. Further amplification to tens of millijoules is performed in a cryogenically cooled "thick disk" Yb:YAG amplifier operating in an active mirror configuration. ...