Drew Schiltz

Point defects in Sc 2 O 3 thin films by ion beam sputtering

We show that the concentration of oxygen interstitials trapped in Sc2O3 films by ion beam sputtering from metal targets can be controlled by modifying deposition conditions. We have identified point defects in the form of oxygen interstitials that are present in Sc2O3 films, in significantly high concentrations, i.e., ∼10(18)  cm(-3). These results show a correlation between the increase of oxygen interstitials and the increase in stress and optical absorption in the films. Sc2O3 films with the lowest stress and optical absorption loss at 1 μm wavelength were obtained when using a low oxygen partial pressure and low beam voltage.

Improvements in the laser damage behavior of Ta2O5/SiO2 interference coatings by modification of the top layer design

We have investigated the properties and laser damage behavior of Ta2O5/SiO2 quarter wave stacks designed for λ=1 μm operation by substituting the Ta2O5 layer by either Y2O3 or HfO2 and the SiO2 by Al2O3 in the top 3 pairs of the multilayer stack. The high reflectors were deposited by dual ion beam sputtering. Laser damage at 1 μm using 350 ps showed enhanced performance when the Ta2O5/SiO2 stack had HfO2 or Y2O3 in its top few pairs.

Modification of multilayer mirror top-layer design for increased laser damage resistance

This work focuses on the optimization of a high reflector design for operation at 1 μm wavelength to achieve a high laser damage threshold when tested at pulse durations of 0.19 and 4 ns. Different designs that modify the standing wave electric field distribution of a quarter wave Ta2O5/SiO2 multilayer dielectric coating are considered. It is found that the addition of an extra SiO2 quarter wave to reduce the peak electric field in the coating, increases the 50% damage probability by over 100% at both pulse durations.

Strategies to increase laser damage performance of Ta_2O_5/SiO_2 mirrors by modifications of the top layer design

Ta₂O₅/SiO₂ high reflection (HR) interference coatings for λ∼1  μm offer superior performance at high irradiance conditions. However, these coatings are not good candidates for high peak power conditions in comparison to HfO₂/SiO₂ multilayer stacks. Here we show that the modification of the top layers design of a quarter wave Ta₂O₅/SiO₂ high reflector leads to 4-5 fold increase in the laser damage fluence compared to a quarter wave (Ta₂O₅/SiO₂)¹⁵ when tested at λ=1.03  μm using pulse durations of 0.19 and 4 ns and peak power densities of 43.5 and 216  GW/cm². One of the designs achieved a laser damage threshold fluence of 174  J/cm² at 4 ns, which is 10% higher than that of a HfO₂/SiO₂ quarter wave design.

Thin film absorption characterization by focus error thermal lensing

A simple, highly sensitive technique for measuring absorbed power in thin film dielectrics based on thermal lensing is demonstrated. Absorption of an amplitude modulated or pulsed incident pump beam by a thin film acts as a heat source that induces thermal lensing in the substrate. A second continuous wave collimated probe beam defocuses after passing through the sample. Determination of absorption is achieved by quantifying the change of the probe beam profile at the focal plane using a four-quadrant detector and cylindrical lenses to generate a focus error signal. This signal is inherently insensitive to deflection, which removes noise contribution from point beam stability. A linear dependence of the focus error signal on the absorbed power is shown for a dynamic range of over 105. This technique was used to measure absorption loss in dielectric thin films deposited on fused silica substrates. In pulsed configuration, a single shot sensitivity of about 20 ppm is demonstrated, providing a unique technique for the characterization of moving targets as found in thin film growth instrumentation.

Laser damage of interference coatings at λ = 1.6 μm with an optical parametric chirped pulse amplifier

Laser damage of interference coatings is investigated with 1.6μm wavelength, 2 picosecond pulses from an optical parametric chirped pulse amplification system. 7 J/cm2 damage thresholds are achieved and deviation from conventional damage models is reported.

Laser Damage Resistant Ta 2 O 5 /HfO 2 /SiO 2 Multilayer Mirrors by Ion Beam Sputtering

We show the threshold fluence for damage of Ta2O5/SiO2 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.