We propose a figure of merit that characterizes the femtosecond laser damage behavior of optical coatings. This figure of merit, based on the complete spatiotemporal evolution of the field in a multilayer system, can be included in optics design. The monochromatic intensity enhancement widely used in “electric field-engineering” is sufficient only in certain structures such as high-reflectivity quarter-wave mirrors. In more complex systems, for example, in group delay dispersion mirrors and frequency tripling mirrors, one should consider the actual (typically smaller) intensity enhancement produced by short pulses and the change (typically increase) of pulse duration within the stack.
When designing femtosecond laser mirrors, one tries to minimize the peak intensity within the high-index layers to increase the laser-induced damage threshold. Typically, the optimization procedure utilizes the electric field distribution in the layer stack generated by monochromatic irradiation. This approach is sufficient for certain structures like high-reflectors based on quarter-wave films. More complex structures require to take into account the exact evolution of the pulse parameters such as peak intensity and duration within the multi-layer system. We exemplify this by discussing a merit function that can be included in femtosecond optics design.
Third harmonic generation (THG) in dielectric films with femtosecond laser pulses is used to study properties of dielectric thin films and stacks thereof below and above the 1-on-1 laser damage threshold. Deviations from the ideal cubic relationship between third-harmonic signal and incident fundamental fluence are a result of several fundamental processes. Their relative contributions are assessed by comparing results from LIDT and conversion efficiency measurements as well as beam profile and pump-probe studies.
We have studied laser induced material modification in a frequency tripling mirror (FTM) consisting of alternating hafnia and silica layers. The third-harmonic signal generated by a train of femtosecond laser pulses (791 nm, 55 fs, 110 MHz) drops over time until it reaches about 20% of the initial value. From the observed changes in reflection and transmission of the mirror a refractive index change of 0.07 was estimated, which occurs in the layer with the highest field enhancement. This index change triggers a drop in the field enhancement, which reduces the efficiency of nonlinear optical processes. The estimated value of ▵n allowed us to explain the 80% reduction in conversion efficiency and as well as an observed decrease in two-photon absorption.
The laser damage behavior of high quality coatings under nanosecond pulse illumination is controlled by statistically distributed defects, whose physical nature and defect mechanisms are still largely unknown. Defect densities are often retrieved by modeling the fluence dependence of the damage probability measured by traditional damage test (TDT) methods, based on ‘damage’ or ‘no damage’ observations. STEREO-LID (Spatio-TEmporally REsolved Optical LaserInduced Damage) allows the determination of the damage fluence (and intensity) in a single test by identifying the initiation of damage both temporally and spatially. The advantages of this test method over the TDT are discussed. In particular, its ability to retrieve detailed defect distribution functions is demonstrated by comparison of results from HfO2 films prepared by ion-assisted electron beam evaporation, ion-beam sputtering, and atomic layer deposition.
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