We present a thorough investigation of surface deformation and thermal properties of high-damage threshold large-area semiconductor saturable absorber mirrors (SESAMs) designed for kilowatt average power laser oscillators. We compare temperature rise, thermal lensing, and surface deformation of standard SESAM samples and substrate-removed SESAMs contacted using different techniques. We demonstrate that for all cases the thermal effects scale linearly with the absorbed power, but the contacting technique critically affects the strength of the temperature rise and the thermal lens of the SESAMs (i.e. the slope of the linear change). Our best SESAMs are fabricated using a novel substrate-transfer direct bonding technique and show excellent surface flatness (with non-measureable radii of curvature (ROC), compared to astigmatic ROCs of up to 10 m for standard SESAMs), order-of-magnitude improved heat removal, and negligible deformation with absorbed power. This is achieved without altering the saturation behavior or the recovery parameters of the samples. These SESAMs will be a key enabling component for the next generation of kilowatt-level ultrafast oscillators.

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Aluminium nitride thin-films (AlN) were fabricated by a DC-magnetron sputtering technique at different background pressures while maintaining the same deposition conditions. The influence of varying sputtering pressure on the structural and optical properties of AlN thin-films was investigated. XRD measurements were utilized to determine the structural properties of the deposited AlN thin-films such as strain, stress, crystallite size, and crystalline density. They confirmed the hexagonal wurtzite structure of AlN thin-films and the increase in the degree of c-axis orientation as the sputtering pressure decreases. The optical properties of AlN thin-films, deposited on glass substrates, were analyzed by means of transmittance and absorption spectra using a UV-Vis spectrophotometer. The results have shown that an increase in the sputtering pressure leads to a shift of the threshold transmittance towards the lower wavelength range. This results in widening the optical bandgap and decreasing both the refractive index and the extinction coefficient. The dispersion of the refractive index is investigated according to the Wemple-DiDomenico single oscillator model and the model parameters (such as effective single oscillator E 0 , dispersion energy E d , zero-frequency dielectric constant e 0 and optical moments) were estimated accordingly.

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