In the femtosecond regime laser damage thresholds are determined by the electric field distribution within the optical components. Especially, for radiation sources with integrated frequency conversion the simultaneous presence of photons with different frequencies introduces additional ionization channels in optical materials by cross excitation and other effects. In this work we report on the pulse delay dependency of the LIDT of HR390/780nm mirrors under simultaneous exposure to fundamental and second harmonic femtosecond radiation. We perform Son 1-tests according to ISO 21254 with the addition of a second harmonic pulse at different fixed pulse energies. To determine the influence of the cross excitation between fundamental and second harmonic radiation, these tests are repeated for different time delays between the two pulses. For the 1on1, single wavelength femtosecond LIDT testing, the Keldysh theory in combination with the Drude Model has been proven to reasonably describe the time dependent electron density in the conduction band, and hence the LIDT. We extend these approaches to the determination of the LIDT for the case of simultaneous interactions of photons of two separate wavelengths.
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