High harmonic cutoff energy scaling and laser intensity measurement with a 1.8 µm laser source

High harmonic generation in gas targets leads to the production of attosecond pulses. The process of high harmonic generation requires that the gas be ionized by an intense femtosecond laser field. The highest photon energy produced is related to the laser intensity times the wavelength squared. This cutoff is reached only if good phase matching is achieved. Using a laser with a wavelength of 1800 nm, we estimate the laser intensity in the gas jet by recording the ion yield, and simultaneously record the high harmonic spectrum. We show that the cutoff energy matches the measured intensity, confirming that good phase matching is achieved to 100 eV. We also use the ion collector to characterize the spatial size of the gas jet and to measure the confocal parameter of the laser beam, parameters that are useful for numerical modelling.