High-order harmonic generation (HHG) is an extreme non-linear optical process in which ultrafast laser pulses are used to produce bursts of coherent radiation with photon energies reaching the soft x-ray region. HHG light sources have enabled ultrafast x-ray experiments in a laboratory environment, as opposed to large-scale facilities such as synchrotrons and free-electron lasers. In HHG, the maximum achievable photon energy scales quadratically with the driving laser wavelength, motivating many efforts to use laser sources with wavelengths extending towards the mid-infrared. However, the total photon flux produced in HHG scales inversely to the 5^th – 6^th power of the driving wavelength, for wavelengths in the visible and near-infrared, making the choice of driving wavelength a delicate balancing act between cutoff energy and total flux. In this work, Emelina and colleagues perform analytical calculations on the HHG scaling behavior, in atomic hydrogen, for wavelengths in the mid- to long-wavelength infrared regime. They show that several effects, including an increased role of the magnetic component of the driving field at longer wavelengths, change the scaling to be even less favorable than at shorter wavelengths. This work provides vital information as researchers seek to develop more energetic and brighter high-order harmonic light sources.

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