Multi-keV ultrahigh-order harmonics produced in gases with subrelativistically intense mid-IR laser pulses

Due to favorable wavelength scaling of the electron ponderomotive energy, high-order harmonic generation in gases is known to benefit from using long-wavelength femtosecond laser sources to produce high-energy photons. In this paper, using an analytical quantum-mechanical approach, taking into account the laser-induced depletion of atomic levels and the magnetic-field effects, we analyze the relative roles of these factors in limiting the generation efficiency and in shaping the spectral profile of the harmonic radiation produced with mid-IR laser pulses in various gaseous targets. Based on these calculations, we estimate the maximum width of the high-harmonic spectrum that can be obtained using the high-power mid-IR laser sources. The feasibility of multi-keV photon generation in gases irradiated by state-of-the-art mid-IR drivers is demonstrated.