We employ the two-band density-matrix equations to calculate high-order
harmonic generation (HHG) and its ellipticity in gapped graphene irradiated by
a femtosecond short-pulse laser under different orientation angles. The
orientation-dependent harmonic spectra show obvious enhancement harmonics. Noi
also focus on the ellipticity of these enhanced harmonics. Utilizing the
recombination trajectory model, the enhanced harmonics are attributed to the
caustic effect, whose orientation dependence originates from the inequivalence
of $\textrm{K}$ points during the electron ionization process for different
orientation angles. Inoltre, the harmonic ellipticity can be well
understood by the phase difference of saddle-point currents. Based on our
teoria, we design a two-color field scheme to generate elliptically polarized
attosecond pulses in gapped graphene. This work may shed light on the
generation of elliptically polarized attosecond pulses in two-dimensional
materials.

Questo articolo esplora i giri e le loro implicazioni.

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