  We revisit the mechanism of high harmonic generation (HHG) from solids by
comparing HHG in laser fields with different ellipticities but constant maximum
amplitude. It is shown that the cutoff of HHG is strongly extended in a
circularly polarized field. Moreover, the harmonic yield with large ellipticity
is comparable to or even higher than that in the linearly polarized field. To
understand the underlying physics, we develop a reciprocal-space-trajectory
method, which explains HHG in solids by a trajectory ensemble from different
ionization times and different initial states in the reciprocal space. We show
that the cutoff extension is related to an additional pre-acceleration step
prior to ionization, which has been overlooked in solids. By analyzing the
trajectories and the time-frequency spectrogram, we show that the HHG in solids
cannot be interpreted in terms of the classical re-collision picture alone.
Instead, the radiation should be described by the electronhole interband
polarization, which leads to the unusual ellipticity dependence. We propose a
new four-step model to understand the mechanism of HHG in solids.
