  Various interference effects are known to exist in the process of high
harmonic generation (HHG) both at the single atom and macroscopic levels. In
particular, the quantum path difference between the long and short trajectories
of electron excursion causes the HHG yield to experience interference-based
temporal and spectral modulations. In solids, due to additional phenomena such
as multi-band superposition and crystal symmetry dependency, the HHG mechanism
appears to be more complicated than in gaseous atoms in identifying
accompanying interference phenomena. Here, we first report experimental data
showing intensity-dependent spectral modulation and broadening of high
harmonics observed from bulk sapphire. Then, by adopting theoretical
simulation, the extraordinary observation is interpreted as a result of the
quantum path interference between the long and short electron/hole
trajectories. Specifically, the long trajectory undergoes an
intensity-dependent redshift, which coherently combines with the short
trajectory to exhibit spectral splitting in an anomalous way of inverse
proportion to the driving laser intensity. This quantum interference may be
extended to higher harmonics with increasing the laser intensity, underpinning
the potential for precise control of the phase matching and modulation even in
the extreme ultraviolet and soft X-ray regime. Further, this approach may act
as a novel tool for probing arbitrary crystals so as to adjust the electron
dynamics of higher harmonics for attosecond spectroscopy.

Optics