  Strong field driven electric currents in condensed matter systems open new
frontiers in petahertz electronics. In this regime new challenges arise as the
role of the band structure and the quantum nature of electron-hole dynamics
have yet to be resolved. Here we reveal the underlying attosecond dynamics that
dictates the temporal evolution of carriers in multi-band solid state systems,
via high harmonic generation (HHG) spectroscopy. We demonstrate that when the
electron-hole relative velocity approaches zero, enhanced quantum interference
leads to the appearance of spectral caustics in the HHG spectrum. Introducing
the role of the dynamical joint density of states (JDOS) we identify its direct
mapping into the spectrum, exhibiting singularities at the spectral caustics.
By probing these singularities, we visualize the structure of multiple
unpopulated high conduction bands. Our results open a new path in the control
and study of attosecond quasi-particle interactions within the field dressed
band structure of crystals.
