  Attosecond metrology sensitive to sub-optical-cycle electronic and structural
dynamics is opening up new avenues for ultrafast spectroscopy of condensed
matter. Using intense lightwaves to precisely control the extremely fast
carrier dynamics in crystals holds great promise for next-generation
electronics and devices operating at petahertz bandwidth. The carrier dynamics
can produce high-order harmonics of the driving light field extending up into
the extreme-ultraviolet region. Here, we introduce polarization-state-resolved
high-harmonic spectroscopy of solids, which provides deeper insights into both
electronic and structural dynamics occuring within a single cycle of light.
Performing high-harmonic generation measurements from silicon and quartz
samples, we demonstrate that the polarization states of high-order harmonics
emitted from solids are not only determined by crystal symmetries, but can be
dynamically controlled, as a consequence of the intertwined interband and
intraband electronic dynamics, responsible for the harmonic generation. We
exploit this symmetry-dynamics duality to efficiently generate circularly
polarized harmonics from elliptically polarized driver pulses. Our experimental
results are supported by ab-initio simulations, providing clear evidence for
the microscopic origin of the phenomenon. This spectroscopy technique might
find important applications in future studies of novel quantum materials such
as strongly correlated materials. Compact sources of bright circularly
polarized harmonics in the extreme-ultraviolet regime will advance our tools
for the spectroscopy of chiral systems, magnetic materials, and 2D materials
with valley selectivity.
