  Strain engineering is a versatile method to boost the carrier mobility of
two-dimensional materials-based electronics and optoelectronic devices. In
addition, strain is ubiquitous during device fabrication via material
deposition on a substrate with a different lattice structure. Here, we show
that the polarization properties of the harmonics in graphene under uniaxial
strain are strongly yet differently affected in the lower and higher orders.
The polarization plane of the lower-order emitted harmonics is rotated -- a
manifestation of Faraday rotation due to the broken symmetry planes. In
contrast, we observe elliptically-polarized higher-order harmonics due to the
intricate interplay of the interband and intraband electron dynamics. The
implications of these findings are twofold: First, we show how the rotation of
the polarization plane of the lower-order harmonics can be used as a probe to
characterize the strain's nature, strength, and angle. Second, we demonstrate
how strain engineering can be used to alter the polarization properties of
higher-order harmonics, relevant for applications in ultrafast chiral-sensitive
studies. Our research opens a promising avenue for strain-tailored polarization
properties of higher-order harmonics in engineered solids.
