  Degenerate minima in momentum space - valleys - provide an additional degree
of freedom that can be used for information transport and storage. Notably,
such minima naturally exist in the band structure of transition metal
dichalcogenides (TMDs). When these atomically thin crystals interact with
intense laser light, the second harmonic generated (SHG) field inherits special
characteristics that reflect not only the broken inversion symmetry in real
space, but also the valley anisotropy in reciprocal space. The latter is
present whenever there exists a valley population imbalance (VPI) between the
two valleys. In this work, it is shown that the temperature-induced changes of
the SHG intensity dependence on the excitation fieldpolarization, is a unique
fingerprint of VPI in TMDs. Analysis of such changes, in particular, enables
the calculation of the valley-induced to intrinsic second order
susceptibilities ratio. Unlike temperature-dependent photoluminescence (PL)
measurements of valley polarization and coherence, the proposed polarization
resolved SHG (PSHG) methodology is insensitive to the excitation field
wavelength, an advantage that renders it ideal for monitoring VPI in large
crystalline or stacked areas comprising different TMDs.
