Nonlinear valleytronics in atomically thin semiconductors

In monolayer Transition Metal Dichalcogenides (TMDs), the interplay between space-inversion and Time-Reversal (TR) symmetry defines the spin-valley degree of freedom: direct transitions at the ±K valleys are energetically degenerate but non-equivalent. Thus, engineering of TR symmetry naturally leads to the field of valleytronics, a new type of information technology that allows to manipulate data at the speed of ultrashort light pulses. However, all the methods that have been developed to date for the detection of a Valley Polarization (VP) are based on linear optics and suffer from severe limitations. In this talk, I will discuss a new method for the detection of broken TR symmetry and VP in TMDs based on nonlinear optics. I will show that the VP can be measured from a rotation in the polarization angle of the Second Harmonic (SH) signal emitted from the ±K valleys of a TMD monolayer. In addition, I will show signatures of valley polarization-electric dipole interference and demonstrate that this is a direct consequence of a phase-mismatch between the intrinsic (broken space-inversion symmetry) and valley (broken TR symmetry) terms of the second order nonlinear susceptibility.