Measuring Valley Susceptibility of Transition Metal Dichalcogenides with Second-harmonic Spectroscopy*

Polarization-resolved (PR) photoluminescence (PL) [1] and Kerr rotation (KR) [2] are commonly used to probe the degree of valley polarization (VP) of transition metal dichalcogenides (TMDs). Recently, second-harmonic (SH) generation was proposed to also probe VP [3], with the advantage over PL and KR of not requiring the material to have a bandgap or strong spin-orbital coupling, and thus extending VP measurements to other materials such as graphene.

In this work, we have develop a technique that uses a single laser beam prepared in a polarization state that simultaneously creates VP and pumps the SH process. By increasing the ellipticity of this pump beam, which translates into a larger VP in the sample, we observed an additional SH signal due to the VP, in good agreement with our theoretical model. Based on a transient analysis within the duration of the laser pulse, our model confirms SH susceptibility induced by VP to be directly proportional to the degree of VP, with the effect an order of magnitude higher for a quasi-resonant VP pump when compared to the off-resonant case.

[1] Nat. Nanotechnol. 7, 490 (2012), Nat. Nanotechnol. 7, 494 (2012).
[2] Phys. Rev. B 90, 161302(R) (2014), Nat. Commun. 6, 8963 (2015).
[3] 2D Mater. 4, 21027 (2017).