Two-color Kerr rotation spectroscopy of a suspended transition-metal dichalcogenide monolayer

We demonstrate the advantage of using two-color Kerr rotation spectroscopy to study the long-lived valley polarization in a suspended WSe2 monolayer. Low-temperature optical measurements under electrostatic gating reveal the high degree of freedom in tailoring the properties of the suspended monolayer by controlling optical interference at the monolayer, strain, and the carrier density of the material. We examine the lifetime of spin-valley polarized carriers by using the reflected light in a two-color pump-probe experiment. The observed dependence of the Kerr rotation lifetimes on the gate voltage and probe laser energy allows us to examine their origins associated with different forms of excitons. Our results show possibilities for analyzing long-lived valley polarization dynamics in gate-tunable suspended monolayers by using Kerr rotation, where we can exclude the influence of substrate perturbations on the carrier dynamics.