posted on 2020-03-18, 20:19authored byYi Wei Ho, Henrique G. Rosa, Ivan Verzhbitskiy, Manuel J. L. F. Rodrigues, Takashi Taniguchi, Kenji Watanabe, Goki Eda, Vitor M. Pereira, José C. Viana-Gomes
A population imbalance at different valleys
of an electronic system
lowers its effective rotational symmetry. We introduce a technique
to measure such imbalance (a valley polarization), which exploits
the unique fingerprints of this symmetry reduction in the polarization-dependent
second-harmonic generation (SHG). We present the principle and detection
scheme in the context of hexagonal two-dimensional crystals, which
include graphene-based systems and the family of transition metal
dichalcogenides, and provide a direct experimental demonstration using
a molybdenum diselenide monolayer with 2H polytype at room temperature.
We deliberately use the simplest possible setup, where a single pulsed
laser beam simultaneously controls the valley imbalance and tracks
the SHG process. We further developed a model of the transient population
dynamics, which analytically describes the valley-induced SHG rotation
in very good agreement with the experimental data. In addition to
providing the first experimental demonstration of the effect, this
work establishes a conceptually simple, compact, and transferable
way of measuring instantaneous valley polarization, with direct applicability
in the nascent field of valleytronics.