Measuring valley polarization in two-dimensional materials with second-harmonic spectroscopy

• Autor(a) principal: Ho, Yi Wei
• Data de Publicação: 2020
• Outros Autores: Rosa, Henrique Guimarães, Verzhbitskiy, Ivan, Rodrigues, Manuel José Lima Ferreira, Taniguchi, Takashi, Watanabe, Kenji, Eda, Goki, Pereira, Vitor Manuel, Viana-Gomes, Jose Carlos
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: https://hdl.handle.net/1822/90340
• topic: Valley polarization; Transition metal dichalcogenides; 2D materials; Second-harmonic generation; Graphene; Ciências Naturais::Ciências Físicas; Science & Technology; Indústria, inovação e infraestruturas

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 - that 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 2H-MoSe$_{2}$ monolayer 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 experiment. In addition to providing the first experimental demonstration of the effect, this work establishes a conceptually simple, com-pact and transferable way of measuring instantaneous valley polarization, with direct applicability in the nascent field of valleytronics.