Measuring Valley Polarization in Two-Dimensional Materials with Second-Harmonic Spectroscopy,ACS Photonics

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Measuring Valley Polarization in Two-Dimensional Materials with Second-Harmonic Spectroscopy
ACS Photonics ( IF 6.5 ) Pub Date : 2020-03-11 , DOI: 10.1021/acsphotonics.0c00174
Yi Wei Ho _{1,

2,

3} , Henrique G. Rosa ₂ , Ivan Verzhbitskiy _{2,

3} , Manuel J. L. F. Rodrigues _{2,

4} , Takashi Taniguchi ₅ , Kenji Watanabe ₅ , Goki Eda _{2,

3,

6} , Vitor M. Pereira _{2,

3} , José C. Viana-Gomes _{2,

3,

4}

Affiliation

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.

中文翻译：

使用二次谐波光谱法测量二维材料中的波谷极化

电子系统不同谷处的人口不平衡会降低其有效的旋转对称性。我们介绍了一种测量这种不平衡（谷底极化）的技术，该技术利用了极化相关的二次谐波生成（SHG）中这种对称性降低的唯一指纹。我们在六角形二维晶体的背景下介绍了原理和检测方案，包括基于石墨烯的系统和过渡金属二卤化碳族，并提供了在室温下使用具有2H多型的二硒化钼单层的直接实验演示。我们故意使用最简单的设置，其中单个脉冲激光束同时控制波谷不平衡并跟踪SHG过程。我们进一步开发了一个瞬态种群动态模型，分析性地描述了谷诱导的SHG旋转，与实验数据非常吻合。除了提供该效应的第一个实验证明之外，这项工作还建立了一种概念上简单，紧凑且可转移的测量瞬时谷极化的方法，可直接应用于Valleytronics的新生领域。

更新日期：2020-04-23

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