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Spatiotemporal Evolution of Coherent Elastic Strain Waves in a Single MoS2 Flake

机译：单个MOS2薄片中相干弹性应变波的时空演变

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We use bright-field imaging in an ultrafast electron microscope to spatiotemporally map the evolution of photoexcited coherent strain waves in a single, micrometer-size flake of MoS2. Following in situ femtosecond photoexcitation, we observe individual wave trains emerge from discrete nanoscale morphological features and propagate in-plane along, specific wave vectors at approximately the speed of sound (7 nm/ps). Over the span of several hundred picoseconds, the 50 GHz wave trains (20 ps periods) are observed to undergo phonon-phonon scattering and wave-train interference, resulting in a transition to larger-scale, incoherent structural dynamics. This incoherent motion further evolves into coherent nanomechanical oscillations over a few nanoseconds, ultimately leading to megahertz, whole-flake multimode resonances having microsecond lifetimes. These results provide insight into the low-frequency structural response of MoS2 to relatively coherent optical photoexcitation by elucidating the origin and the evolution of high-velocity, gigahertz strain waves.
机译：我们在超快电子显微镜中使用明亮场成像，以时尚地图在单个微米尺寸的MOS2中的光屏蔽相干应变波的演变。遵循原位飞秒光透镜，我们观察各个波列车从离散纳米级形态特征中出现并在平面内传播，特定波矢量在大约声速（7nm / ps）。在几百个皮秒的范围内，观察到50GHz波列（20 ps时段）经历声子位散射和波动串干扰，导致转变为大规模，不连贯的结构动态。这种非相干的运动进一步发展成相干的纳米力学振荡，超过几纳秒，最终导致Megahertz，全鳞片多模共振具有微秒的寿命。这些结果通过阐明高速的起源和演变，吉格赫兹应变波的起源和进化提供了对MOS2的低频结构响应的洞察。

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