Electron-electron scattering on the order of a few to tens of femtoseconds plays a crucial role in the ultrafast electron dynamics of conventional metals. When mid-infrared light is used for driving and the period of light field is comparable to the scattering time in metals, unique light-driven states and nonlinear optical responses associated with the scattering process are expected to occur. Here, we use high-harmonics spectroscopy to investigate the effect of electron-electron scattering on the electron dynamics in thin film $2\mathrm{H}\text{−}{\mathrm{NbSe}}_2$ driven by a mid-infrared field. We observed odd-order high harmonics up to 9th order as well as a broadband emission from hot electrons in the energy range from 1.5 to 4.0 eV. The electron-electron scattering time in 2H-${\mathrm{NbSe}}_2$ was estimated from the broadband emission to be almost the same as the period of the mid-infrared light field. A comparison between experimental results and a numerical calculation reveals that competition and cooperation between the driving and scattering enhances the nonperturbative behavior of high harmonics in metals, causing a highly nonequilibrium electronic state corresponding to several thousand Kelvin.

中文翻译：

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高次谐波探测 2H−NbSe2 金属相中的超快电子-电子散射

几到几十飞秒量级的电子-电子散射在传统金属的超快电子动力学中起着至关重要的作用。当使用中红外光进行驱动并且光场周期与金属中的散射时间相当时，预计会发生与散射过程相关的独特的光驱动状态和非线性光学响应。在这里，我们使用高次谐波光谱来研究电子-电子散射对薄膜中电子动力学的影响$2\mathrm{H}\text{-}{\mathrm{硒化铌}}_2$由中红外场驱动。我们观察到高达 9 阶的奇数次高次谐波以及能量范围为 1.5 至 4.0 eV 的热电子的宽带发射。 2H-电子-电子散射时间${\mathrm{硒化铌}}_2$根据宽带发射估计，其周期几乎与中红外光场的周期相同。实验结果与数值计算的比较表明，驱动和散射之间的竞争与合作增强了金属中高次谐波的非微扰行为，导致相当于数千开尔文的高度非平衡电子态。