High-order harmonic generation (HHG) in plasmas induced by ultrashort, relativistic-intensity laser pulses on solid surfaces can provide an efficient source of attosecond pulses and opens routes toward new regimes of laser–matter interactions, x-ray generation, laser particle acceleration, and relativistic nonlinear optics. However, field intensities in the range of $I_{\text{rel}}\sim {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$ are typically needed to achieve the relativistic regime of HHG in experiments with near-infrared laser pulses. Here, we show that, in the mid-infrared range, due to the ${\lambda }^{-2}$ scaling of $I_{\text{rel}}$ with the driver wavelength $\lambda$, relativistic HHG can be observed at much lower levels of laser field intensities. High-peak-power 80-fs, 3.9-μm pulses are focused in our experiments on a solid surface to provide field intensities in the range of ${10}^{17}\mathrm{W}/{\mathrm{cm}}^2$. Remarkably, this level of field intensities, considered as low by the standards of relativistic optics in the near infrared, is shown to be sufficient for generation of high-order harmonics with signature properties of relativistic HHG—beam directionality, spectra with extended plateaus, and a high HHG yield sustained for both $p$- and $s$-polarized driver fields.

中文翻译：

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相对论强度低于100-fs的中红外脉冲从固体表面等离子体产生高次谐波

固体表面上超短的相对论强度激光脉冲在等离子体中产生的高次谐波产生（HHG）可以提供有效的阿秒脉冲源，并开辟通往新的激光物质相互作用，X射线产生，激光粒子加速机制的途径，以及相对论非线性光学。但是，场强在${\mathrm{一世}}_{\text{rel}}〜{10}^{19}\mathrm{w\; ^}/{\mathrm{厘米}}^{\mathrm{2个}}$在使用近红外激光脉冲的实验中，通常需要达到HHG的相对论状态。在这里，我们表明，在中红外范围内，由于${\lambda }^{-\mathrm{2个}}$ 缩放 ${\mathrm{一世}}_{\text{rel}}$ 与驱动器波长 $\lambda$相对论的HHG可以在低得多的激光场强度下观察到。在我们的实验中，高峰值功率的80fs，3.9μm脉冲集中在固体表面上，以提供在${10}^{17}\mathrm{w\; ^}/{\mathrm{厘米}}^{\mathrm{2个}}$。值得注意的是，这一水平的场强被相对论光学的近红外标准认为是低的，已显示出足以产生具有相对论HHG特征的高阶谐波（光束方向性，具有扩展的平稳期的光谱，以及双方的HHG产量都很高$p$- 和 $s$-极化的驱动程序字段。