Opt Lett. 2018 Nov 15;43(22):5571-5574. doi: 10.1364/OL.43.005571.
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∼10 W/cm 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 λ scaling of I with the driver wavelength λ, 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 W/cm. 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.
基于固体表面的超短、相对论强度激光脉冲产生的高阶谐波（HHG）可以提供高效的阿秒脉冲源，并为激光物质相互作用、X 射线产生、激光粒子加速和相对论非线性光学的新领域开辟道路。然而，在近红外激光脉冲实验中，要实现 HHG 的相对论 regime，通常需要达到 I∼10 W/cm 的场强范围。在这里，我们表明，在中红外范围内，由于驱动波长 λ 的 λ 缩放，在低得多的激光场强度下就可以观察到相对论 HHG。我们的实验中使用了高峰值功率的 80fs、3.9μm 脉冲，聚焦在固体表面上，以提供 10 W/cm 的场强范围。值得注意的是，这种场强水平，在近红外的相对论光学标准下被认为是低的，但其足以产生具有相对论 HHG 特征的高阶谐波，包括光束方向性、谱线具有扩展平台，以及对于 p 极化和 s 极化驱动场都具有高的 HHG 产率。
