We investigate the orientation dependence of high-order harmonic generation (HHG) in graphene by solving the semiconductor Bloch equations. The tight-binding approximation including up to the third-nearest-neighbor atoms is used to model the graphene. Our simulations show that the orientation dependence of HHG shows different patterns for different harmonics. With increasing the laser intensity, these patterns become similar except for a double-peak structure shown in lower-order harmonics. The double-peak structure appears at high laser intensity due to the contribution of the harmonic perpendicular to the laser polarization direction. The harmonic yield is decomposed as the contributions of the nearest-neighbor interaction, the second-nearest-neighbor interaction, and the third-nearest-neighbor interaction. Our results show that the harmonic yields are mainly contributed by the nearest-neighbor interaction, but the third-nearest-neighbor interaction also plays an important role and modifies the orientation dependence.

中文翻译：

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石墨烯中高次谐波产生的方向依赖性

我们通过求解半导体布洛赫方程来研究石墨烯中高次谐波生成 (HHG) 的方向依赖性。包含最多第三个最近邻原子的紧束缚近似用于模拟石墨烯。我们的模拟表明，HHG 的方向依赖性显示出不同谐波的不同模式。随着激光强度的增加，除了在低阶谐波中显示的双峰结构之外，这些图案变得相似。由于垂直于激光偏振方向的谐波的贡献，双峰结构出现在高激光强度下。谐波产率被分解为最近邻相互作用、第二最近邻相互作用和第三最近邻相互作用的贡献。