The Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan.
Photon Science Center, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
Phys Rev Lett. 2018 Jun 15;120(24):243903. doi: 10.1103/PhysRevLett.120.243903.
The polarization property of high harmonics from gallium selenide is investigated using linearly polarized midinfrared laser pulses. With a high electric field, the perpendicular polarization component of the odd harmonics emerges, which is not present with a low electric field and cannot be explained by the perturbative nonlinear optics. A two-dimensional single-band model is developed to show that the anisotropic curvature of an energy band of solids, which is pronounced in an outer part of the Brillouin zone, induces the generation of the perpendicular odd harmonics. This model is validated by three-dimensional quantum mechanical simulations, which reproduce the orientation dependence of the odd-order harmonics. The quantum mechanical simulations also reveal that the odd- and even-order harmonics are produced predominantly by the intraband current and interband polarization, respectively. These experimental and theoretical demonstrations clearly show a strong link between the band structure of a solid and the polarization property of the odd-order harmonics.
我们利用线性偏振中红外激光脉冲研究了硒化镓中的高次谐波的偏振特性。在高电场下，出现了奇数谐波的垂直偏振分量，而在低电场下则没有这种分量，也不能用微扰非线性光学来解释。我们建立了一个二维单能带模型，表明固体能带的各向异性曲率（在布里渊区的外部区域表现明显）导致了垂直奇数谐波的产生。这个模型通过三维量子力学模拟得到了验证，模拟结果再现了奇数阶谐波的取向依赖性。量子力学模拟还表明，奇数阶和偶数阶谐波主要分别由带内电流和带间极化产生。这些实验和理论的证明清楚地表明，固体的能带结构和奇数阶谐波的偏振特性之间存在着很强的联系。
