基于h-BN的方向依赖高次谐波产生(特邀)

Orientation-dependent High Harmonic Generation from h-BN(Invited)

利用紧束缚电子能带结构并结合求解两能带半导体Bloch方程的方法,研究了单层h-BN的高次谐波产生过程。结果显示平行于驱动光偏振方向的谐波分量在截止区表现出异常的晶格取向依赖关系,当光子能量超过7.31 eV时,奇次谐波的取向依赖相对于较低级次的奇次谐波取向依赖表现出明显的30°角度偏移。确定了能够发生偏移的那些谐波光子能量与布里渊区M点处的载流子动力学相关。进一步的时频分析表明,当驱动光偏振沿zigzag方向时,截止区谐波仅在特定极性的激光半周期内产生,这导致了zigzag方向谐波的急剧减弱,从而造成了截止区晶格取向分布的偏移。这一现象对利用高次谐波光谱重建电子能带结构具有重要意义。

The absorption and emission of light by matter plays a crucial role in the development of science and technology.High Harmonic Generation(HHG)presents an extremely nonlinear optical radiation induced by the interaction of intense laser fields with matter.Over the past two decades,HHG in gaseous materials has been extensively studied and regarded as a vital tool for advancements in ultrafast science.The fundamentals of gaseous HHG can be explained by the semi-classical three-step model,the understanding of microscopic processes in HHG has laid the foundation for atto-second physics and metrology,including the ability to probe atomic structure and dynamics and molecular systems.Recently,there have been reports of HHG occurring in various solid-phase materials,the targets of solid-state high harmonic studies have been extended from bulk metals,semiconductors,and insulators to low-dimensional nanostructures.Two-dimensional materials can neglect the propagation effects in the direction of laser propagation,and thus become ideal materials for the study of high-harmonic carrier dynamics.An important phenomenon of HHG in solids is the anisotropy.Due to the modulation of the lattice symmetry,the harmonic signals generated by the driving light polarized along the different directions of the crystal are quite different,it has shown potential applications such as reconstructing crystal band structure,measuring Berry curvature,and investigating topological phase transitions.In this paper,we have investigated the process of HHG from monolayer h-BN by using the tight binding energy band and solving the two-band SBEs.We show that the yield of harmonics displays a periodicity of 60°as the azimuthal angle between the h-BN and driving field are varied,consistent with the symmetry of the laser and the crystal.Notably,an intriguing pattern in the orientation-dependent HHG is observed.Specifically,we decompose the high harmonics into components parallel and perpendicular to the driving light,the parallel component of the odd-order harmonics in the cut-off region exhibits an angular shift of 30°compared to the other orders,and this angular shift is independent of the change of driving light intensity.Comparison of the harmonic spectra of the driving light polarization along the zigzag direction and the armchair direction reveals that the harmonic spectrum has a sharp decrease(cut-off region)for harmonics above H17 when the driving field is along the zigzag direction.The harmonic intensities below the H17 are stronger in the zigzag direction than in the armchair direction,and the sharp decrease in the zigzag direction harmonics at the H17 results in the harmonic intensities being less than those in the armchair direction harmonics.Therefore,we believe that the angular shift of the intensity modulation of odd-order harmonic parallel components are related to the cut-off of the zigzag directional harmonic spectrum.Furthermore,we found the energy at which this angle shift occurs is strongly correlated with the bandgap energy of h-BN,especially when close to the M-momentum channel bandgap.Through detailed analyses,we determine that the phase shift in the intensity modulation of H17 arises due to the interference of different momentum channels and the interference of different polarity half-periods.We believe that this phenomenon is not coincidental,that the effect of energy band structure on harmonics is significant.The angular shift of odd-order harmonics holds important potential for developing techniques to probe the energy band structure of solids through HHG.