该研究团队利用时域近场干涉技术对超高速双曲射线脉冲进行了分析，发现超高速双曲射线脉冲在超薄hBN层内的曲折反射轨迹，产生了脉冲条纹向后移动和跳跃行为的错觉。这些射线是由双曲波导模式的相干拍频产生的，但可能被错误地解释为负群速度和反向能量流。此外，之字形反射沿轨迹产生纳米级(60 nm)和超快(40 fs)时空光学涡流，为光-物质相互作用的手性时空控制提供了机会。
实验证据进一步支持了模拟结果，凸显了双曲射线反射在分子振动吸收纳米光谱学中的巨大潜力。这一发现为小型化、片上光学光谱仪和超快速光学操纵技术的发展铺平了道路。
据悉，传统薄材料中的光衍射极限限制了特定频率下波导模式的数量。然而，层状范德华（vdW）材料如六方氮化硼（hBN）能突破这一局限。它们的介电各向异性特点，即在一个光轴上呈现正介电常数，而在另一个光轴上呈现负介电常数，使得双曲射线能在材料内部传播，并产生无限数量以双曲色散为特征的亚衍射模式。这一特性为光学应用开辟了新的可能。
附：英文原文
Title: Spatiotemporal beating and vortices of van der Waals hyperbolic polaritons
Author: Zhang, Tianning, Yan, Qizhi, Yang, Xiaosheng, Ma, Weiliang, Chen, Runkun, Zhang, Xin, Janzen, Eli, Edgar, James H., Qiu, Cheng-Wei, Zhang, Xinliang, Li, Peining
Issue&Volume: 2024-3-11
Abstract: In conventional thin materials, the diffraction limit of light constrains the number of waveguide modes that can exist at a given frequency. However, layered van der Waals (vdW) materials, such as hexagonal boron nitride (hBN), can surpass this limitation due to their dielectric anisotropy, exhibiting positive permittivity along one optic axis and negativity along the other. This enables the propagation of hyperbolic rays within the material bulk and an unlimited number of subdiffractional modes characterized by hyperbolic dispersion. By employing time-domain near-field interferometry to analyze ultrafast hyperbolic ray pulses in thin hBN, we showed that their zigzag reflection trajectories bound within the hBN layer create an illusion of backward-moving and leaping behavior of pulse fringes. These rays result from the coherent beating of hyperbolic waveguide modes but could be mistakenly interpreted as negative group velocities and backward energy flow. Moreover, the zigzag reflections produce nanoscale (60 nm) and ultrafast (40 fs) spatiotemporal optical vortices along the trajectory, presenting opportunities to chiral spatiotemporal control of light–matter interactions. Supported by experimental evidence, our simulations highlight the potential of hyperbolic ray reflections for molecular vibrational absorption nanospectroscopy. The results pave the way for miniaturized, on-chip optical spectrometers, and ultrafast optical manipulation.
DOI: 10.1073/pnas.2319465121
Source: https://www.pnas.org/doi/abs/10.1073/pnas.2319465121