Manipulating the polarization of light at the nanoscale is key to the development of next-generation optoelectronic devices. This is typically done via waveplates using optically anisotropic crystals, with thicknesses on the order of the wavelength. Here, using a novel ultrafast electron-beam-based technique sensitive to transient near fields at THz frequencies, we observe a giant anisotropy in the linear optical response in the semimetal WTe₂ and demonstrate that one can tune the THz polarization using a 50 nm thick film, acting as a broadband wave plate with thickness 3 orders of magnitude smaller than the wavelength. The observed circular deflections of the electron beam are consistent with simulations tracking the trajectory of the electron beam in the near field of the THz pulse. This finding offers a promising approach to enable atomically thin THz polarization control using anisotropic semimetals and defines new approaches for characterizing THz near-field optical response at far-subwavelength length scales.

中文翻译：

---

超薄各向异性半金属中的巨大太赫兹双折射


在纳米尺度上操纵光的偏振是开发下一代光电器件的关键。这通常是通过使用光学各向异性晶体的波片来完成的，其厚度约为波长。在这里，使用一种对太赫兹频率瞬态近场敏感的新型超快电子束技术，我们观察到半金属 WTe ₂ 中线性光学响应的​​巨大各向异性，并证明可以调谐太赫兹使用50 nm厚的薄膜进行偏振，充当厚度比波长小3个数量级的宽带波片。观察到的电子束圆形偏转与跟踪太赫兹脉冲近场中电子束轨迹的模拟一致。这一发现提供了一种很有前途的方法，可以使用各向异性半金属实现原子级薄的太赫兹偏振控制，并定义了在远亚波长尺度上表征太赫兹近场光学响应的​​新方法。