近日，首都师范大学张亮亮团队研究了突然自动聚焦激光器的定向强太赫兹发射。该研究于2025年7月2日发表在《物理评论A》杂志上。

空气中的双色激光诱导等离子体灯丝可以作为宽带太赫兹（THz）脉冲的桌面传感器。地球上无处不在的空气促进了它的广泛应用，特别是在无线通信和遥感领域，利用基于空气的方案的独特优势，太赫兹辐射可以通过泵浦激光在空气中传播。然而，太赫兹辐射模式不可避免地具有一个锥形角轮廓，在传播轴上有一个倾角。因此，太赫兹能量的集中、传播的方向性和信号解调的精度都明显降低，极大地限制了其直接应用。

通过实验，研究组成功地消除了不利的圆锥形轮廓，同时利用突然自动调焦的主题，将焦点处太赫兹的方向性和强度提高了17倍。当从实验记录的荧光图像中读取空气灯丝长度时，他们使用远场模型的模拟很好地再现了实验观察结果。更重要的是，他们发现由于忽略高斯光束的guy相位，广泛采用的脱相长度被明显高估了30倍，这不能解释即使灯丝长度小于脱相长度时观测到的锥形太赫兹辐射。采用突然自聚焦光束时，消相长度增加了5倍，可以很好地克服消相效应，消除了不利的锥形轮廓，增强了太赫兹强度。这种方案很容易实现，因为可以通过在输入高斯光束上施加空间光调制器来实现突然自动调焦光束。

附：英文原文

Title: Directional intense terahertz emission by abruptly autofocusing lasers

Author: Xiao-Ran Zheng, Nan Li, Wei-Min Wang, Rui Zhang, Cun-Lin Zhang, Liang-Liang Zhang

Issue&Volume: 2025/07/02

Abstract: Two-color laser induced plasma filamentation in air could serve as tabletop sources of broadband terahertz (THz) pulses. Ubiquitous air in the earth facilitates its widespread utilities, particularly, in wireless communication and remote sensing, exploiting the unique advantage of the air-based scheme that the THz sources can be delivered over standoff distances via the pump laser propagation in air. However, the THz emission pattern inevitably has a conical angular profile with a dip in the propagation axis. Therefore, THz energy concentration, propagation directionality, and accuracy of signal demodulation are significantly impaired, greatly limiting its direct applications. Here, we successfully eliminate the unfavorable conical profile by experiments and meanwhile enhance THz directionality and intensity by 17 folds at the focus by use of abruptly autofocusing laser beams. Our simulations with a far-field model well reproduce the experimental observations, when the air filament length is read from the experimentally recorded fluorescence images. More importantly, we find that because the Gouy phase of Gaussian beam is ignored, the widely adopted dephasing length is significantly overestimated by 30 folds, which cannot explain the observed conical THz emission even when the filament length is shorter than the dephasing length. While the abruptly autofocusing beams are employed, the dephasing length is increased by five folds and the dephasing effect can be well overcome, which causes elimination of the unfavorable conical profile and enhancement of the THz intensity. This scheme is easily accessible since the abruptly autofocusing beam can be achieved by imposing a spatial light modulator on the input Gaussian beam.

DOI: 10.1103/b57m-txqz

Source: https://journals.aps.org/pra/abstract/10.1103/b57m-txqz