Opt Lett. 2018 Oct 15;43(20):4953-4956. doi: 10.1364/OL.43.004953.
The plasma arising due to the propagation of a filamenting ultrafast laser pulse in air contains currents driven by the pulse that generate radiated electromagnetic fields. We report absolutely calibrated measurements of the frequency spectrum of microwaves radiated by the filament plasma from 2-40 GHz. The emission pattern of the electric field spectrum is mapped as a function of air pressure from atmosphere to 0.5 Torr. For fixed laser pulse energy, duration, and focal geometry, we observe that decreasing the air pressure by a factor of approximately 10 increases the amplitude of the electric field waveform by a factor of about 40. As the air pressure decreases, the lower frequency components (<10 GHz) increase in amplitude faster than those at higher frequencies (>20 GHz). To the best of our knowledge, this behavior has not been observed before, is not predicted by existing theory, and implies the existence of a radiation mechanism in the plasma distinct from that which emits at terahertz frequencies.
在空气中传播的丝状超快激光脉冲会产生等离子体，其中包含由脉冲驱动的电流，这些电流会产生辐射电磁场。我们报告了丝状等离子体辐射的微波频谱的绝对校准测量结果，频率范围为 2-40GHz。作为空气压力从大气压到 0.5 托的函数，我们绘制了电场频谱的发射模式。对于固定的激光脉冲能量、持续时间和聚焦几何形状，我们观察到，空气压力降低约 10 倍会使电场波形的幅度增加约 40 倍。随着空气压力的降低，较低频率分量（<10GHz）的幅度增加速度比较高频率分量（>20GHz）快。据我们所知，这种行为以前没有观察到，也没有现有理论预测，这意味着在等离子体中存在一种不同于发射太赫兹频率的辐射机制。
