In this study, we present a method for free-space beam shaping and steering based on a silicon optical phased array, which addresses the theoretical limitation of traditional bulk optics. We theoretically analyze the beam propagation properties with changes in the applied phase. Different beam profiles can be shaped by varying the phase combination, while a high-order quasi-Bessel beam can be generated with a cubic change to the phase modulation. The simulated results are validated further experimentally, and they match one another well. Beam steering can be achieved with a field of view as large as 140°, which has potential benefits for practical applications. The presented method is expected to have broad application prospects for optical communications, free-space optical interconnects, and light detection and ranging.

一定强度的飞秒激光聚焦于空气能生成空气等离子体并诱导生成冲击波。为了观察该冲击波传播特性，引入了超快时间分辨涡旋滤波成像技术，并对观测到的冲击波动力学过程进行了分析。实验探测到泵浦能量为1.5 mJ的飞秒激光经过透镜聚焦到空气中产生等离子体空气冲击波，分析了在3~15 μs时间段冲击波的动态演化过程。结果表明，飞秒激光等离子体空气冲击波在传输时以不对称的球形形状向外扩散，且沿着激光传播方向的传播速度与背着激光传播方向的传播速度不同，分别为372 m/s和341 m/s。这一观察结果与传统的点爆炸模型的对称情形不同，尝试对该不对称动力学过程进行了合理解释。

将泵浦—探测技术的时间分辨能力引入到光学涡旋日冕仪成像系统中，从而在实现微弱相位型物体无背景成像的同时对其做时间分辨诊断。将该技术应用于飞秒激光激发的空气等离子体的诊断。实验结果表明，利用该技术可以清晰地观察到空气等离子体的生成过程，且实验结果与数值模拟结果符合较好。另外，还观测到空气等离子体的两个衰减过程，并对其进行了双指数拟合分析。其中，具有较快时间尺度（~65 ps）的过程主要由电子与正离子复合导致，具有较慢时间尺度（~810 ps）的衰减过程主要由自由电子在氧气分子上的附着所致。

We experimentally investigated the forward 353.8 nm radiation from plasma filaments in pure nitrogen gas pumped by intense circularly polarized 800 nm femtosecond laser pulses. This emission line corresponds to the B2Σu+(v′=4)?X2Σg+(v=3) transition of nitrogen ions. In the presence of an external seeding pulse, the 353.8 nm signal was amplified by 3 orders of magnitude. Thanks to the much enhanced intensity, we performed time-resolved measurement of the amplified 353.8 nm emission based on the sum-frequency generation technique. It was revealed that the built-up time and duration of these emissions are both inversely proportional to the gas pressure, while the radiation peak power grows up nearly quadratically with pressure, indicating that the 353.8 nm radiation is of the nature of superradiance.