强场太赫兹超短涡旋脉冲的高效产生及其检测技术研究

Motivated by the significant demands from the intense and nonlinear terahertz science and applications, our project aims to generate directly intense terahertz ultrashort vortex pulses through nonlinear frequency conversion with high conversion efficiency. For this purpose, we will firstly be engaged ourselves to develop a novel pump source, which can output two synchronous temporally 1560 nm pulses: one with a narrow bandwidth of ~4 nm without any spiral phase, and another with a broad bandwidth of ~36 nm and a spiral phase. This pump source is compact and cheap, but is easy to scale up its output pulse energy. Basing on this source, we then turns focus to generate terahertz ultrashort vortex pulses via the difference frequency generation by use of an organic crystal DSTMS as terahertz emitter due to its large nonlinear coefficient. It is predicted that the generated terahertz vortex pulses will possess a focused electric field of more than 10 MV/cm, a pulse duration of less than 1 ps, a topological charge value of 1, and a spectral region from 0.4 to 5.0 THz, while the optical-to-terahertz conversion efficiency will be up to 1%. This kind of intense terahertz vortex field has never reported publicly up to date. Correspondingly, in order to realize coherent detection without distortion for the intense terahertz field, we will also develop a novel electric-optic sampler for terahertz free-space temporal domain spectroscopy. It combines spatially encoded spectral interferometry with single-probe terahertz electric-optic sampling, which results in realizing single-shot terahertz coherent detection with high signal-noise ratio. We believe, the above work will fill in the gap in generating directly intense terahertz vortex field, and open an avenue to develop a table-top terahertz source with 100 MV/cm, GV/cm even stronger vortex electric field. Our novel electric-optic detection will provide with very powerful supports for intense terahertz science and its applications.

为满足强场、非线性太赫兹(THz)科学领域许多重要研究和应用需求，本项目将研究通过非线性光学差频高效直接产生强场THz超短涡旋脉冲。1）发展新型台式泵浦系统：能同时输出毫焦耳级、1560nm附近的一皮秒窄带非涡旋脉冲和一宽带涡旋啁啾脉冲，且脉冲能量有巨大升级空间。2）研究以上述脉冲泵浦DSTMS晶体高效直接产生10MV/cm级的亚皮秒THz涡旋脉冲，谱区为0.4~5.0 THz，拓扑荷值为1，转换效率达0.01量级，该结果至今未见报道。3）研究一种基于空间-光谱二维干涉与单光束THz电光取样技术相结合的新型时域光谱技术，实现对强场THz超短脉冲单次高信噪比的时域光谱检测。研究结果将解决发展100MV/cm、GV/cm甚至更高量级的台式THz超短涡旋脉冲系统关键科学技术问题，填补目前直接产生强场THz超短涡旋脉冲的技术空白，并为强场THz脉冲的应用提供非常有效的相干检测手段。

本项目围绕强场太赫兹 (THz) 超短涡旋脉冲的产生及其检测技术展开研究。首先在国际上首次在无需使用任何THz波段调制或模式转换器件的情况下实验上直接实现基于台式红外激光泵浦的THz涡旋脉冲。脉冲聚焦场强约2 MV/cm, 脉宽~2 ps，谱区为0.4~5.0 THz，拓扑荷取决于所用的Q片。这为高效产生强THz涡旋脉冲提供了一条有效的技术途径。其次，发展了一种THz时域光谱单次测量技术。与现有的技术比较，我们的测量信噪比高出一个数量级。可测量的THz场引入相位高达1000。该成果为THz强场领域的研究和应用提供强有力的检测手段，可推广、产业化。第三是成功发展了一种基于太赫兹时域光谱的单次THz脉冲偏振测量技术。该技术只需单次测量，无需光学扫描，因而测量信噪比高，高效、省时，抗干扰能力很强，具有重要的应用价值。第四，将光谱剪切技术与二步相移技术相结合，成功发展出一种新型的超短激光脉冲的电场恢复重建技术。该技术对于测量复杂时域形状或窄带激光脉冲测量能力有明显提升，测量的稳定性和信噪比都显著优于现有同类装置，该装置已接近商品化，可推广、产业化。第五，发展高强度圆偏THz场产生及其偏振控制技术。所产生的圆偏THz场能量是同条件下线偏双色场产生THz强度的2.5倍。该方案相比于传统的非共线圆偏双色场方技术具备很好的抗空气扰动、设备机械振动等的外界环境干扰的能力，有助于提升THz源的稳定性。另外，还利用多丝技术有效减少饱和效应的限制，大幅提升THz的输出功率。并调节多丝间的相对时间延迟，可灵活控制THz波的偏振态。最后，发展一种同时具有高时间分辨、高空间分辨、高帧频的单次多幅成像技术。有效摄影频率达15 Tfps、时间分辨50 fs，同时空间分辨达到2 um。该成果被著名光学工程SPIE网站任务是开创了史无前例的摄影频率，可推广、产业化，具有重要的应用价值。