Generation of sub-picosecond electron bunches by strong terahertz fields for high gradient electron acceleration and ultrafast electron diffractive imaging

基于加速器的超亮电子源通过直接观察控制结构跃迁的原子运动，极大地帮助创建了原子解析结构动力学的新领域[Siwick ' 03, Dwyer ' 06, Zewail ' 06]。为了捕捉化学和生物反应中最快的核运动，时间分辨率的前沿已经进入飞秒范围。因此，需要非常短的电子束与适当的泵浦和探测激光器紧密同步，精度很少飞秒。实现短束的主要挑战是库仑斥力，它会导致束在到达样品之前扩散。排斥可以通过牺牲束荷来克服[Baum ' 13]。为了保持足够的信噪比，需要增加重复率，这可能与不可逆或半可逆样品，特别是有机样品不相容。或者，可以使用重聚空腔[Oudheusden ' 10]，但这需要高电磁场梯度。现代加速器通常由多厘米尺度的微波信号驱动，加速度梯度在100 MV/m以下，受材料场致击穿的限制。通过将加速场频率提高到太赫兹范围，可以使用更高的场梯度和更小的器件，增加压缩力并减少自斥力的时间尺度[Siwick ' 03]，从而实现更短的束。太赫兹脉冲也可以光产生，消除困扰微波器件的时间抖动在几百fs水平[Gao ' 12, Chatelain ' 12]。支持基础设施也减少了，使科学界更容易获得这些设备。在这里，我们解决了这些问题，并通过开发太赫兹驱动的加速器技术来推动分辨率的前沿，该技术能够构建具有低于100 fs分辨率的紧凑型太赫兹超快电子衍射仪（THz-UED）。该项目的基础是德国和俄罗斯团队高效产生太赫兹脉冲的演示[Bodrov'13, Wu' 14, Vicario'14, Vicario'15, Wu'16, Ahr'17]，以及高梯度加速器结构理论的发展[Kuzikov'10, Kuzikov'16]和基于太赫兹的光电枪的原理证明演示[Huang '16]和LINACs [Nanni '15]。德国团队最近的工作表明，强场太赫兹辐射可以用来加速、压缩、聚焦和诊断持续时间和内在激光同步在1 - 100 fs范围内的电子束[Fallahi'16, Zhang'17]。将开发用于时间分辨率低于100秒的太赫兹- ued的实用电子枪，并用于提高当前能力之外的分辨率，例如声子参与氧化钒的绝缘体-金属相变[O' callahan '14]，以及蛋白质和DNA的超快动力学，被认为发生在10秒的时间尺度上。

Ultrabright electron sources based on accelerators have helped greatly to create the new field of atomically-resolved structural dynamics by enabling direct observation of atomic motions governing structural transitions [Siwick’03, Dwyer’06, Zewail’06]. The frontiers of temporal resolution have moved into the femtosecond regime in order to capture the fastest nuclear motions involved in chemical and biological reactions. Therefore, very short electron bunches tightly synchronized to appropriate pump and probe lasers with few femtosecond precision are needed. A primary challenge in achieving short bunches is Coulombic repulsion which causes bunch spreading before reaching the sample. Repulsion can be overcome by sacrificing bunch charge [Baum’13]. To maintain sufficient signal-to-noise calls for increased repetition rate which may become incompatible with irreversible or semi-reversible samples, especially organic samples. Alternatively, a rebunching cavity [Oudheusden’10] can be used, but this requires high electromagnetic field gradients. Usually, modern accelerators are driven by microwave signals with multi-centimeter length scales and have acceleration gradients below 100 MV/m limited by field-induced breakdown of materials. By increasing acceleration field frequencies to the THz regime, higher field gradients and smaller devices can be used, increasing compressive forces and reducing time-scales for self-repulsion [Siwick’03] allowing for shorter bunches. THz pulses can also be optically generated, eliminating timing jitter plaguing microwave devices at the few hundred fs level [Gao’12, Chatelain’12]. Supporting infrastructure is also reduced, making devices more accessible to the scientific community.Here, we address these issues and push the resolution frontier by developing THz-driven accelerator technology which enables the construction of a compact terahertz-based ultrafast electron diffractometer (THz-UED) with sub-100 fs resolution. The groundwork for this project has been laid by demonstrations from both German and Russian teams of highly efficient generation of THz pulses [Bodrov'13, Wu’14, Vicario'14, Vicario'15, Wu'16, Ahr'17] as well as development of theory of high-gradient accelerator structures [Kuzikov'10, Kuzikov'16] and proof-of-principle demonstrations of THz-based photoguns [Huang’16] and LINACs [Nanni’15]. Recent work by the German team indicates that strong-field THz radiation can be used to accelerate, compress, focus and diagnose electron bunches with durations and intrinsic laser synchronization in the 1 - 100 fs range [Fallahi'16, Zhang'17]. Practical electron guns for THz-UED with sub-100 fs temporal resolution will be developed and used to enhance resolution beyond current capabilities, such as phonon involvement in the insulator-metal phase transition of Vanadium-Oxide [O'Callahan'14] and ultrafast dynamics in proteins and DNA, thought to occur on a 10 fs time scale.