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转变的原子运动，极大地帮助创建了原子分辨结构动力学的新领域[Siwick‘03，Dwyer’06，Zewail‘06]。为了捕捉化学反应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，需要与适当的泵浦和探测激光器紧密同步的非常短的电子束，而飞秒精度很低。实现短束的一个主要挑战是库仑排斥，这会导致束在到达样品之前扩散。排斥力可以通过牺牲束团电荷来克服[Baum‘13]。为了保持足够的信噪比，需要提高重复频率，这可能会与不可逆或半可逆样品，特别是有机样品不相容。或者，也可以使用再聚束腔[Oudheusden‘10]，但这需要很高的电磁场梯度。通常，现代加速器是由多厘米长的微波信号驱动的，受材料场致击穿的限制，加速度梯度低于100 mV/m。通过将加速场频率增加到太赫兹区域，可以使用更高的场梯度和更小的装置，从而增加压缩力并减少自排斥的时间尺度[Siwick‘03]，从而允许更短的束流。太赫兹脉冲也可以光学产生，消除了几百飞秒级别上困扰微波器件的定时抖动[Gao‘12，Chatelain’12]。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前沿，该技术能够构建分辨率低于100 fS的紧凑型太赫兹超快电子衍射仪(THz-UED)。这个项目的基础是来自德国和俄罗斯团队的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]，以及基于THz的光枪[Huang‘16]和LINAC[Nanni’15]的原理验证演示。德国团队最近的工作表明，强场太赫兹辐射可用于加速、压缩、聚焦和诊断持续时间在1-100秒范围内的电子束和本征激光同步[Fallahi‘16，Zhang’17]。将开发用于THz-UED的时间分辨率低于100fs的实用电子枪，并用于提高分辨率，超出目前的能力，例如声子参与钒氧化物的绝缘体-金属相变[O‘Callahan’14]，以及蛋白质和DNA中的超快动力学，据信发生在10fs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.