Measurement station for ultrafast terahertz-driven photoemission spectroscopy

该仪器基本上结合了定制的强场太赫兹（THz）激发与时间分辨角分辨光电子能谱（tr-ARPES）实验。它将使我们能够以子周期时间分辨率和足够快的速度观察到各种材料中的超快非平衡动力学，这些材料在强大的“自下而上”激发每种材料的基本，最低能量模式的独特条件下具有THz辐射。狄拉克材料中的能流和超快非平衡态发展等物理问题（例如石墨烯、3D狄拉克半金属、拓扑绝缘体）、磁性材料（如稀土正铁氧体）、Peierls半金属（铋或砷）和货车德瓦耳斯异质结构（例如WSe 2）将在材料中最低能量模式直接激发的独特条件下进行研究（费米能级周围的传导电流、红外活性光学声子、磁振子）。材料的“自下而上”THz激发将避免电子或晶格子系统的寄生激发，例如在超快光学激发的情况下，导致电子带-带跃迁和晶格的拉曼激发。使用ARPES，我们将能够直接观察自下而上的能量沉积到材料中对能带结构和布居的影响。此外，在外部强太赫兹场的存在下，光电子本身的效果将被研究，通过在“较慢”（多）太赫兹驱动场的光电子条纹，相比通常使用的红外信号。这将为我们提供独特的访问更长的时间尺度的光电子动力学。我们的装置将由飞秒激光器提供动力，该飞秒激光器以100 kHz的重复率提供5 mJ能量和1030 nm中心波长的脉冲。激光器将配备光转换级，允许从EUV到THz的发射。激光器的高重复频率使得能够在足够短的采集时间内采集时间和角度分辨的光电发射数据，以防止样品表面退化。这将使我们能够执行我们的实验上广泛的一类“脆弱”的材料，如（半）金属，半导体，磁性材料，分子装饰的表面等作为电子分析仪，动量显微镜将被使用，其中，除了信息的能量动量依赖的光电发射电子状态，也允许在成像模式下操作样品检查。一个可变温度的样品阶段将允许我们，特别是，研究材料的动力学经历相变。5 mJ的驱动激光器的相对高的脉冲能量将使得能够经由优化的光学整流产生强场THz，产生具有在0.1 - 1 MV/cm范围内的电场强度和具有0.1 - 40 THz频率内容的单周期和多周期THz脉冲。

The proposed instrument essentially combines tailored strong-field terahertz (THz) excitation with time-resolved angular-resolved photoemission spectroscopy (tr-ARPES) experiment. It will allow us to observe, with sub-cycle time resolution and fast enough to prevent the surface degradation, the ultrafast nonequilibrium dynamics in a wide class of materials, under unique conditions of powerful “bottom-up” excitation of each material’s fundamental, lowest-energy modes with THz radiation. Physical problems such as energy flow and the development of ultrafast non-equilibrium in Dirac materials (e.g. graphene, 3D Dirac semimetals, topological insulators), magnetic materials (e.g. rare-earth orthoferrites), Peierls semimetals (bismuth or arsenic) and van der Waals heterostructures (e.g. WSe2) will be studied, under the unique conditions of direct excitation of lowest-energy modes in the materials (conduction currents around the Fermi level, infrared-active optical phonons, magnons). “Bottom-up” THz excitation of materials will avoid the parasitic excitations of electronic or lattice subsystems, such as in the case of ultrafast optical excitation, leading to electronic band-to-band transitions and Raman excitation of the lattice. Using ARPES, we will thus be able to directly observe the effect of bottom-up energy deposition into the material on bandstructure and population. Further, the effect of photoemission itself in the presence of external strong THz-fields will be studied, by photoelectron streaking in “slower” (multi-)THz driving fields, as compared to typically used infrared signals. This will provide us unique access to longer timescales in photoemission dynamics. Our setup will be powered by a femtosecond laser delivering the pulses of 5 mJ energy and central wavelength of 1030 nm, at a repetition rate of 100 kHz. The laser will be equipped with light conversion stages allowing emission from EUV to THz. The high repetition rate of the laser enables the acquisition of time- and angle-resolved photoemission data in short enough acquisition times to prevent sample surface degradation. This will allow us to perform our experiments on a wide class of “vulnerable” materials, such as (semi-)metals, semiconductors, magnetic materials, molecular-decorated surfaces etc. As an electron analyzer, the momentum microscope will be used, which, apart from the information on the energy-momentum dependency of the photoemitting electronic state, also allows operation in an imaging mode for sample inspection. A variable temperature sample stage will allow us, in particular, to study the dynamics of materials undergoing phase transitions. The relatively high pulse energy of the driving laser of 5 mJ will enable strong-field THz generation via optimized optical rectification, yielding single- and multi-cycle THz pulses with electric field strength in the rage 0.1 – 1 MV/cm, and with the frequency content 0.1 – 40 THz.