Measurement station for ultrafast terahertz-driven photoemission spectroscopy

该仪器实质上结合了定制的强场太赫兹(THz)激发和时间分辨角度分辨光电子能谱(TR-ARPES)实验。它将使我们能够以亚周期时间分辨率和足够快的速度观察到一大类材料中的超快非平衡动力学，在独特的条件下，用太赫兹辐射对每种材料的基本、最低能量模式进行强大的自下而上的激发。在直接激发材料中最低能量模式(费米能级附近的传导电流、红外活性光学声子、磁子)的独特条件下，将研究Dirac材料(例如石墨烯、3D Dirac半金属、拓扑绝缘体)、磁性材料(例如稀土铁氧体)、Peierls半金属(铋或砷)和van der Waals异质结构(例如WSe2)中的能量流动和超快非平衡发展等物理问题。材料的“自下而上”太赫兹激发将避免电子或晶格子系统的寄生激发，例如在超快光激发的情况下，导致电子带到带的跃迁和晶格的拉曼激发。因此，使用ARPES，我们将能够直接观察到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，与通常使用的红外信号相比，将通过光电子在较慢的(多)太赫兹驱动场中的光电子条纹来研究存在外部强太赫兹场时的光电发射本身的影响。这将为我们提供独特的途径，了解光电发射动力学中更长的时间尺度。我们的装置将由飞秒激光供电，以100千赫的重复频率发射能量为5MJ、中心波长为1030 nm的脉冲。激光器将配备光转换阶段，允许从极紫外光发射到太赫兹。激光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。这将使我们能够在一大类“脆弱”材料上进行实验，如(半)金属、半导体、磁性材料、分子装饰表面等。作为一种电子分析器，动量显微镜将被使用，除了关于光电发射电子态的能量-动量依赖的信息外，它还允许在成像模式下操作以进行样品检查。变温样品台将特别使我们能够研究经历相变的材料的动力学。5mJ的驱动激光具有较高的脉冲能量，通过优化的光整流，可以产生强场太赫兹脉冲，产生场强在0.1-1 mV/cm，频率在0.1-40THz的单周和多周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.