SR-XTRS-2DLayMat · Self-Referenced XUV Transient Reflectivity Spectroscopy: Attosecond Electron Dynamics of 2D Layered Materials

Self-Referenced XUV Transient Reflectivity Spectroscopy: Attosecond Electron Dynamics of 2D Layered Materials

The aim of the project is to advance the method of attosecond extreme ultra-violet (XUV) transient reflectivity spectroscopy (XTRS) beyond its current state of the art with the aim to better understand ultrafast electron dynamics in solid state materials, especially novel layered and 2D materials. Since solid state materials are truly many-body systems, reaching high densities, electrons can quickly exchange their energy, spin and momentum with other electrons or the crystal lattice. These fast relaxation processes require sophisticated time-resolved spectroscopy methods with resolutions at the attosecond range to be understood in their fundamental behavior. Attosecond XTRS uses extremely short laser pulses of attosecond duration in the XUV spectral range, where core-level transitions are available. These core-level transitions, in which an electron is excited from a highly localized core-orbital at element-characteristic wavelengths into unoccupied valence states can give us a localized picture into the complex electronic structure of solids, aiming in the understanding and interpretation of experimental results. The method is closely related to attosecond XUV transient absorption spectroscopy (XTAS): While samples are investigated and spectroscopic data is recorded in a transmission geometry in XTAS, a reflection spectroscopy is used in XTRS. This has several advantages over the transmission geometry including the availability of higher quality samples, larger signal contrasts and surface sensitivity, to name a few. However, the method comes with a significant drawback, which has so far hindered its widespread usage. This is due to the increased complexity in the data analysis due to the mixing of dispersive and absorptive parts (real and imaginary) of the optical response (dipole response function). The proposed solution to this problem within this action is to implement a self-referenced, interferometric approach, which will allow to directly reconstruct the spectral amplitude and phase of the optical response in an XTRS experiment and thus recover the seperation of real and imaginary part of the dipole response function. Additionally, the proposed optical setup will allow to use a fast modification of the XUV spectrum to apply lock-in approaches to XTRS to increase the signal-to-noise ratio achievable in state-of-the-art XUV beamlines. Both parts will be crucial to further our understanding of ultrafast carrier dynamics in solid materials. We set out to apply these methods to few- and monolayer materials, such as heterojunctions of transition metal dichalcogenides and study their (coherent) carrier transport properties, interactions with the lattice and spin dynamics with attosecond resolution. To set out to this goal we plan to address these phenomena in prototypical samples and apply our knowledge gained there to novel and more unfamiliar materials. The advancement of XTRS will help to further establish this method as a powerful tool to understand carrier dynamics in solid state materials.

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