Non-equilibrium Dynamics of Light-Matter Interactions at Low-Dimensional Nano-Structured Metal-Dielectric Interfaces Probed by Soft X-ray Absorption Spectroscopy

• Applicant: Themistoklis Sidiropoulos, Ph.D.
• Subject Area: Experimental Condensed Matter Physics ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 558545993

Future quantum technologies will rely on an efficient and fast manipulation of correlated states in condensed matter systems. To date, the control of correlated states in equilibrium is well advanced, but practical operating conditions are restricted to low temperatures and GHz frequencies. In contrast, using light to generate correlated states, operating speeds can be increased to PHz frequencies and through a strong interaction of light with matter many novel quantum phenomena emerge even at room temperature. While correlated states emerging out of non-equilibrium conditions have been demonstrated, their control remains challenging due the general weak interaction of light with matter, the excitation of quasiparticles, which interact with the neighbouring electromagnetic environment, and the ultrashort time scales involved. A consequence of the weak interaction of light with matter is the need for strong optical fields, which can be achieved by high power lasers or by focussing light to small volumes using nano-photonic geometries. Yet, in dielectric materials the confinement of light is diffraction-limited, thus only weak light-matter coupling occurs, which merely enhances and accelerates optical processes. Current research is therefore exploring hybrid metal-dielectric geometries to focus light to sub-wavelength scales, enabling strong light-matter interactions and paving the way for future light-based quantum technologies. The development of novel light sources based on the process of high-harmonic generation (HHG) in noble gases, reach soft X-ray photon energies with pulse lengths below 100as, which enabled the observation of light-field driven electron dynamics. In addition to exploiting the unprecedented temporal resolution, the broadband spectrum provides access to the energetic redistribution of optically excited carriers and their coupling to quasiparticles in real-time. The broadband HHG spectra cover absorption edges of multiple elements, which allow following the transfer of photo-excited charges across interfaces. Thus, HHG spectroscopy in the in X-ray spectral region gives access to the energetic redistribution of non-equilibrium many-body systems in the weak and strong coupling regime at metal-2D interfaces with unprecedented temporal resolution. The aim of the proposal is to study the energy transfer dynamics between coupled light-matter states in the weak and strong coupling regime. To this end we will prepare nanostructured metals that show plasmon resonances in the NIR. We will transfer a 2D material on the nanostructured metal and identify the energy transfer mechanism of hot electrons across such an interface. Precise control of the plasmon resonance will enable a strong coupling to excitons in the 2D material. We will probe the time- and energy-dependent carrier occupation in each material with a high harmonic soft X-ray light source in order to identify the mechanisms that limit an efficient coupling.

• DFG Programme: Research Grants