The main results are a) Photoinduced demagnetization of magnetic insulators paves the way for launching ultrafast dynamics of spins, which can't be reached in terms of conventional methods to modulate the microscopic magnetism. Owing to the simple honeycomb crystal structure, the ruthenium-based compound α-RuCl3 provides an attractive platform to explore the physics of electronic correlations, unconventional magnetism and opto-magnetic effects in correlated insulators. It is illustrated that α-RuCl3 accommodates essential ingredients of the Kitaev model owing to the interplay of electron correlations and magnetic interactions, facilitating a variety of exotic quantum phases. These findings provide physical insights into the coupling between the electronic and magnetic degrees of freedom in α-RuCl3 and shed light on suppressing the long-range magnetic orders and reaching a proximate spin liquid phase for two-dimensional magnets on an ultrafast timescale.
b) High harmonic generation (HHG) has recently been established as a powerful method for probing ultrafast electron dynamics in solids. However, it remains unknown if HHG can be similarly applied for probing lattice distortions such as phonons. Specifically, it is unclear if the extreme nonlinearity of HHG can contribute to enhanced temporal resolution or sensitivity for probing lattice dynamics. We presented a pump-probe and multidimensional spectroscopy approach that relies on carrier-envelope-phases sensitivity, in which HHG is highly sensitive for phonon dynamics. We further showed that in the regime where the excited phonon period and the pulse duration are of the same order of magnitude, the HHG process becomes sensitive to the carrier envelope phase (CEP) of the driving field, even though the pulse duration is so long that no such sensitivity is observed in the absence of coherent phonons. The degree of CEP sensitivity versus pump-probe delay is shown to be a highly selective measure for instantaneous structural changes in the lattice, providing an approach for ultrafast multidimensional HHG spectroscopy.
In addition, we theoretically investigated the role of collective coherent vibrations in HHG in a wide range of solids (e.g. hBN, graphite, 2H-MoS2, and diamond). We predicted that phonon-assisted high harmonic yields can be enhanced significantly compared to the phonon-free case – up to a factor of ~20 for the transverse optical phonon in bulk hBN. We also showed that the emitted harmonics strongly depend on the character of the pumped vibrational modes. Through state-of-the-art ab initio calculations, we elucidate the physical origin behind the HHG yield enhancement – phonon-assisted photoinduced carrier doping, which plays a paramount role in both intraband and interband dynamics. Our work showed a direct path for understanding phonon-mediated nonlinear optical processes in materials and provides a significant knob to engineer and control solid-state high harmonics.
This work was published in renowned Journals including Science Advances, Proc. Natl. Acad. Sci. U.S.A. Nano Letters and Physical Review Letters. Five more papers are under review and will be published soon. Furthermore, the researcher participated in several dissemination activities i.e. conferences, seminars, invited talks and workshops in Germany and China (2020-2022) Progress beyond the state of the art, expected results until the end of the project and potential impacts (including the social and economic impact and the wider societal implications of the project so far). Following the results that we have achieved, we aim to propose new way to manipulate the topological and electronic properties in the low-dimensional materials via non-equilibrium methods, such as the classic light and quantum light. When we apply the lase to solid states, the light will couple to electronic degrees of freedom directly or the quasiparticles. Therefore, we will distinguish two levels of accuracy of modeling the light field: either neglecting or keeping its quantum nature.
According to the activities outlined in the proposal, we report the researcher attending five conferences and workshops, including, Quantum Dynamics in Tailored Intense Fields (QUTIF), Max-Born-Institut, Berlin (Feb. 26-28, 2020). More over acknowledgement of EU funding was also made visible in all slides and talks presented by the researcher as stated in the GA. Furthermore, during the fellowship, the researcher participated in the activities organized by the MPSD Theory Department for the DESY. Public events such as Science Night or Girl’s Days did not take place due to the COVID-19 related restrictions.
