QUMATTO · Quantum Materials Probed with Attosecond Optoelectronics

Quantum Materials Probed with Attosecond Optoelectronics

The feasibility to sculpt light at the level of individual oscillations has allowed sub-femtosecond (1fs = 10-15s) monitoring and control of electron dynamics in gas phase [Rev. Mod. Phys. 81 (2009)]. Recently, such technology and the theory that underpins it is being transferred into the solid state. The project QUMATTO combines theory and experiment to bring strong-field physics into lightwave control of quantum properties in crystals. One of the key aims of this project is the use of strong, polarization-tailored fields with spatial symmetry matching the symmetry of the crystal lattice in order to modify the electronic and topological properties of the crystal on ultrafast timescales. During the Action, we demonstrated few-femtosecond control over the bandstructure properties of hexagonal materials using trefoil-symmetric laser fields. In particular, we realized non-resonant control of the valley pseudospin in a hexagonal boron nitride monolayer [arXiv:2303.13044], as well as in bulk MoS2 [arXiv:2302.12564]. Probing of the electronic dynamics in the laser-modified material is achieved via high harmonic generation (HHG), where the sub-laser-cycle electron motion is mapped onto the properties of the light emitted by the oscillating dipole of the driven system [Nat. Phys. 15 (2019)]. Current theoretical modelling of HHG in solids is framed in the reciprocal space, and based on the generation of non-linear currents through intraband electron motion and interband electron-hole recombination processes [Phys. Rev. Lett. 113 (2014); Nature 523 (2015)]. Yet, this framework fails for materials with complex band structures, e.g., very dense or with multiple band crossings. During the Action, we performed HHG experiments in bulk ReS2, a complex material with a dense and flat band structure. The HHG yield showed a strong intensity-dependent anisotropy, in contrast to other prototypical materials like MgO or ZnO. To explain this, we developed an orbital-based framework of HHG in crystals that allowed us to interpret such sharp anisotropic features as interference of currents generated by the different atoms in the unit cell, as well as to show that the contribution of particular atoms to the HHG emission can be enhanced or suppressed by tuning the laser parameters [arXiv:2309.06290], providing an unprecedented atomic perspective of strong-field dynamics in crystals. The duration of the Action was of 17.5 months due to an early termination of the Agreement.

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