During the reporting period we achieved the set scientific goals. A major breakthrough was the experimental demonstration of few-femtosecond controllable band structure engineering using polarization-tailored strong fields in hexagonal boron nitride (hBN) and MoS2. We used a light field composed of a circularly-polarized pulse and its counter-rotating second harmonic, generating a trefoil shape in space that matched the symmetry of the crystal lattice. By changing the time delay between the two pulses, the trefoil rotates in space, allowing to control fundamental symmetries of the dressed system, i.e. inversion and time-reversal (see Figure). In this way, we were able to: (i) realize the light-dressed analogue of the Haldane topological model in hBN, where the valley band gaps can be controlled on sub-laser-cycle timescales, (ii) demonstrate valley polarization in bulk MoS2, extending valleytronics to inversion-symmetric materials. These milestones were achieved in collaboration with the groups of, respectively, (i) Matthias Kling at Max Planck Institute for Quantum Optics (Munich, Germany) and (ii) Jens Biegert at the Institute of Photonic Sciences (ICFO, Barcelona, Spain).
Another milestone was achieved by combining angle-resolved HHG experiments performed on ReS2 flakes and a newly developed real-space framework of high harmonic generation in solids. This allowed us to resolve the attosecond dynamics responsible for high-harmonic emission on each atom of the unit cell of a ReS2 crystal, thus with picometer resolution. We demonstrated that interferences between the atomic currents in the lattice crucially determine the macroscopic emission, and can be manipulated with the parameters of the driving field (intensity and polarization), allowing to select high-harmonic emission from particular atomic orbitals.
Other highlights of this project included: (i) a theoretical proposal for a coherent control scheme that uses time and polarization-controlled short pulses to create an all-optical valley switch operating on less than 100fs, (ii) an extension of the reconstruction of attosecond beating by interference of two photon transitions (RABBIT) technique to solids, (iii, work in progress) HHG experiments on TiSe2 flakes to probe the (2x2x2) charge density wave transition occurring at cold temperatures (~200K). Also, we provided theoretical support to HHG measurements in atomic gases and solids using the codes developed in [R.E.F. Silva et al., Phys. Rev. B 100 (2019); S. Patchkovskii et al., Comput. Phys. Commun. 199 (2016)].
In the reporting period, we produced 5 peer-reviewed articles and another 5 that are currently under review. The results were disseminated in four scientific conferences, four seminars and two news reports.
