Pseudospin-selective Floquet band engineering in black phosphorus.

Time-periodic light field has emerged as a control knob for manipulating quantum states in solid-state materials 1-3 , cold atoms 4 and photonic systems 5 through hybridization with photon-dressed Floquet states 6 in the strong-coupling limit, dubbed Floquet engineering. Such interaction leads to tailored properties of quantum materials 7-11 , for example, modifications of the topological properties of Dirac materials 12,13 and modulation of the optical response 14-16 . Despite extensive research interests over the past decade 3,8,17-20 , there is no experimental evidence of momentum-resolved Floquet band engineering of semiconductors, which is a crucial step to extend Floquet engineering to a wide range of solid-state materials. Here, on the basis of time and angle-resolved photoemission spectroscopy measurements, we report experimental signatures of Floquet band engineering in a model semiconductor, black phosphorus. On near-resonance pumping at a photon energy of 340-440 meV, a strong band renormalization is observed near the band edges. In particular, light-induced dynamical gap opening is resolved at the resonance points, which emerges simultaneously with the Floquet sidebands. Moreover, the band renormalization shows a strong selection rule favouring pump polarization along the armchair direction, suggesting pseudospin selectivity for the Floquetband engineering as enforced by the lattice symmetry. Our work demonstrates pseudospin-selective Floquet band engineering in black phosphorus and provides important guiding principles for Floquet engineering of semiconductors.