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.

Automated summary

Time-periodic light field has been used to manipulate quantum states in solid-state materials, cold atoms, and photonic systems. This is achieved through interaction with photon-dressed Floquet states in the strong-coupling limit, known as Floquet engineering. In this study, experimental evidence of momentum-resolved Floquet band engineering in a model semiconductor, black phosphorus, is reported using time and angle-resolved photoemission spectroscopy measurements. Strong band renormalization and light-induced dynamical gap opening are observed near the band edges under near-resonance pumping, along with the emergence of Floquet sidebands. The band renormalization shows a selection rule favoring pump polarization along the armchair direction, indicating pseudospin selectivity for Floquet band engineering enforced by lattice symmetry. This work demonstrates pseudospin-selective Floquet band engineering in black phosphorus and provides important guiding principles for Floquet engineering of semiconductors.