Papers
paper
Long-lived coherence between incoherent excitons revealed by time-resolved angle-resolved photoemission spectroscopy: An exact solution
June 2025
We investigate the exciton dynamics in an exactly solvable two-band model for semiconductors. The model incorporates light-matter, electron-electron, and electron-phonon interactions, and captures exciton formation as well as the transition from the coherent to the incoherent regime. We analyze excitonic polarization, populations, and coherences, with special focus on their impact in time-resolved angle-resolved photoemission spectroscopy (TR-ARPES). For nonresonant pumping with below-gap photon energies, TR-ARPES spectra reveal distinct excitonic replica and quantum beats persisting in the incoherent regime. These are due to a coherence between different species of incoherent excitons. Such type of coherence is resistant to phonon dephasing, indicating that it follows different dynamics than those governing the coherences considered so far.
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Observation of Floquet–Bloch states in monolayer graphene
May 2025
Floquet engineering enables the manipulation of quantum phases of matter through periodic driving. It has been implemented across different platforms, ranging from photonic systems to optical lattices of ultracold atoms. In solids, coherent light–matter interaction induced by periodic driving leads to hybridization of Bloch electrons with photons, resulting in the formation of replica bands known as Floquet–Bloch states. These states have been observed in several materials, and their properties have been linked to a range of predicted phase transitions. However, direct energy and momentum-resolved observation of these states remains limited to a few. Here we report the direct observation of Floquet–Bloch states in monolayer epitaxial graphene. By using time-resolved and angle-resolved photoemission spectroscopy with mid-infrared pump excitation, we detected replicas of the Dirac cone. The dependence of these replica bands on pump polarization shows that they originate from the scattering between Floquet–Bloch states and photon-dressed free-electron-like photoemission final states, known as Volkov states. Our method can potentially be used to directly observe Floquet–Bloch states at large momenta in other quantum materials.
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paper
Light-matter interactions in layered materials and heterostructures: from moiré physics and magneto-optical effects to ultrafast dynamics and hybrid meta-photonics
April 2025
Layered two-dimensional (2D) materials have revolutionized how we approach light–matter interactions, offering unprecedented optical and electronic properties with the potential for vertical heterostructures and manipulation of spin–valley degrees of freedom. The discovery of moiré physics in twisted heterostructures has further unlocked new possibilities for controlling the band structure of tailored semiconductor heterostructures. In parallel, the integration of 2D materials with hybrid photonic structures and ultrafast studies on their optical and spin–valley properties has revealed a wealth of novel physical phenomena. This perspective highlights the recent advances in our understanding of light–matter interactions in moiré and 2D systems, with a particular emphasis on ultrafast processes and the integration of these materials into photonic platforms. We explore the implications for optoelectronics and emerging photonic technologies, positioning 2D materials as a transformative tool for next-generation devices.
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