Yunquan Liu

Rotational Doppler effect is the angular counterpart of the linear Doppler effect: a receiver rotating in a wave field with an azimuthally varying phase can experience a shifted frequency of the wave. Here, we theoretically reveal a novel ultrafast rotational Doppler effect arising from electron motion during strong-field ionization driven by optical vortices. Using the strong-field approximation, we show that the resulting above-threshold-ionization rings in momentum space are twisted by the spiral phase of the optical vortices. The underlying mechanism is that photoelectrons emitted in different directions experience different effective frequencies of the driving laser field. The frequency shift induced by this ultrafast rotational Doppler effect depends sensitively on the position of the atoms relative to the phase singularity of the optical vortices. Our results have broad implications for investigating electron–light interactions in spatially structured strong laser fields.

Free-electron interactions with optical near fields are conventionally described as one-dimensional energy ladders, in which multicolor photon-exchange pathways can become spectrally degenerate. In this work, we show that coupling a single free electron to commensurate two-color optical vortices can unfold this degeneracy, realizing a tunable synthetic lattice spanned by the electron energy and orbital angular momentum (OAM). The resulting single-particle dynamics map the free-electron light interaction onto an effective tight-binding Hamiltonian whose primitive hopping vectors and coupling anisotropy are controlled by the optical OAM charges and near-field amplitudes. We use this construction to emulate a Lifshitz transition in the synthetic quasienergy band structure, identified by a topological reconnection of the zero-quasienergy contour and the associated Van Hove singularity in the density of states. We further show that the generated electron state exhibits tunable intraparticle energy-OAM mode nonseparability, which serves as a diagnostic of the synthetic-lattice geometry. These results establish free electrons as a tunable platform for synthetic-band engineering, combining electron-wavefunction control with ultrafast electron-microscopy and nanoscale near-field techniques.

Generating ever-shorter and brighter light pulses is a central goal of ultrafast science, enabling coherent control and observation of electron dynamics on their natural timescale. State-of-the-art isolated attosecond pulse generation currently achieves pulse durations of 40-50 attoseconds. Here we demonstrate isolated attosecond light pulses with durations of 18 attoseconds, via high-order harmonic generation driven by a post-compressed industrial-grade Yb laser system. The high-harmonic spectrum spans photon energies from 50 to 320 eV, covering the carbon K-edge, with a calibrated photon flux exceeding 10¹² photons per second. Pulse durations were characterized by angle-resolved photoelectron streaking in helium and optimized using a series of filters with different thicknesses to compensate the attochirp. We further developed a robust, fast-converging pulse-retrieval algorithm capable of reconstructing broadband isolated attosecond pulses and double pulses. These results establish Yb-laser-driven high-order harmonic generation as a powerful platform for bright isolated attosecond sources and next-generation ultrafast spectroscopy.

Three-level systems exhibit quantum interference effects absent in two-level systems, making them important for quantum optics. Here, we study the coherent interaction of a $\mathrm{\ensuremath{\Lambda}}$-type three-level system with free electrons shaped by optical near fields. By treating the electron train as a quantum drive, we show that the interplay between electron modulation and the three-level system's transition pathways induces tunable interference patterns. This interaction effectively realizes electron-mediated coherent population trapping. We identify a regime that enables complete population transfer between the two lower states and the preparation of a high-coherence superposition, manifested as dark states. In particular, these driven-dissipative steady states are independent of the initial state. Our Letter proposes shaped free electrons as a platform for steady-state coherent control of three-level systems, enabling atomic-scale state engineering.

The role of edge states in van der Waals heterostructures, specifically whether they enhance or inhibit charge recombination, has remained unclear due to a lack of direct nanoscale spatial and temporal probes. Using time-resolved photoemission electron microscopy with combined femtosecond temporal and nanometer spatial resolution, we observe that edge states in a WSe ₂ - ReS ₂ heterojunction significantly slow interlayer electron relaxation. In contrast to isolated monolayer ReS ₂ , where edges act as efficient recombination centers, the same edge regions in the heterostructure strongly suppress interlayer recombination and markedly prolong the carrier lifetime at the heterointerface. First-principles calculations attribute this functional reversal to an edge-localized flat band in ReS ₂ that is energetically aligned with the valence-band maximum of WSe ₂ . This configuration introduces momentum mismatch and spatial separation that effectively impede direct electron–hole recombination across the interface. Our results demonstrate that edge states are not merely structural defects but play a crucial role in governing interlayer electron dynamics through heterostructure engineering. This work provides a new design strategy for controlling charge separation and transport in two-dimensional optoelectronic devices via tailored interfacial band structure.

Individual nanostructures with high quality factors ( Q factors) are highly desirable for applications such as deterministic single-photon sources, quantum light generation, and single-molecule spectroscopy, where strong light–matter interaction and nanoscale field confinement are required. However, achieving high Q factors in deeply subwavelength plasmonic systems remains fundamentally limited by strong radiative damping. Here, we demonstrate that the Q factor of plasmonic modes can be significantly enhanced in a coupled gold nanoantenna system by engineering quasi-bound states in the continuum (quasi-BICs). The formation of quasi-BICs, confirmed by quasi-normal mode analysis, enables strong suppression of radiative loss through modal interference. As a result, the Q factor reaches up to 40, representing a substantial improvement over conventional plasmonic resonances at the nanoscale. Furthermore, the Purcell factor for dipole emitters placed in the structure reaches values as high as ∼358, indicating strong enhancement of light–matter interaction. These results provide a practical strategy for optimizing plasmonic nanoantenna systems, particularly for applications in single-photon emission and nanoscale light sources.

We study tunable, nanoscale, femtosecond coherent radiation based on a coupled nanowire pair structure, which is transversely excited by a strong, linearly polarized laser pulse. The structure can function as a nanoscale undulator: the electrons moving through the nanogap are driven by a spatially periodic, transverse optical near field. We show that the near field can actively shape the electron wave function by inducing both a periodic oscillation and a quantum squeezing of its width. We then validate this theoretical framework by numerically solving the relativistically corrected time-dependent Schrödinger equation. The generated femtosecond pulse trains can be spectrally, temporally, and spatially controlled. This framework establishes the transverse optical near-field interaction as a novel mechanism for spatiotemporally shaping electron wave functions, thereby illuminating a path to a versatile platform for an on-chip femtosecond coherent light source and applications in free-electron quantum optics.

Autler-Townes splitting (ATS) is a hallmark phenomenon of strong light-matter interaction, in which the resonant photoelectron energy spectrum splits into distinct peaks when driven by a strong electromagnetic field. We investigate the Autler-Townes Splitting (ATS) in K₂ molecules with quantum light. The triple splitting structures originating from three-photon ionization are analyzed under different types of driving fields, including bright squeezed vacuum (BSV), phase-squeezed coherent (PSC) light and amplitude-squeezed coherent (ASC) light by numerically solving the time-dependent Schrödinger equation. Compared with the results with the coherent-state (classical) light drive, we show that the ATS dynamics is sensitive with the squeezing parameter of quantum light, leading to modifications of the subband structures and splitting magnitudes. In particular, a BSV pulse exhibits a higher splitting efficiency than a classical pulse of the same mean intensity. This work has implications on ultrafast coherent control of molecular reactions with quantum light.

The influence of the quantum state of light on strong-field ionization has emerged as a frontier in attosecond physics. Here, we develop a quantum-optical extension of the quantum-trajectory Monte Carlo framework to study the ionization of hydrogen atoms driven by an orthogonally polarized two-color quantum field, composed of an 800 nm coherent field and a 1600 nm squeezed coherent field. We show that the quantum properties of light significantly influence the subcycle dynamics of photoelectron wave packets. We systematically analyze how the quantum statistics of light influence the photoelectron phase, momentum, yield, and number distribution. Our results reveal a direct correspondence between the quantum fluctuations of the driving field and the electron final state, unveiling an avenue for controlling electron wave packet interference via the quantum properties of light.