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A gauge-invariant formulation of interband and intraband currents in solids
摘要： Experiments and simulations in solid-state high harmonic generation often make use of the distinction between interband and intraband currents. These two contributions to the total current have been associated with qualitatively different processes as well as physically measurable signatures, for example, in the spectral phase of harmonic emission. However, it was recently argued [P. F?ldi, Phys. Rev. B 96, 035112 (2017)] that these quantities can depend on the gauge employed in calculations. Since physical quantities are expected to have gauge-independent values, this raises the question of whether the decomposition of the total current into interband and intraband contributions is physically meaningful or merely a feature of a particular mathematical representation of nature. In this article, we explore this apparent ambiguity. We show that a closely related issue arises when calculating instantaneous band populations. In both the case of inter/intraband currents and the case of instantaneous band populations, we propose definitions that are gauge invariant and thus allow these quantities to be calculated consistently in any gauge.
关键词： gauge invariance,solid-state,interband currents,band populations,intraband currents,high harmonic generation
更新于2025-09-09 09:28:46
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High-harmonic generation from solids
摘要： High-harmonic generation in atomic gases has been studied for decades, and has formed the basis of attosecond science. Observation of high-order harmonics from bulk crystals was, however, reported much more recently, in 2010. This Review surveys the subsequent efforts aimed at understanding the microscopic mechanism of solid-state harmonics in terms of what it can tell us about the electronic structure of the source materials, how it can be used to probe driven ultrafast dynamics and its prospects for novel, compact short-wavelength light sources. Although most of this work has focused on bulk materials as the source, recent experiments have investigated high-harmonic generation from engineered structures, which could form flexible platforms for attosecond photonics.
关键词： ultrafast dynamics,electronic structure,high-harmonic generation,solids,short-wavelength light sources,attosecond science
更新于2025-09-09 09:28:46
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Vortex Dynamics and Optical Vortices || Ultrashort Extreme Ultraviolet Vortices
摘要： Optical vortices are very attractive because they transport a well-defined orbital angular momentum (OAM) associated with the singularity of the beam. These singular beams, commonly generated in the optical regime, are used in a wide range of applications: communication, micromanipulation, microscopy, among others. The production of OAM beams in the extreme ultraviolet (XUV) and X-ray regimes is of great interest as it allows to extend the applications of optical vortices down to the nanometric scale. Several proposals have been explored in order to generate XUV vortices in synchrotrons and FEL facilities. Here, we study the generation of XUV vortices through high-order harmonic generation (HHG). HHG is a unique source of coherent radiation extending from the XUV to the soft X-ray regime, emitted in the form of attosecond pulses. When driving HHG by OAM beams, highly charged XUV vortices with unprecedented spatiotemporal properties are emitted in the form of helical attosecond beams. In this chapter, we revise our theoretical work in the generation of XUV vortices by HHG. In particular, we illustrate in detail the role of macroscopic phase matching of high-order harmonics when driven by OAM beams, which allows to control the production of attosecond beams carrying OAM.
关键词： attosecond helical beams,ultrafast phenomena,orbital angular momentum,attosecond pulses,extreme ultraviolet vortices,high harmonic generation,nonlinear optics,phase matching
更新于2025-09-09 09:28:46
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Interferometry of dipole phase in high harmonics from solids
摘要： Understanding the temporal profiles of high harmonics is essential for their applications in attosecond science. Microscopically, the dipole phase plays an important role in determining the high-harmonic emission phase. In gas-phase high-harmonic generation, the tunnel-ionized electron spends much of its travel time in the continuum—far from the parent ion, where it accumulates the dipole phase. Therefore, the atomic dipole phase is largely independent of the target atom. In solid-state high-harmonic generation, since the driven electron experiences a periodic potential during the entire travel time, the dipole phase may depend on the electronic structures of source materials. Here, we employ an interferometric method to characterize high harmonics from magnesium oxide and quartz crystals. We measure material-dependent intensity-induced high-harmonic phase delays that we attribute to the intensity-induced changes in the dipole phase originating from the interband polarization. The material-dependent dipole phase can provide a robust platform for high-harmonic spectroscopy of solids.
关键词： dipole phase,solids,high-harmonic generation,attosecond science,interferometry
更新于2025-09-09 09:28:46
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[IEEE 2018 43rd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz 2018) - Nagoya (2018.9.9-2018.9.14)] 2018 43rd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Terahertz Quasiparticle Acceleration: From Electron-Hole Collisions To Lightwave Valleytronics
摘要： Intense lightwaves can accelerate quasiparticles inside solids. This strong-field light-matter interaction results in the emission of high-harmonic or high-order sideband radiation. While the former process relies on a complex coupling between simultaneously driven interband polarization and intraband currents, high-order sidebands originate from a ballistic acceleration of the quasiparticles within the bands. This mechanism allows for the implementation of a quasiparticle collider in order to study those entities in close analogy to conventional collision experiments. Accelerating electrons and holes in a monolayer of transition metal dichalcogenides extends this scheme to internal quantum degrees of freedom. Our experiments show a lightwave-induced switching of the valley pseudospin, paving the way to ultimately fast valleytronics.
关键词： valley pseudospin,terahertz,high-harmonic generation,lightwave valleytronics,electron-hole collisions,quasiparticle acceleration,high-order sideband radiation,transition metal dichalcogenides
更新于2025-09-04 15:30:14
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Optimization and Characterization of High-Harmonic Generation for Probing Solid Density Plasmas
摘要： The creation of high energy density plasma states produced during laser–solid interaction on a sub-picosecond timescale opens a way to create astrophysical plasmas in the lab to investigate their properties, such as the frequency-dependent refractive index. Available probes to measure absorption and phase-changes given by the complex refractive index of the plasma state are extreme-UV (EUV) and soft X-ray (XUV) ultra-short pulses from high harmonic generation (HHG). For demanding imaging applications such as single-shot measurements of solid density plasmas, the HHG probe has to be optimized in photon number and characterized in intensity and wavefront stability from shot-to-shot. In an experiment, a coherent EUV source based on HHG driven by a compact diode-pumped laser is optimized in photons per pulse for argon and xenon, and the shot-to-shot intensity stability and wavefront changes are characterized. The experimental results are compared to an analytical model estimating the HHG yield, showing good agreement. The obtained values are compared to available data for solid density plasmas to con?rm the feasibility of HHG as a probe.
关键词： pump-probe,XUV,diode-pumped lasers,high energy density physics,high-harmonic generation
更新于2025-09-04 15:30:14