Diabatic Mechanisms of Higher-Order Harmonic Generation in Solid-State Materials under High-Intensity Electric Fields

Dephasing Dynamics Accessed by High Harmonic Generation: Determination of Electron-Hole Decoherence of Dirac Fermions

We reveal the critical effect of ultrashort dephasing on the polarization of high harmonic generation in Dirac fermions. As the elliptically polarized laser pulse falls in or slightly beyond the multiphoton regime, the elliptically polarized high harmonic generation is produced and exhibits a characteristic polarimetry of the polarization ellipse, which is found to depend on the decoherence time T2. T2 could then be determined to be a few femtoseconds directly from the experimentally observed polarimetry of high harmonics. This shows a sharp contrast with the semimetal regime of higher pump intensity, where the polarimetry is irrelevant to T2. An access to the dephasing dynamics would extend the prospect of high harmonic generation into the metrology of a femtosecond dynamic process in the coherent quantum control.

Strategies for designing low thermal quenching upconverting temperature sensors

The Er³⁺/Yb³⁺:NaGd(WO₄)₂ phosphors and the phosphor-in-glass (PIG) have been synthesized employing a typical approach to investigate their structural, morphological and optical properties. Several PIG samples containing different amounts of NaGd(WO₄)₂ phosphor have been manufactured by sintering the phosphor and glass [TeO₂-WO₃-ZnO-TiO₂] frit together at 550 °C, and its impact on the luminescence characteristics has been extensively studied. It has been noticed that the upconversion (UC) emission spectra of PIG under 980 nm excitation display similar characteristic emission peaks to the phosphors. The maximum absolute sensitivity of the phosphor and PIG is 17.3 × 10^-3 K^-1 @ 473 K and the maximum value of relative sensitivity is 10.0 × 10^-3 K^-1 @ 296 K and 10.7 × 10^-3 K^-1 @ 298 K, respectively. However, thermal resolution at room temperature has been improved in the case of PIG as compared to the NaGd(WO₄)₂ phosphor. As compared to the Er³⁺/Yb³⁺ codoped phosphor and glass, the less thermal quenching of luminescence has been observed in PIG.

Effect of stacking configuration on high harmonic generation from bilayer hexagonal boron nitride

High harmonic generation from bilayer h-BN materials with different stacking configurations is theoretically investigated by solving the extended multiband semiconductor Bloch equations in strong laser fields. We find that the harmonic intensity of AA'-stacking bilayer h-BN is one order of magnitude higher than that of AA-stacking bilayer h-BN in high energy region. The theoretical analysis shows that with broken mirror symmetry in AA'-stacking, electrons have much more opportunities to transit between each layer. The enhancement in harmonic efficiency originates from additional transition channels of the carriers. Moreover, the harmonic emission can be dynamically manipulated by controlling the carrier envelope phase of the driving laser and the enhanced harmonics can be utilized to achieve single intense attosecond pulse.

Gate-tunable quantum pathways of high harmonic generation in graphene

Under strong laser fields, electrons in solids radiate high-harmonic fields by travelling through quantum pathways in Bloch bands in the sub-laser-cycle timescales. Understanding these pathways in the momentum space through the high-harmonic radiation can enable an all-optical ultrafast probe to observe coherent lightwave-driven processes and measure electronic structures as recently demonstrated for semiconductors. However, such demonstration has been largely limited for semimetals because the absence of the bandgap hinders an experimental characterization of the exact pathways. In this study, by combining electrostatic control of chemical potentials with HHG measurement, we resolve quantum pathways of massless Dirac fermions in graphene under strong laser fields. Electrical modulation of HHG reveals quantum interference between the multi-photon interband excitation channels. As the light-matter interaction deviates beyond the perturbative regime, elliptically polarized laser fields efficiently drive massless Dirac fermions via an intricate coupling between the interband and intraband transitions, which is corroborated by our theoretical calculations. Our findings pave the way for strong-laser-field tomography of Dirac electrons in various quantum semimetals and their ultrafast electronics with a gate control.

Under strong laser fields, materials exhibit extreme non-linear optical response, such as high harmonic generation. These higher harmonics provide insights into electron behaviour in materials in sub-laser cycle timescale. Here, Cha et al study higher harmonic generation resulting from the laser driven motion of massless Dirac fermions in graphene.

Bidirectional residual current in monolayer graphene under few-cycle laser irradiation

By numerically solving the time-dependent Schrödinger equation and semiconductor Bloch equations, the light-induced residual current in monolayer graphene driven by a circularly polarized few-cycle laser is investigated. An evident current direction reversal is disclosed when the amplitude of the driving electric field exceeds a certain threshold value, which is absent in recent investigation [Nature550, 224 (2017)10.1038/nature23900]. Here the internal physical mechanism for the current reversal is inter-optical-cycle interference under a suitable long laser wavelength. Moreover, the reversal-related laser field amplitude depends sensitively on the ratio of ponderomotive energy to photon energy.

Anomalous Temperature Dependence of High-Harmonic Generation in Mott Insulators

We reveal the crucial effect of strong spin-charge coupling on high-harmonic generation (HHG) in Mott insulators. In a system with antiferromagnetic correlations, the HHG signal is drastically enhanced with decreasing temperature, even though the gap increases and the production of charge carriers is suppressed. This anomalous behavior, which has also been observed in recent HHG experiments on Ca_{2}RuO_{4}, originates from a cooperative effect between the spin-charge coupling and the thermal ensemble, as well as the strongly temperature-dependent coherence between charge carriers. We argue that the peculiar temperature dependence of HHG is a generic feature of Mott insulators, which can be controlled via the Coulomb interaction and dimensionality of the system. Our results demonstrate that correlations between different degrees of freedom, which are a characteristic feature of strongly correlated solids, have significant and nontrivial effects on nonlinear optical responses.

Analysis of low-frequency THz emission from monolayer graphene irradiated by a long two-color laser pulse

Terahertz (THz) radiations from graphene are expected to provide a powerful light source for their wide applications. However, their conversion efficiencies are limited with either long-duration or few-cycle single-color laser pulses. Here, we theoretically demonstrate that THz waves can be efficiently generated from monolayer graphene by using a long-duration two-color laser pulse at normal incidence. Our simulated results show that low-frequency THz emissions are sensitive to the phase difference between two colors, the laser intensity, and the fundamental wavelength. Their dependence on these parameters can be very well reproduced by asymmetry parameters accounting for electron populations of conduction and valence bands. On the contrary, a newly defined σ parameter including the Landau-Zener tunneling probability cannot precisely predict such dependence. Furthermore, the waveform of THz electric field driven by two-color laser pulses exhibits the typical feature of a half-cycle pulse.

All-Optical Probe of Three-Dimensional Topological Insulators Based on High-Harmonic Generation by Circularly Polarized Laser Fields

We report the observation of an anomalous nonlinear optical response of the prototypical three-dimensional topological insulator bismuth selenide through the process of high-order harmonic generation. We find that the generation efficiency increases as the laser polarization is changed from linear to elliptical, and it becomes maximum for circular polarization. With the aid of a microscopic theory and a detailed analysis of the measured spectra, we reveal that such anomalous enhancement encodes the characteristic topology of the band structure that originates from the interplay of strong spin-orbit coupling and time-reversal symmetry protection. The implications are in ultrafast probing of topological phase transitions, light-field driven dissipationless electronics, and quantum computation.

High-harmonic generation in Weyl semimetal β-WP2 crystals

As a quantum material, Weyl semimetal has a series of electronic-band-structure features, including Weyl points with left and right chirality and corresponding Berry curvature, which have been observed in experiments. These band-structure features also lead to some unique nonlinear properties, especially high-order harmonic generation (HHG) due to the dynamic process of electrons under strong laser excitation, which has remained unexplored previously. Herein, we obtain effective HHG in type-II Weyl semimetal β-WP2 crystals, where both odd and even orders are observed, with spectra extending into the vacuum ultraviolet region (190 nm, 10th order), even under fairly low femtosecond laser intensity. In-depth studies have interpreted that odd-order harmonics come from the Bloch electron oscillation, while even orders are attributed to Bloch oscillations under the "spike-like" Berry curvature at Weyl points. With crystallographic orientation-dependent HHG spectra, we further quantitatively retrieved the electronic band structure and Berry curvature of β-WP2. These findings may open the door for exploiting metallic/semimetallic states as solid platforms for deep ultraviolet radiation and offer an all-optical and pragmatic solution to characterize the complicated multiband electronic structure and Berry curvature of quantum topological materials.

Inter-half-cycle spectral interference in high-order harmonic generation from monolayer MoS2

The enhancement of even-order harmonics near the cut-off of high-order harmonic spectra from monolayer MoS₂ has been experimentally observed recently by several groups. Here we demonstrate that this enhancement can be interpreted as a result of spectral interference between half-cycles with opposite polarity by adopting a fully quantum mechanical calculation. We found that, due to the energy modulation induced by Berry connections, only half-cycles with the same polarity can generate high-order harmonics near the cut-off frequency, thus the lack of destructive interference leads to the enhanced intensity of the corresponding even-order harmonics. The explanation is supported by the frequency shift of the measured harmonic peaks. Our finding revealed the role of inter-half-cycle interference in high-harmonic generation (HHG) from non-centrosymmetric materials.

Propagation Induced Dephasing in Semiconductor High-Harmonic Generation

The influence of propagation on the nonperturbative high-harmonic features in long-wavelength strong pulse excited semiconductors is studied using a fully microscopic approach. For sample lengths exceeding the wavelength of the exciting light, it is shown that the propagation effectively acts as a very strong additional dephasing that reduces the relative height of the emission plateau up to six orders of magnitude. This propagation induced dephasing clarifies the need to use extremely short polarization decay times for the quantitative analysis of experimental observations.

Control of High-Harmonic Generation by Tuning the Electronic Structure and Carrier Injection

High-harmonic generation (HHG), which is the generation of light with multiple optical harmonics, is an unconventional nonlinear optical phenomenon beyond the perturbation regime. HHG, which was initially observed in gaseous media, has recently been demonstrated in solid-state materials. Determining how to control such extreme nonlinear optical phenomena is a challenging subject. Here, we demonstrate the control of HHG through tuning the electronic structure and carrier injection using single-walled carbon nanotubes (SWCNTs). We reveal systematic changes in the high-harmonic spectra of SWCNTs with a series of electronic structures ranging from a metal structure to a semiconductor structure. We demonstrate enhancement or reduction of harmonic generation by more than 1 order of magnitude by tuning the electron and hole injection into the semiconductor SWCNTs through electrolyte gating. These results open a path toward the control of HHG in the context of field-effect transistor devices.

Dual Resonant Sum Frequency Generations from Two-Dimensional Materials

We propose dual resonant optical sum frequency generation (SFG), where the two most singular resonances could be selected, and report for the monolayer (1L-) WSe₂ when one (ω₁) of two excitation pulses is resonant to A exciton and their sum frequency (ω₁ + ω₂) to D exciton. The dual resonant SFG confirms that, under an irradiation of ω₁ and ω₂ pulses with the same fluence of ∼1.4 × 10¹⁰ W/m², its signal intensity could be enhanced about 20 times higher than the resonant SHG (i.e., 2ω₁ to the D excitonic absorption). Further, the dual resonant SFG intensity of 1L-WSe₂ is found to be 1 order of magnitude higher than the single resonant SFG intensity of 1L-WS₂ under the same condition of two-pulse irradiation. Finally, observations of the dual resonant SFG are thoroughly examined using real-time time-dependent density functional theory (rt-TDDFT), and the relevant nonlinear optical characteristics are scrutinized using the Greenwood-Kubo formalism.

Imperfect Recollisions in High-Harmonic Generation in Solids

We theoretically investigate high-harmonic generation in hexagonal boron nitride with linearly polarized laser pulses. We show that imperfect recollisions between electron-hole pairs in the crystal give rise to an electron-hole-pair polarization energy that leads to a double-peak structure in the subcycle emission profiles. An extended recollision model (ERM) is developed that allows for such imperfect recollisions, as well as effects related to Berry connections, Berry curvatures, and transition-dipole phases. The ERM illuminates the distinct spectrotemporal characteristics of harmonics emitted parallel and perpendicularly to the laser polarization direction. Imperfect recollisions are a general phenomenon and a manifestation of the spatially delocalized nature of the real-space wave packet; they arise naturally in systems with large Berry curvatures, or in any system driven by elliptically polarized light.

Interband resonant high-harmonic generation by valley polarized electron-hole pairs

High-harmonic generation in solids is a unique tool to investigate the electron dynamics in strong light fields. The systematic study in monolayer materials is required to deepen the insight into the fundamental mechanism of high-harmonic generation. Here we demonstrated nonperturbative high harmonics up to 18th order in monolayer transition metal dichalcogenides. We found the enhancement in the even-order high harmonics which is attributed to the resonance to the band nesting energy. The symmetry analysis shows that the valley polarization and anisotropic band structure lead to polarization of the high-harmonic radiation. The calculation based on the three-step model in solids revealed that the electron-hole polarization driven to the band nesting region should contribute to the high harmonic radiation, where the electrons and holes generated at neighboring lattice sites are taken into account. Our findings open the way for attosecond science with monolayer materials having widely tunable electronic structures.

Fiber-amplifier-pumped, 1-MHz, 1-µJ, 2.1-µm, femtosecond OPA with chirped-pulse DFG front-end

High-repetition-rate, high-power, few-cycle mid-infrared lasers with carrier-envelope phase (CEP) stabilization are ideal driving sources for studying strong-field nonlinear processes, such as strong-field driven electron emission, solid-state high-harmonic generation, and nonlinear microscopy. Here, we report on a 1-MHz, 1-μJ, femtosecond, 2.1-µm optical parametric amplifier (OPA), pumped by a Yb-doped fiber chirped-pulse amplifier (CPA) and seeded by a chirped-pulse difference-frequency generation (DFG) front-end providing positively chirped 2.1-μm signal pulses. The home-built multi-stage 1030-nm Yb-doped fiber CPA pump laser generates >55-μJ near-transform-limited (245-fs) pulses at 1 MHz repetition rate using a novel 4-pass all-fiber stretcher/front-end for careful dispersion/spectral management. The chirped-pulse DFG scheme is achieved by wave-mixing the 1030 nm pump pulse with a dispersive wave at 645-735 nm generated in a photonic crystal fiber, allowing passive CEP stability of the 2.1-µm pulses. The 2.1-μm pulse is amplified to 1 μJ in a two-stage dispersion-managed optical parametric amplifier (OPA) with a pump energy of ~21 μJ resulting in 95-fs pulses with nice beam profile without additional pulse compression. Multi-μJ, sub-30 fs pulses can be obtained at full pump energy and additional dispersion compensation. The fiber-amplifier-based mid-infrared OPA can be directly applied to high-harmonic generation in solids and optical-field-driven nanophotonic devices and is a compact front-end for a future high-power, high-repetition-rate, long wavelengths CEP-stabilized source for gas-phase high-order harmonic generation.

Up/down conversion luminescence and energy transfer of Er3+/Tb3+ activated NaGd(WO4)2 green emitting phosphors

A series of double scheelite-type tungstate green phosphors NaGd(WO₄)₂:Er³⁺, Tb³⁺ were synthesized by a hydrothermal route and subsequent calcination process, and they were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectrometry (EDS), photoluminescence spectroscopy and fluorescence lifetime measurements. The phosphors take on octahedral microcrystals with a mean side length of ~2 μm. In the single doped phosphors system, the energy transfer processes from WO₄^2- to Er³⁺ or Tb³⁺ were discussed. The quenching concentrations of Er³⁺ and Tb³⁺ are 0.05 and 0.07, respectively. The critical distances for Er³⁺ and Tb³⁺ ions are calculated to be 14.28 Å and 12.76 Å, respectively. When doping Er³⁺/Tb³⁺ is applied in the single compound, the concentration quenching effect of Tb³⁺ ions occurs via a resonant-type dipole-dipole interaction as well as that of Er³⁺ ions. Under the excitation with ultraviolet (378 nm) or infrared (980 nm) light, the Er³⁺/Tb³⁺ co-doped NaGd(WO₄)₂ phosphors emit strong green emission. The obtained samples with bright emission intensity and appropriate decay time are suitable for use as green phosphors in the near ultraviolet LEDs and bioimaging applications.

High-Harmonic Generation in Mott Insulators

Using Floquet dynamical mean-field theory, we study the high-harmonic generation in the time-periodic steady states of wide-gap Mott insulators under ac driving. In the strong-field regime, the harmonic intensity exhibits multiple plateaus, whose cutoff energies ε_{cut}=U+mE_{0} scale with the Coulomb interaction U and the maximum field strength E_{0}. In this regime, the created doublons and holons are localized because of the strong field and the mth plateau originates from the recombination of mth nearest-neighbor doublon-holon pairs. In the weak-field regime, there is only a single plateau in the intensity, which originates from the recombination of itinerant doublons and holons. Here, ε_{cut}=Δ_{gap}+αE_{0}, with Δ_{gap} the band gap and α>1. We demonstrate that the Mott insulator shows a stronger high-harmonic intensity than a semiconductor model with the same dispersion as the Mott insulator, even if the semiconductor bands are broadened by impurity scattering to mimic the incoherent scattering in the Mott insulator.

Harmonic Quantum Coherence of Multiple Excitons in PbS/CdS Core-Shell Nanocrystals

The generation and recombination dynamics of multiple excitons in nanocrystals (NCs) have attracted much attention from the viewpoints of fundamental physics and device applications. However, the quantum coherence of multiple exciton states in NCs still remains unclear due to a lack of experimental support. Here, we report the first observation of harmonic dipole oscillations in PbS/CdS core-shell NCs using a phase-locked interference detection method for transient absorption. From the ultrafast coherent dynamics and excitation-photon-fluence dependence of the oscillations, we found that multiple excitons cause the harmonic dipole oscillations with ω, 2ω, and 3ω oscillations, even though the excitation pulse energy is set to the exciton resonance frequency, ω. This observation is closely related to the quantum coherence of multiple exciton states in NCs, providing important insights into multiple exciton generation mechanisms.

Quantum-trajectory analysis for charge transfer in solid materials induced by strong laser fields

We investigate the dependence of charge transfer on the intensity of driving laser field when SiO₂ crystal is irradiated by an 800 nm laser. It is surprising that the direction of charge transfer undergoes a sudden reversal when the driving laser intensity exceeds critical values with different carrier-envelope phases. By applying quantum-trajectory analysis, we find that the Bloch oscillation plays an important role in charge transfer in solids. Also, we study the interaction of a strong laser with gallium nitride (GaN), which is widely used in optoelectronics. A pump-probe scheme is applied to control the quantum trajectories of the electrons in the conduction band. The signal of charge transfer is controlled successfully by means of a theoretically proposed approach.

Impact of the Electronic Band Structure in High-Harmonic Generation Spectra of Solids

An accurate analytic model describing the microscopic mechanism of high-harmonic generation (HHG) in solids is derived. Extensive first-principles simulations within a time-dependent density-functional framework corroborate the conclusions of the model. Our results reveal that (i) the emitted HHG spectra are highly anisotropic and laser-polarization dependent even for cubic crystals; (ii) the harmonic emission is enhanced by the inhomogeneity of the electron-nuclei potential; the yield is increased for heavier atoms; and (iii) the cutoff photon energy is driver-wavelength independent. Moreover, we show that it is possible to predict the laser polarization for optimal HHG in bulk crystals solely from the knowledge of their electronic band structure. Our results pave the way to better control and optimize HHG in solids by engineering their band structure.

Plasmon-assisted high-harmonic generation in graphene

High-harmonic generation in condensed-matter systems is both a source of fundamental insight into quantum electron motion and a promising candidate to realize compact ultraviolet and ultrafast light sources. While graphene is anticipated to efficiently generate high-order harmonics due to its anharmonic charge-carrier dispersion, experiments performed on extended samples using THz illumination have revealed only a weak effect. The situation is further complicated by the enormous electromagnetic field intensities required by this highly nonperturbative nonlinear optical phenomenon. Here we argue that the large light intensity required for high-harmonic generation to occur can be reached by exploiting localized plasmons in doped graphene nanostructures. We demonstrate through rigorous time-domain simulations that the synergistic combination of strong plasmonic near-field enhancement and a pronounced intrinsic nonlinearity result in efficient broadband high-harmonic generation within a single material. Our results support the strong potential of nanostructured graphene as a robust, electrically tunable platform for high-harmonic generation.