High Harmonic Generation Research Articles

High harmonic generation in organic molecules: a time-dependent density functional theory (TDDFT) approach

High harmonic generation in organic molecules: a time-dependent density functional theory (TDDFT) approach

Amplify high harmonic generation via anharmonic phonons in PbTe

Amplify high harmonic generation via anharmonic phonons in PbTe

High-harmonic generation from subwavelength silicon films

Abstract Recent years have witnessed significant developments in the study of nonlinear properties of various materials at the nanoscale. Often, experimental results on harmonic generation are reported without the benefit of suitable theoretical models that allow assessment of conversion efficiencies compared to the material’s intrinsic properties. Here, we report experimental observations of even and odd harmonics up to the 7th, generated from a suspended subwavelength silicon film resonant in the UV range at 210 nm, the current limit of our detection system, using peak power densities of order 3 TW/cm2. We also highlight the time-varying properties of the dielectric function of silicon, which exhibits large changes under intense illumination. We explain the experimental data with a time domain, hydrodynamic-Maxwell approach broadly applicable to most optical materials. Our approach accounts simultaneously for surface and magnetic nonlinearities that generate even optical harmonics, as well as linear and nonlinear material dispersions beyond the third order to account for odd optical harmonics, plasma formation, and a phase locking mechanism that makes the generation of high harmonics possible deep into the UV range, where semiconductors like silicon start operating in a metallic regime.

Second harmonic generation from bound-state in the continuum-hosted few-layers van der Waals metasurface

Abstract Monolayer transition metals dichalcogenides (TMDs) have been coupled to bound-state in the continuum (BIC) hosted dielectric structures to attain high second harmonic generation (SHG). However, the transvers electric modes are strongly localized in the waveguides result in fairly weak exciton-photon coupling in monolayer TMD placed on the surface. To achieve SHG in few-layers TMDs based BIC-inspired structure is a challenge. Here, we report BIC in few-layers TMDs metasurface with high quality factor (Q-factor), tunability, and modes-upholding in different environments. The metasurface sustains BIC at different thickness of the meta-atoms, which is highly desired for maintaining the accuracy in fabrications. Next, we calculate the SHG efficiency from few-layers TMD metasurface around BIC wavelengths. The high conversion efficiency in this work is 1.47 × 10−4 for 6 mW incident power. Moreover, our design is highly thin and can be used for various linear and non-linear applications in optics. This study will provide a new route to next generation post-silicon metasurfaces.

Computational microscopy with coherent diffractive imaging and ptychography.

Microscopy and crystallography are two essential experimental methodologies for advancing modern science. They complement one another, with microscopy typically relying on lenses to image thelocal structuresof samples, and crystallography using diffraction to determine the global atomic structure of crystals. Over the past two decades, computational microscopy, encompassing coherent diffractive imaging (CDI) and ptychography, has advanced rapidly, unifying microscopy and crystallography to overcome their limitations. Here, I review the innovative developments in CDI and ptychography, which achieve exceptional imaging capabilities across nine orders of magnitude in length scales, from resolving atomic structures in materials at sub-ångstrom resolution to quantitative phase imaging of centimetre-sized tissues, using the same principle and similar computational algorithms. These methods have been applied to determine the 3D atomic structures of crystal defects and amorphous materials, visualize oxygen vacancies in high-temperature superconductors and capture ultrafast dynamics. They have also been used for nanoscale imaging of magnetic, quantum and energy materials, nanomaterials, integrated circuits and biological specimens. By harnessing fourth-generation synchrotron radiation, X-ray-free electron lasers, high-harmonic generation, electron microscopes, optical microscopes, cutting-edge detectors and deep learning, CDI and ptychography are poised to make even greater contributions to multidisciplinary sciences in the years to come.

Mechanical Twisting-Induced Enhancement of Second-Order Optical Nonlinearity in a Flexible Molecular Crystal.

Flexible molecular crystals are essential for advancing smart materials, providing unique functionality and adaptability for applications in next-generation electronics, pharmaceuticals, and energy storage. However, the optical applications of flexible molecular crystals have been largely restricted to linear optics, with nonlinear optical (NLO) properties rarely explored. Herein, we report on the application of mechanical twisting of flexible molecular crystals for second-order nonlinear optics. The crystal formed through the self-assembly of the model compound 9-anthraldehyde (AA) features an intrinsic chiral and noncentrosymmetric structure, demonstrating high efficiency second harmonic generation (SHG) and NLO circular dichroism, which could be greatly enhanced by macroscopic mechanical twisting. The anisotropic molecular stacking imparts the AA crystal with mechanical flexibility of combined elastic bending and plastic twisting. The isochiral mechanical twisting could greatly enhance the SHG intensities by an order of magnitude depending on their M- or P-configuration. Meanwhile, the SHG circular dichroism factor gSHG-CD of the isochiral twisted crystal is greatly increased, achieving the highest reported NLO anisotropy factor among organic NLO materials. These boosted NLO performances of SHG intensity and nonlinear chiroptical response are expected to greatly expand the photonic applications of flexible molecular crystals.

Research on Opening Reignition Characteristics and Suppression Measures of 750 kV AC Filter Circuit Breakers

The operation of converter valves in converter stations often results in high reactive power consumption and harmonic generation, necessitating measures to maintain reactive power balance and ensure power quality. To achieve this, the filter bank circuit breaker is frequently switched on and off during daily operation. In recent years, multiple incidents of circuit breaker breakdown during the opening process have been reported. In this study, power systems computer-aided design (PSCAD)/electromagnetic transients including DC (EMTDC) V5.0 electromagnetic transient simulation software is used to simulate and calculate the overvoltage and inrush current under different configurations of circuit breaker operating mechanism dispersion, opening phase angle, and operating speed. Additionally, the suppression effects of two measures are compared: “phase selection opening” and “phase selection opening combined with controlled opening speed” to mitigate overvoltage and inrush current. The results demonstrate that for BP11/13 filters, HP24/36 filters, and HP3 filters, the combined strategy of “phase-selective opening with controlled opening speed” is more effective in suppressing inrush current and overvoltage. However, for SC filters, the suppression effect of this combined strategy is not significant. Considering economic and practical factors, it is more reasonable to adopt the phase-selective opening measure for SC filters. These findings provide guidance for ensuring the safe operation of AC filter circuit breakers.

Influence of point defects on laser-induced excitation in silicon

We studied the influence of defect states on the laser excitation process in silicon using time-dependent density functional theory. We assumed two types of point defects: interstitial oxygen and silicon vacancies. We found that the photoabsorption efficiency increased with defect density in both cases owing to the color center. These defects distorted the crystal structure, thereby relaxing the selection rules and changing the indirect gap to direct. At low laser intensities, the defect states dominated the absorption process. However, as the laser intensity increased, the excitation efficiency approached that of crystalline silicon. In addition, we observed that the excitation efficiency did not scale linearly with the pulse length. Notably, in the case of Si vacancies, saturable absorption reduced photoabsorption significantly. Our results suggest that the existence of a defect induces the even-order high-harmonics generation. The enhancement of even-order high-harmonics generation by the silicon vacancy is larger than the interstitial oxygen. Published by the American Physical Society 2024

Enhanced high-harmonic generation with spectral tunability in nonlocal metasurfaces enabled by the excitation of quasiguided modes

Enhanced high-harmonic generation with spectral tunability in nonlocal metasurfaces enabled by the excitation of quasiguided modes

Quantum Electrodynamics in High-Harmonic Generation: Multitrajectory Ehrenfest and Exact Quantum Analysis.

High-harmonic generation (HHG) is a nonlinear process in which a material sample is irradiated by intense laser pulses, causing the emission of high harmonics of incident light. HHG has historically been explained by theories employing a classical electromagnetic field, successfully capturing its spectral and temporal characteristics. However, recent research indicates that quantum-optical effects naturally exist or can be artificially induced in HHG, such as entanglement between emitted harmonics. Even though the fundamental equations of motion for quantum electrodynamics (QED) are well-known, a unifying framework for solving them to explore HHG is missing. So far, numerical solutions have employed a wide range of basis-sets, methods, and untested approximations. Based on methods originally developed for cavity polaritonics, here we formulate a numerically accurate QED model consisting of a single active electron and a single quantized photon mode. Our framework can, in principle, be extended to higher electronic dimensions and multiple photon modes to be employed in ab initio codes for realistic physical systems. We employ it as a model of an atom interacting with a photon mode and predict a characteristic minimum structure in the HHG yield vs phase-squeezing. We find that this phenomenon, which can be used for novel ultrafast quantum spectroscopies, is partially captured by a multitrajectory Ehrenfest dynamics approach, with the exact minima position sensitive to the level of theory. On the one hand, this motivates using multitrajectory approaches as an alternative for costly exact calculations. On the other hand, it suggests an inherent limitation of the multitrajectory formalism, indicating the presence of entanglement and true quantum effects (especially prominent for atomic and molecular resonances). Our work creates a roadmap for a universal formalism of QED-HHG that can be employed for benchmarking approximate theories, predicting novel phenomena for advancing quantum applications, and for the measurements of entanglement and entropy.

Laser excitation of the 1S–2S transition in singly-ionized helium

Laser spectroscopy of atomic hydrogen and hydrogen-like atoms is a powerful tool for tests of fundamental physics. The 1S–2S transition of hydrogen in particular is a cornerstone for stringent Quantum Electrodynamics (QED) tests and for an accurate determination of the Rydberg constant. We report laser excitation of the 1S–2S transition in singly-ionized helium (3He+), a hydrogen-like ion with much higher sensitivity to QED than hydrogen itself. The transition requires two-photon excitation in the challenging extreme ultraviolet wavelength range, which we achieve with a tabletop coherent laser system suitable for precision spectroscopy. The transition is excited by combining an ultrafast amplified pulse at 790 nm (derived from a frequency comb laser) with its 25th harmonic at 32 nm (produced by high-harmonic generation). The results are well described by our simulations and we achieve a sizable 2S excitation fraction of 10−4 per pulse, paving the way for future precision studies.

Three-dimensional bonding anisotropy of bulk hexagonal metal titanium demonstrated by high harmonic generation

High harmonic generation (HHG) in solid-state materials is an emerging field of photonics research that can unveil the detailed electronic structure of materials, bond strengths and scattering processes of electrons. Although HHG in semiconducting and insulating materials has been intensively investigated both experimentally and theoretically, metals have rarely been explored because the strong screening effect of high-density free electrons is considered to significantly weaken the HHG signal. Here, we investigated HHG upon infrared excitation in bulk hexagonal metal titanium (Ti), a typical building block for practical lightweight structural materials. By analyzing the polarization dependence, the approach revealed the three-dimensional (3D) anisotropy in the electronic states. The results demonstrated the potential of HHG spectroscopy for characterizing 3D bonding anisotropy in metallic systems that are of fundamental importance for designing lightweight and strong structural materials.

Neural-network enabled octave-spanning coherent diffraction imaging

Ultrafast lasers, providing the shortest pulses worldwide, have been playing a vital role in the ultrafast imaging technology. The temporal resolution has been increasing rapidly in recent years but finally reaches its limit—the pulse width approaches photoperiods, causing significant broadening of spectral bandwidth. The state-of-the-art high harmonics generation based attosecond lasers, with pulse widths reaching ∼50 attoseconds, present octave-spanning spectra. This brings a major challenge to traditional imaging methods, as they result in unbearable chromatic aberrations. To address this challenge, we propose the neural-network approach for broadband imaging and demonstrate its effectiveness empirically by facilitating rapid coherent diffractive imaging under octave-spanning supercontinuum illumination. The proposed method remains effective when deployed with three-octave-spanning spectra, supporting both continuous and comb-like profiles as indicated by simulations. Such lensless imaging method, applicable to both extreme ultraviolet and soft x-ray sources, potentially provides an approach to attosecond imaging.

MIR laser CEP estimation using machine learning concepts in bulk high harmonic generation

Monitoring the carrier-envelope phase (CEP) is of paramount importance for experiments involving few-cycle intense laser fields. Common measurement techniques include f-2f interferometry or stereo-ATI setups. Here we demonstrate a new concept, both by simulations and by experiments, for CEP estimation in the mid-infrared regime using machine learning (ML) techniques that rely on the observation of the spectrum of high harmonic generation (HHG) in bulk material. Once the ML model is trained, the method provides a way for cheap and compact in-situ CEP tagging. This technique can complement other CEP monitoring methods, can capture the complex correlation between the CEP and the observable HHG spectra, and is readily generalizable for any laser wavelengths.

Curvature-assisted high harmonic generation in strongly-driven superconductors

The dynamic of the order parameter in superconductors (SCs) under strong driving is inherently nonlinear, but utilization for nonlinear optics and high harmonic generation (HHG) is hindered by the weak coupling of SCs to transverse homogeneous driving fields. When the superconducting coherence length is large enough to allow for curvature modulations without breaking the superconducting phase, the coupling of the order parameter to the driving-field vector potential changes. Here, we show that with the help of full numerical simulations, mesoscopic curvatures or bending of strongly driven type-II SC structures result in highly nonlinear THz superconducting currents, which lead to far-field HHG extending up to the tens harmonics. It is shown that even and odd harmonics can be controllably emitted by a transport current. The driving mechanism is a nonlinear steering of supercurrent by geometric and finite-size effects. At the same time, the SC state remains fully coherent over the whole sample and within the driving time. The phase matching of the emitted harmonics is discussed.

Continuous relativistic high-harmonic generation from a kHz liquid-sheet plasma mirror

Continuous relativistic high-harmonic generation from a kHz liquid-sheet plasma mirror

High harmonic generation spectra from polyacetylene in condensed phase

In this work, we investigate high-harmonic generation (HHG) spectra in polyacetylene with two relevant bands by numerically solving the time-dependent Schrödinger equation. Using the Su-Schrieffer-Heeger (SSH) model, the problem reduces to a 2 × 2 tight-binding Bloch Hamiltonian. Additionally, coupling to the laser field requires solving the time-dependent Schrödinger equation numerically in reciprocal space. The HHG spectra are obtained by performing a Fourier transform in two consecutive steps: first on the lattice momentum and then on time. The k -integration of velocity is carried out to ensure the coherent addition of the momentum contributions of all electrons within the Brillouin zone. The time integration of the acceleration yields the HHG spectra. Our findings reveal the well-known signatures of high harmonics from solids.

High harmonic generation in altermagnets

High harmonic generation in altermagnets

Cascaded high harmonic generation in mixture of argon and helium: Achieving a broad photon spectrum

In this study, we explore high harmonic generation (HHG) in pure argon (Ar) gas and a mixture of argon and helium (He). We investigate phase-matching conditions and interference effects in both single and mixed-gas systems. By varying the gas pressure in the Ar–He mixture, we optimize the harmonic spectrum, achieving a broad range from H17 to H43, corresponding to photon energies of 25 eV to 75 eV. We attribute the spectrum broadening to a cascaded HHG process: argon-generated extreme ultraviolet photons excite helium's outer electrons, which are then driven by the fundamental laser field to contribute to HHG. This finding aligns with previous studies, showing that mixing gases with low and high ionization potentials can enhance HHG. The results offer a broad HHG spectrum ideal for ultra-fast spectroscopy and high-resolution imaging applications.

Enhancement of high harmonic generation in liquid water by resonant excitation in the mid-infrared

Abstract We study high harmonic generation (HHG) in the visible spectral range generated in a flat liquid jet of H2O, excited by intense mid-infrared (MIR) radiation around 3 μm, which is nearly resonant with the OH vibrational modes. By introducing a weak excitation pulse prior to the intense MIR driver pulse for HHG, we observed an enhancement of the 5th, 7th, and 9th harmonics occurring approximately 2 ps after excitation and persisting for more than 120 ps, which is completely absent in the case of non-resonant D2O. These results suggest that the enhancement is caused by ultrafast heating through vibrational excitation.