Harmonic Emission Research Articles

Efficient angular dispersed high-order harmonic generation by dual laser pulses interacting with a solid target

High-order harmonics emitted from relativistic laser–solid interaction can be used as a bright extreme ultraviolet source. However, such a source usually has mixed frequencies along the emission direction. Through theoretical and numerical studies, we propose a scheme to realize monochromatic harmonic emission along individual directions by using dual laser pulses with orthogonal polarizations, where a relativistic P-polarized laser is used as a pump laser in combination with a moderate S-polarized laser as a seed laser. We find that harmonics of the S-polarized laser can be efficiently generated with several orders of magnitude enhancement and spontaneous angular dispersion depending on the harmonic order. Our scheme may be beneficial to future applications requiring monochromatic radiation along a specific direction or multi-chromatic radiations in different directions.

Tunable spectral continuous shift of high-order harmonic generation in atoms by a plasmon-assisted shaping pulse

We delve into the phenomenon of high-order harmonic generation within a helium atom under the influence of a plasmon-assisted shaping pulse. Our findings reveal an intriguing manipulation of the frequency peak position in the harmonic emission by adjusting the absolute phase parameter within the frequency domain of the shaping pulse. This phenomenon holds potential significance for experimental setups necessitating precisely tuned single harmonics. Notably, we observe a modulated shift in the created harmonic photon energy, spanning an impressive range of 1.2 eV. This frequency peak shift is rooted in the asymmetry exhibited by the rising and falling edges of the laser pulse, directly influencing the position of the peak frequency emission. Our study quantifies the dependence of this tuning range and the asymmetry of the laser pulse, offering valuable insights into the underlying mechanisms driving this phenomenon. Furthermore, our investigation uncovers the emergence of semi-integer order harmonics as the phase parameter is altered. We attribute this discovery to the intricate interference between harmonics generated by the primary and secondary return cores. This observation introduces an innovative approach for generating semi-integer order harmonics, thus expanding our understanding of high-order harmonic generation. Ultimately, our work contributes to the broader comprehension of complex phenomena in laser-matter interactions and provides a foundation for harnessing these effects in various applications, particularly those involving precise spectral control and the generation of unique harmonic patterns.

Challenges of Large Converter-Fed Synchronous Machines for Variable-Speed Pumped Hydro Storage

The green energy transition of electrical energy production is leading to an increasing share of total energy production for volatile renewable energy sources, mainly wind and solar power. To handle this volatile production, flexible and efficient energy storage is required. The development of high-power converters has enabled the generation of variable-speed pumped hydro storage power plants, combining the so-far-unequalled energy storage capacity of classical pumped-storage hydro power plants and the recently increased operation requirements. The introduction of large-scale converters has led to new challenges in the overall design of power plant systems. This paper intended to take a closer look at a large-scale converter-fed synchronous generator, especially the distribution of the current and voltage harmonics caused by the converter in the implemented generation system. Thereby, holistic design considerations for an ideal loss distribution as well as possible measures to limit the effects of harmonic coupling at the generator shaft and bearings are discussed. Furthermore, basic considerations of harmonic emission to the connected network are described. These topics are addressed by analyzing on-site measurements at an 85 MVA converter-fed synchronous generator with a voltage source inverter, underpinned with the theoretical background.

Molecular orbital tomography of HOMO and HOMO-1 of the nonlinear molecule H2O: The study on multi-orbital effects

Molecular orbital tomography (MOT) through high-order harmonic generation (HHG) is a crucial method for probing individual molecular orbitals. Previous studies mainly focused on linear molecules such as N2, and CO, where valence orbitals were imaged by detecting harmonic emission at various alignments. In this study, we investigate the HHG from the water molecules, with the aim of exploring MOT for nonlinear molecules. By employing the time-dependent density functional theory, we obtain HHG spectra for different angles between the laser polarization and molecular symmetry axis. By manipulating the scanning planes (planes perpendicular to the laser propagation direction and contain the field polarization direction), specific orbitals can be selectively imaged. Particularly, a well-reconstructed highest occupied molecular orbital (HOMO) is obtained when the laser scanning plane approximately aligns with the nodal plane of HOMO-1. However, when the laser polarization deviates from the nodal plane, multiple orbitals contribute to the HHG process, and we visualize the orbital distortions resulting from such effects. Furthermore, HOMO-1, can be reconstructed when the laser scanning plane aligns with the nodal plane of HOMO. This research significantly advances the development of MOT and opens up intriguing possibilities for reconstructing orbitals of more complex molecules.

High repetition rate relativistic laser-solid-plasma interaction platform featuring simultaneous particle and radiation detection.

We report on a uniquely designed high repetition rate relativistic laser-solid-plasma interaction platform, featuring the first simultaneous measurement of emitted high-order harmonics, relativistic electrons, and low divergence proton beams. This versatile setup enables detailed parametric studies of the particle and radiation spatio-spectral beam properties under a wide range of controlled interaction conditions, such as pulse duration and plasma density gradient. Its array of complementary diagnostics unlocks the potential to unravel interdependencies among the observables and should aid in further understanding the complex collective dynamics at play during laser-plasma interactions and in optimizing the secondary beam properties for applications.

Modifying the Electron Dynamics in High-Order Harmonic Generation via a Two-Color Laser Field

We investigate the harmonic emission from bichromatic periodic potential by numerically solving the time-dependent Schrödinger equation in the velocity gauge. The results show that the harmonic minimum is sensitive to the wavelength. Moreover, distinct crystal momentum states contribute differently to harmonic generation. In momentum space, the electron dynamics reveal a close relationship between the spectral minimum and the electron distribution in higher conduction bands. Additionally, by introducing an ultraviolet pulse to the fundamental laser field, the suppression of the harmonic minimum occurs as a result of heightened electron populations in higher conduction bands. This work sheds light on the harmonic emission originating from a solid with a two-atom basis.

Evaluation the results of electromagnetic compatibility of a prototype device for ear stimulation in patients with tinnitus.

<b><br>Aim:</b> The aim of the study was to evaluate the results of electromagnetic compatibility of a prototype device for ear stimulation in patients with tinnitus.</br> <b><br>Material and methods:</b> The electromagnetic compatibility tests of the prototype device for electro- and magnetostimulation of the hearing organ were carried out at the Center for Attestation and Certification Tests OBAC Sp. z o. o. in Gliwice in 2020. The following product standards were used: PN-EN 60601-1-2:2015-11 - medical electrical equipment (general requirements for basic safety and essential functioning; PN-EN 55011:2012 - industrial, scientific and medical equipment [characteristics of radio frequency disturbances] ) PN-EN 61000-3-2:2014-10 - electromagnetic compatibility (EMC), permissible levels of harmonic current emissions (phase load current ≤ 16 A).</br> <b><br>Results:</b> The level of expanded uncertainty in the measurement of conducted disturbances in the range of 0.150-30MHz does not exceed the level specified in the PN-EN-55016-4-2:2011 standard. In the study of the emission of radiated disturbances up to 1GHz in the frequency range of 30-1000MHz (PN-EN 55011:2012 standard), it was found that the setting of the EUT during the tests was in accordance with the requirements of the standard. The level of expanded uncertainty in the measurement of radiated disturbances in the 30-1000MHz range does not exceed the level specified in the PN-EN 55016-4-2:2011 standard. The measured current harmonic levels (phase power supply current ≤16A) with a frequency range of 50Hz-2kHz do not exceed the permissible levels specified in the PN-EN 61000-3-2:2014-10 standard. The test of resistance to the magnetic field at the frequency of the power grid (PN-EN 61000-4-8: 2010 standard also showed that the setting of the EUT during the tests was in accordance with the requirements of the standard and the result was positive.</br> <b><br>Conclusions:</b> Testing of immunity to radiated radio frequency electromagnetic field (PN-EN 61000-4-3:2007 +A1:2008+A2:2011 standard) and testing of resistance to magnetic field at power frequency (PN-EN 61000-4-8 standard :2010) did not exceed the level specified in the standard and showed a positive result. The measured harmonic levels of the network current (phase supply current ≤16A) with the frequency range 50Hz-2kHz do not exceed the permissible levels specified in the PN-EN 61000- 3-2:2014-10 standard for a class A device.</br&gt.

Valley-Selective High Harmonic Generation and Polarization Induced by an Orthogonal Two-Color Laser Field

The valley pseudospin properties of electrons in two-dimensional hexagonal materials result in many fascinating physical phenomena, which opens up the new field of valleytronics. The valley-contrasting physics aims at distinguishing the valley degree of freedom based on valley-dependent effects. Here, we theoretically demonstrate that both of the valley-selective high harmonic generation and valley-selective electronic excitation can be achieved by using an orthogonal two-color (OTC) laser field in gapped graphene. It is shown that the asymmetry degrees of harmonic yields in the plateaus, cutoff energies of generated harmonics and electron populations from two different valleys can be precisely controlled by the relative phase of the OTC laser field. Thus, the selectivity of the dominant valley for the harmonic radiation and electronic polarization can be switched by adjusting the relative phase of the OTC laser field. Our work offers an all-optical route to produce the valley-resolved high harmonic emissions and manipulate the ultrafast valley polarization on a femtosecond timescale in condensed matter.

An Anisotropic Density Turbulence Model from the Sun to 1 au Derived from Radio Observations

Solar radio bursts are strongly affected by radio-wave scattering on density inhomogeneities, changing their observed time characteristics, sizes, and positions. The same turbulence causes angular broadening and scintillation of galactic and extragalactic compact radio sources observed through the solar atmosphere. Using large-scale simulations of radio-wave transport, the characteristics of anisotropic density turbulence from 0.1 R ⊙ to 1 au are explored. For the first time, a profile of heliospheric density fluctuations is deduced that accounts for the properties of extrasolar radio sources, solar radio bursts, and in situ density fluctuation measurements in the solar wind at 1 au. The radial profile of the spectrum-weighted mean wavenumber of density fluctuations (a quantity proportional to the scattering rate of radio waves) is found to have a broad maximum at around (4–7) R ⊙, where the slow solar wind becomes supersonic. The level of density fluctuations at the inner scale (which is consistent with the proton resonance scale) decreases with heliocentric distance as cm−6. Due to scattering, the apparent positions of solar burst sources observed at frequencies between 0.1 and 300 MHz are computed to be essentially cospatial and to have comparable sizes, for both fundamental and harmonic emission. Anisotropic scattering is found to account for the shortest solar radio burst decay times observed, and the required wavenumber anisotropy is q ∥/q ⊥ = 0.25–0.4, depending on whether fundamental or harmonic emission is involved. The deduced radio-wave scattering rate paves the way to quantify intrinsic solar radio burst characteristics.

Calibration of quantitative rescattering model for simulating vortex high-order harmonic generation driven by Laguerre–Gaussian beam with nonzero orbital angular momentum

We calibrate the macroscopic vortex high-order harmonic generation (HHG) obtained by the quantitative rescattering (QRS) model to compute single-atom induced dipoles against that by solving the time-dependent Schrödinger equation (TDSE). We show that the QRS perfectly agrees with the TDSE under the favorable phase-matching condition, and the QRS can accurately predict the main features in the spatial profiles of vortex HHG if the phase-matching condition is not good. We uncover that harmonic emissions from short and long trajectories are adjusted by the phase-matching condition through the time-frequency analysis and the QRS can simulate the vortex HHG accurately only when the interference between two trajectories is absent. This work confirms that it is an efficient way to employ the QRS model in the single-atom response for precisely simulating the macroscopic vortex HHG.

Bright Second Harmonic Emission from Photonic Crystal Vertical Cavity

AbstractThis study presents photonic vertical cavities consisting of nonlinear materials embedded in photonic crystals (PhCs) for resonantly enhancing second harmonic generation (SHG). Previous attempts at SHG in such structures have been limited to efficiencies of 10−7 to 10−5, but a high SHG efficiency of 0.28% by constructing a vertical cavity with a lithium niobate membrane placed between two PhCs is demonstrated, which exhibits high‐quality resonances. The results open up new possibilities for compact laser frequency converters that can have a revolutionary impact on the fields of nonlinear optics and photonics.

Distribution Network Mixed-harmonic Separation Method Based on Characteristic Vector

The threat of harmonic pollution has become increasingly serious with the rapid increase in the scale and complexity of distribution networks. However, the harmonic source variety of distribution networks and the phenomena about harmonic characteristics become vague after harmonic mixture because harmonic governance and responsibility division cause much inconvenience. To address this point, a distribution network mixed-harmonic separation method based on characteristic vector is developed in this study. This method realizes the division of the current working state, harmonic emission levels, and harmonic governance responsibility of each harmonic source by equivalent analysis and reverse thinking. The results are verified through an experiment.

Ultra-sensitive terahertz introduced harmonic emission towards terahertz detection

The sixth-generation (6G) technology has garnered significant interest from researchers due to its improved spectral and energy efficiency. However, a major challenge lies in the strong attenuation of terahertz waves in the air, necessitating the development of ultra-sensitive detectors. In recent years, there has been gradual progress in the development of harmonic signal detection methods, offering a new solution for terahertz wave detection with a low detection threshold and high sensitivity. In our study, we proposed a highly sensitive detection approach involving a 1040 nm femtosecond laser and broadband terahertz waves incident on SnSe2 nanosheets with a few layers, obtained through mechanical exfoliation. Utilizing confocal microscopy, the distinct harmonic wave can be observed, with the intensity of the harmonic signal varying based on the polarization angle difference between the laser and terahertz waves. Our estimation suggests that this detection sensitivity could be around ten times higher than that of existing detectors. This approach, which involves THz-induced harmonic generation without the need for high voltage or intense THz/laser exposure, holds immense potential for weak signal detection applications and carries significant importance for the advancement of 6G technology.

Comparison of harmonic emission in LV side of a large grid connected PV power plant

The use of solar photovoltaic (PV) in distribution networks has increased considerably in recent years. Although they have many advantages, PV systems can also result in complex power quality issues in distribution networks, like harmonic distortion, that can interfere with loads and controllers. Harmonic emission from different inverters and their aggregation is thus of interest. In this work, harmonic emission of two PV string inverters and two substations operating in a large PV power plant is presented. The distribution of the full range of current harmonics is analyzed, as well as the correlation between the values of each harmonic and the power level produced, by the inverter or substation, and the correlation between voltage and current harmonics. Significant differences are observed in the harmonic emission of the inverters, despite being under the same operating loading and grid conditions. The results obtained for the substations are in line to those for the inverters. This study was performed in a grid connected 12 MW PV power plant operating in Europe.

Contribution of HOMO-1 to the high-order harmonic generation by a two-component laser field

The contributions of two energetically highest molecular orbitals to the harmonic emission rate are analysed for a two-component laser field. For diatomic molecules exposed to the elliptically polarised field, the emission from the highest-occupied molecular orbital (HOMO) is dominant for various molecular orientations with respect to the laser field. However, the contribution of the lower molecular orbital (HOMO-1) can become significant or even dominant for some molecular orientations. We introduce the ratio of the coherent over the incoherent sum of the HOMO and HOMO-1 contributions as a quantitative measure of the significance of the particular molecular orbital. Also, the gaseous medium response is different for the left and right elliptically polarised light and the molecular characteristics are imprinted into this difference. Moreover, for the orthogonally polarised two-colour (OTC) laser field the relative contributions of HOMO and HOMO-1 depend to a great extent on the relative phase between the field components. The importance of the HOMO-1 can be assessed by the relative error which is made if the harmonic spectra are obtained only with the HOMO contribution. Finally, we investigate the interference of the contributions of two highest molecular orbitals. We show that, for the OTC field, the destructive interference depends linearly on the intensity of the field components. Also, the interference minima shift towards the higher energies with the increase of the component wavelength.

Two-color high-harmonic generation from relativistic plasma mirrors.

High-intensity laser solid interactions are capable of generating attosecond light bursts via high-harmonic generation-most work focuses on single beam interactions. In this paper, we perform a numerical investigation on the role of wavelength and polarization in relativistic, high-harmonic generation from normal-incidence, two-beam interactions off plasma mirrors. We find that the two-beam harmonic-generation mechanism is a robust process described by a set of well-defined selection rules. We demonstrate that the emitted harmonics from normal-incidence interactions exhibit an intensity optimization when the incident fields are of equal intensity for two-color circularly polarized fields.

Harmonic and supraharmonic emissions of fast charging infrastructure - field measurements in LV grids

Harmonic and supraharmonic emissions of fast charging infrastructure - field measurements in LV grids

Verification of tool for allocation of harmonic current emissions considering frequency-dependent impedance

Verification of tool for allocation of harmonic current emissions considering frequency-dependent impedance

Assessment of harmonic emission level of customer installations considering actual level of cancellation

Assessment of harmonic emission level of customer installations considering actual level of cancellation

Electron bunch dynamics and emission in particle-in-cell simulations of relativistic laser–solid interactions: On density artifacts, collisions, and numerical dispersion

Sub-optical-cycle dynamics of dense electron bunches in relativistic-intensity laser–solid interactions lead to the emission of high-order harmonics and attosecond light pulses. The capacity of particle-in-cell simulations to accurately model these dynamics is essential for the prediction of emission properties because the attosecond pulse intensity depends on the electron density distribution at the time of emission and on the temporal distribution of individual electron Lorentz-factors in an emitting electron bunch. Here, we show that in one-dimensional collisionless simulations, the peak density of the emitting electron bunch increases with the increase in the spatial resolution of the simulation grid. When collisions are added to the model, the peak electron density becomes independent of the spatial resolution. Collisions are shown to increase the spread of the peaks of Lorentz-factors of emitting electrons in time, especially in the regimes far from optimum generation conditions, thus leading to lower intensities of attosecond pulses as compared to those obtained in collisionless simulations.