Harmonic Radiation Research Articles

Нелинейно-оптические свойства кристаллов ниобата лития, легированных цинком

The coefficients of the second order nonlinear optical dij tensor were calculated by the method developed on the basis of the dielectrics theory of Phillipson and Van Wetchen and the Levin charge coupling model for lithium niobate crystals doped with zinc (3.43–5.84 mol %). In LiNbO3 : Zn crystals, the concentration dependences of the nonlinear optical coefficients showed several regions with maxima and minima; the most noticeable changes in the nonlinear optical coefficients were observed at threshold concentrations of zinc oxide in the melt. Doping with zinc increases the conversion efficiency of the second harmonic radiation, such the crystals are more promising for use in nonlinear optics than a nominally pure lithium niobate crystal.

Critical Laser Intensity of Phase-Matched High-Order Harmonic Generation in Noble Gases

The efficient generation of high-order harmonic radiation has been a challenging task since the early days of strong-field physics. An essential requirement to achieve efficient high-order harmonic generation inside a gas medium is the phase matching of the high-order harmonic radiation and the incident laser pulse. The dominant contribution to the wave–vector mismatch Δk is associated with the ionization probability of the medium. In this work, we derive two analytical formulas to calculate the critical intensity of a general linearly polarized laser pulse that obey the phase-matching condition Δk=0. The analytic formulas are valid in the tunneling regime (ADK model) and the regime of the tunnel and multi-photon ionization (PPT model), respectively. We compare our results to numerical computations and discuss the scaling of the critical intensity depending on the pulse duration and the wavelength of a realistic incident laser pulse. The analytical approach demonstrated in this work is highly accurate and can compete with the existing numerical computational methods by an error of less than 1% and a decrease in the computation time of approximately 4 to 6 orders of magnitude. This enables complex theoretical studies of the efficiency scaling in HHG or to consider the effects of ground state depletion efficiently.

Time‐modulated multi‐tone antenna array for wireless information and power transfer

SummaryThis paper aims to design a multi‐tone radiator exploiting harmonic radiation characteristic of time‐modulated antenna array for wireless information and power transmission (WIPT) and multi‐operational WIPT receiver. The time‐modulated linear antenna array (TMLAA) radiating simultaneously at modulating and harmonic frequencies separated in multiple switching periods is utilized as a multi‐tone radiating system. TMLAA with suitable power in harmonic bands generates multi‐tone radiation by employing an optimized switching sequence. The ON and OFF time instants of the TMLAA are optimized to suppress sidelobe level and enhance sideband level. A population‐based optimization algorithm, teaching learning based optimization (TLBO), is employed to optimize the ON and OFF instants of the TMLAA to suppress sidelobe level and enhance sideband level. TLBO is utilized for a 16‐element TMLAA with minimizing cost function to achieve the above objectives. The 16‐element TMLAA with optimized switching radiates multi‐tone beams with a minimum of 5 dB peak power difference exploited as WIPT. The purpose of multi‐operation is accomplished by utilizing the Wilkinson power divider in the receiver system; its power dividing capability is analyzed using applied wave research (AWR). A voltage doubler type rectifier is modeled for DC generation and is tested using AWR. The DC generating capability is tested for fixed power available at various frequencies, and the fixed desired frequency with various available power levels is tested. The results show that the designed circuitry provides a maximum of 80% power conversion efficiency (PCE).

Study on a Multi-Point Differential Feeding Strategy for Design of Filtering Patch Antennas With Stopband Enhancement

In this article, a novel multi-point differential feeding technique is introduced and applied in the design of filtering patch antennas with a wide upper stopband. Different from the traditional differential feeding structure, the proposed method has four physical feeding points on the patch. Our analysis results indicate that the proposed feeding approach can not only effectively excite the desired fundamental TM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\mathbf {01}}$ </tex-math></inline-formula> mode but also flexibly allow for the elimination of high-order modes by selecting suitable feeding positions. Just relying on this feeding method, unwanted harmonic radiations can be totally removed up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4{f} _{\mathbf {0}}$ </tex-math></inline-formula> without any additional elements. For demonstration, a prototype of a stacked patch antenna with filtering response has been then designed. With resorting to the proposed feeding scheme, both driven and stacked patches are indeed able to support good excitation of TM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\mathbf {01}}$ </tex-math></inline-formula> mode with other harmonics to be suppressed, leading to a wide upper stopband performance. In addition, several radiation nulls are generated in the stopband to improve the filtering selectivity. Measured results agree well with the simulated ones, indicating that the designed differential antenna possesses enhanced fractional impedance bandwidths, high in-band gains, low cross polarization, as well as intrinsic wide stopband rejection, further verifying the proposed feeding concept.

Cubic Nonlinearity of Tellurite and Chalcogenide Glasses: Terahertz-Field-Induced Second Harmonic Generation vs. Optical Kerr Effect

Third-order nonlinear susceptibilities χ3−2ω;ω,ω,0 and χ3−ω;ω,−ω,ω responsible for electric-field-induced second harmonic generation and the optical Kerr effect were measured and directly compared for tellurite and chalcogenide glasses. The nonlinear coefficients were found by measuring the second harmonic radiation from samples under the action of an external field of terahertz pulses and by the classical z-scan technique, respectively. The influence of ambient air and helium gas on second harmonic generation was analyzed. It was demonstrated that both susceptibilities χ3−2ω;ω,ω,0 and χ3−ω;ω,−ω,ω have close absolute values, which shows the dominant role of nonresonant electronic nonlinearity in the studied glasses.

High-order harmonic spectroscopy in an ionized high-density target

We use high-order harmonic spectroscopy to study ionization dynamics in a macroscopic target with tunable density, spanning over six orders of magnitude. In an in situ pump-probe experiment, the target is prepared at different densities with varying degrees of laser-induced ionization. High-order harmonic radiation is generated in the pre-ionized target, and a steepening in the decrease of the harmonic yield is observed for increasing pre-ionization, allowing not only to identify the contributing quantum paths during high-order harmonic generation but also in determining the amount of ionization within the target. The measurements allow probing of ionization dynamics in laser-induced plasma with high spatio-temporal resolution and are specifically of interest for the optimization of the harmonic generation process in high-density targets with number densities of up to 1022 cm−3.

Lattice design for an improved angular dispersion-induced microbunching scheme

In this paper, an improved angular dispersion-induced microbunching (ADM) scheme [1] is studied to generate fully coherent soft x-ray radiation pulses in an electron storage ring light source with a limited energy modulation amplitude. This scheme releases the coupling between the dispersion function and the momentum compaction in the original ADM scheme. The lattice structure of the improved ADM is simple, which only adds a chicane following the dogleg compared to the original one. With the chicane, even higher harmonic radiation can be generated. Numerical simulation results show that soft x-ray radiation with photon energy covering the water window range can be produced.

Grating-Lobe Suppression for Periodic Leaky-Wave Antennas at the Full Array Level

The scanning range of a periodic leaky-wave antenna (P-LWA) is limited by the appearance of grating lobes, which are brought by the radiation of higher order space harmonics. The proposed method in this letter manipulates the phases of space harmonics and suppresses the grating lobes at the full array level. The P-LWAs operate as subarrays with their perturbations successively shifted along the propagation direction. Feeding the P-LWAs with specific phases, the radiation of the desired space harmonic is retained while radiation of other space harmonics is cancelled out. The proposed method is capable of achieving a large scanning range with a relatively small waveguide index (defined as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${n}_g = {{{\lambda }_0} \mathord{/ {\vphantom {{{\lambda }_0} {{\lambda }_g}}} } {{\lambda }_g}}$</tex-math></inline-formula> ). Specifically, the single-beam full-space scanning can be realized with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${n}_g$</tex-math></inline-formula> less than 2, which is the lower limit for existing works. Based on the proposed method, A prototype consisting of three subarrays was designed with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${n}_g \!=\! 1.426.$</tex-math></inline-formula> A main lobe pointing to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$41.1^\circ $</tex-math></inline-formula> was realized with two grating lobes suppressed by more than 11 dB at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ - 6.5^\circ $</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ - 61.8^\circ $</tex-math></inline-formula> . The sidelobes brought by the slow-wave radiation were also obviously suppressed. Measured results agree well with the simulation and the theoretical prediction.

Power leakage minimization in time‐modulated linear antenna array with optimized switching strategies employing modified harmony search algorithm

Time-modulated linear antenna array (TMLAA) with periodic time modulation of antenna elements radiates infinite inevitable harmonic beams. The infinite harmonic sideband radiation is a wastage of power unless the power is exploited. This paper aims to minimize inevitable power wastage in the harmonic beams radiated from TMLAA by employing an optimized periodic timing sequence. The optimization of the timing sequence is conducted employing harmony search (HS) to suppress the power wastage, However, conventional HS achieves an acceptable sidelobe level (SLL) below – 30 dB. Nevertheless, a modified harmony search algorithm (MHSA) is proposed to attain more power in the desired fundamental beam. The algorithm is validated using an array of 4-, 10-, and 16-element uniform half-wavelength spacing TMLAA. The optimization is conducted on the ON and OFF time instants and the real current of the elements. The simulation results exhibit that HS and MHSA perform potentially alike to suppress the SLL of fundamental below –30 dB. However, the further suppression of the sideband levels employing MHSA justifies the modification proposed in this work. The MHSA performance in terms of radiation parameters is compared with the HS and also with previous works to show the potentiality of the modification.

Controlling LIPSS formation on Ni surface using near-infrared laser beam and its low-order harmonics

Ultrafast laser ablation of metal surfaces is a common physical process used to generate periodic surface structures with tuneable multifunctionality for various applications in science and technology. In this work, the formation of femtosecond laser-induced periodic surface structures (LIPSSs) is investigated with 50 kHz repetition rate laser of 40 fs pulse duration and a central wavelength () along with its second () and third ) harmonic components. The variation in the LIPSS periodicity is demonstrated by changing the wavelength of the ablating laser pulses. LIPSS with periodicity ∼0.85 m at the fundamental ablating pulses, ∼0.42 m periodicity at the second harmonic, and ∼0.35 m periodicity at the third harmonic were observed. The third harmonic radiation was generated in an air-filament with minimal temporal delay with respect to the fundamental driving pulses to structure Nickel surfaces at ambient conditions. The approach demonstrates the potential for generating very fine LIPSS on materials’ surfaces using the efficient high-order harmonics of an infrared fundamental laser beam.

CSES Satellite Observations of 50 Hz Power Line Radiation Over Mainland China and Its Response to COVID‐19

AbstractWe present a low‐altitude satellite survey of power line harmonic radiation (PLHR) at 50 Hz over Mainland China. We analyzed the month‐to‐month variation pattern in PLHR occurrence rate and further analyzed its correlation with some influencing factors (i.e., solar radiation, lightning flashes, and electricity consumption) using CSES satellite electric field data from 2019 to 2021. We also investigate the response of PLHR occurrence rate to COVID‐19. The statistical results show the dayside PLHR occurrence rate decreasing from winter to summer solstice and increasing from summer to winter solstice, which indicates it is controlled by the solar radiation. The nightside variation is more complex, which may be due to many sources that could influence the nightside lower ionosphere. The PLHR occurrence rate significantly decreased over Mainland China in February 2020, which is because of the significant decrease in electricity consumption due to the suspension of industrial production caused by COVID‐19.

High-order vortex harmonics generation by bi-circular Laguerre-Gaussian laser fields with relativistic plasmas

Vortex beams with ultra-high brilliance can greatly enrich the light and matter interaction process and even shed light on the unexpected information in relativistic nonlinear optics. Thus, we propose a scheme for relativistic intense vortex harmonic radiation by use of bi-circular Laguerre–Gaussian lasers irradiating relativistic plasmas. Three-dimensional particle-in-cell simulation results show that the emitted harmonics own controllable spin and orbital angular momentum simultaneously, which can be attributed to the vortex mirror model and the related conservation laws. Meanwhile, the conversion efficiency of harmonic generation can be tuned through adjusting the intensity ratio of the two driving field components.

High-harmonic spectroscopy of coherent lattice dynamics in graphene

High-harmonic spectroscopy of solids is a powerful tool, which provides access to both electronic structure and ultrafast electronic response of solids, from their band structure and density of states to phase transitions, including the emergence of the topological edge states, to the PetaHertz electronic response. However, in spite of these successes, high-harmonic spectroscopy has hardly been applied to analyze the role of coherent femtosecond lattice vibrations in the attosecond electronic response. Here we study coherent phonon excitations in monolayer graphene to show how high-harmonic spectroscopy can be used to detect the influence of coherent lattice dynamics, particularly longitudinal and transverse optical phonon modes, on the electronic response. Coherent excitation of the in-plane phonon modes results in the appearance of sidebands in the spectrum of the emitted harmonic radiation. We show that the spectral positions and the polarization of the sideband emission offer a sensitive probe of the dynamical symmetries associated with the excited phonon modes. Our work brings the key advantage of high-harmonic spectroscopy---the combination of subfemtosecond to tens of femtoseconds temporal resolution---to the problem of probing phonon-driven electronic response and its dependence on the dynamical symmetries in solids.

Reconfigurable Vector Vortex Beams Using Spoof Surface Plasmon Ring Resonators

In this article, a spoof surface plasmon (SSP) ring resonator to generate dynamically reconfigurable vortex beams is proposed. According to the phase constant control of the SSP and the harmonic radiation principle, the tunable cylindrical vector vortex beams (VVBs) with different topological charges can be obtained. The tunable system that operates by electronic control would utilize varactors to change the wavenumber of SPP on the ring resonator and realize the reconfigurable VVBs modes at a fixed frequency. All VVBs have excellent orbital angular momentum (OAM) purity (above 90%). Interestingly, the antenna also simultaneously radiates the scalar vortex beams with tunable OAM modes. Both numerical simulations and experimental characterizations confirm the theoretical predictions. Because the designed antenna has the advantages of simple feeding network, easy bias circuit, as well as miniaturization and integration compatibility, we anticipate that the antenna will enable a wide range of applications for future wireless communication and imaging technologies.

Coherent control of extreme ultraviolet emission generated through frustrated tunneling ionization

Coherent extreme ultraviolet (EUV) line emissions can be generated from an atom excited through frustrated tunneling ionization (FTI). The phase variation of the EUV emission in a generation medium along the propagation direction is a critical parameter that determines the phase-matching condition of this new light source. Here we show that the EUV emission sensitively depends on the intensity and phase of a driving laser pulse and the target position. Angle-resolved EUV spectra measured at different target positions and the carrier-envelope phases of the laser pulse exhibit an intensity modulation, showing similar behavior to that of a long-trajectory high harmonic radiation. The four-step model developed for the FTI emission accurately describes the coherent control of the EUV emission. These findings are essential ingredients for developing coherent extreme ultraviolet sources with high photon flux and for utilizing the FTI emission in time-resolved spectroscopy.

Tunable enhancement of harmonic radiation in coupled quantum wells

Tunable enhancement of harmonic radiation in coupled quantum wells

High power terahertz radiation generated by beam-plasma system in multi-filament regime

In the particle-in-cell simulation study of this paper, it is found that the high-power terahertz radiation at twice the plasma frequency can be generated by the interaction between a weakly relativistic, large-radius, focused electron beam and an overdense plasma. The terahertz radiation is emitted from a multi-filament structure in the electron beam, which is caused by the coupling of the longitudinal two-stream instability and the transverse filamentation instability. The analysis of the k space of the plasma waves indicates that the radiation can be explained by the mode coupling among a forward oblique mode, a backward oblique mode, and a second harmonic radiation mode. The influences of the beam Lorentz factor and plasma density on the radiation power, efficiency, and coherence are also investigated. The present results not only contribute to the development of novel megawatt-level narrowband terahertz radiation sources, but also propose a potential terahertz radiation mechanism for the fundamental research of the beam-plasma system.

Beam Asymmetry, Divergence and Energy Spread Effects on the Radiation from Planar Undulators

The theoretical study of the effect of electron beam parameters, in particular, the emittance and its asymmetry on the radiation from relativistic electrons in undulators is conducted both analytically and numerically. The reasons for the odd and even harmonic generation and radiation are explored. The difference in the underlying physical reasons for the spontaneous and stimulated radiation of harmonics in free electron lasers (FELs) is elucidated. The generalized forms of the special functions of the Bessel and Airy type are employed to account analytically for the off-axis and angular effects together with the effect of the beam energy spread. A comparative analysis of the radiation spectra for undulators with different beams is performed. The examples of the radiation at SPARC and LEUTL are given. The effect of the asymmetry of the beam on the radiation properties is analyzed. The alternative theoretical approaches of other authors are also employed for the analytical calculation of the harmonic powers in FELs. The results are compared with existing experimental data.

Optical second-and third harmonic radiation generation in a laser-produced plasma

The harmonic generation of a laser in gases is strictly dependent on the ionization dynamics. In this work, we study optical second and third harmonic radiation generation of a laser in an ionizing gas incorporating the electron-ion recombination effects. Neutral gas is irradiated by an intense laser field which generates free-electrons by tunnel ionization. If the laser pulse is long enough, the electron-ion recombination effects need to be accounted for laser dynamics in plasmas. The laser is assumed to have intensity near the tunnel ionization threshold so as to get plasma density varying with time, which in turn gives rise to strong second and third harmonic radiation fields. Here we demonstrate enhanced second- and third-harmonic generation in tunnel-ionizing gas by an intense laser pulse incorporating the effect of electron-ion recombination. The results show that the inclusion of electron-ion recombination reduces the laser defocusing and hence enhances the harmonic radiation amplitudes. A systematic numerical analysis has been done to understand the underlined physics.

Seeding with a harmonic optical klystron resonator configuration in a high repetition rate free electron laser

The generation of highly coherent radiation in a high repetition rate free electron laser driven by a superconducting linear accelerator has become a topic of growing interest. External seeding schemes like high-gain harmonic generation (HGHG) and echo-enabled harmonic generation are proven to be able to generate coherent radiation in the extreme ultraviolet and x-ray range. However, the repetition rate of current laser systems with sufficient power to modulate the electron beam is limited to the kilohertz range. Recently, to achieve seeding at a high repetition rate, an optical resonator scheme has been introduced to recirculate the radiation in the modulator to seed the electron bunches. In this paper, two harmonic optical klystron resonator configurations either starting with a seed laser or starting from shot noise are studied. With the harmonic optical klystron as the seeding source, the efficiency of harmonic radiation generation at a comparatively high harmonic can be significantly enhanced when compared with a standard single-stage HGHG. Simulation results show that highly coherent, stable, and high repetition rate pulses in the water window range could be generated by the proposed seeding schemes. Some practical considerations including beam energy chirp effects and the power density effects on mirrors are discussed.