The K-4GSR project targets a 4.0 GeV 4th generation synchrotron radiation source with ∼ 800 m circumference and 4 GeV beam energy. It features a 7 Multi-Bend Achromat (MBA) lattice structure with a beam emittance of less than 100 pm. It employs a 4 GeV full-energy injection using a booster installed in the same tunnel with the storage ring. The booster ramps the 200 MeV injection beam to 4 GeV beam energy for the storage ring injection and operates at 2 Hz. The booster consists of 60 combined-function dipoles, and 66 quadrupoles, with additional sextupoles and correctors. Combined-function dipoles are chosen as bending lattice elements to minimize the number of magnets and the total costs. In this study, the 2D pole face optimization, the 3D FEM analysis, and a detailed reference orbit, which is slightly different from a pure bending orbit are analyzed.

PAL-XFEL is planning to install second hard X-ray undulator line (HX2) to meet the high beamtime demand from the users. The photon energy range for the second hard X-ray beam line is from 2~ to 11 keV which is lower than the first hard X-ray photon energy range (2 ~ 20 keV). The required undulator parameters are 35 mm period, max Keff=3.48 at 9.00 mm gap, ~ 3.0 m magnetic length with phase error less than 5 degrees. In addition to the existing conventional undulator design, horizontal gap vertical polarized undulator (HGVPU) concept is also being considered. HGVPU is well developed by LCLS-II team and applied in LCLS-II. In this report, we summarize the VPU design for PAL-XFEL HX2, and reports progress in the prototyping.

Ever since the advent of quantum mechanics, tunneling has been an intriguing topic and consequently extensively studied and utilized. Investigating both theoretically and experimentally the nonadiabatic tunneling in strong-field ionization across a wide range of laser intensities, we unravel under-the-barrierrecollision dynamics leading to Freeman resonances (FR). The under-the-barrier-recollision model, which goes beyond the traditional direct multiphoton transition description, predicts distinct features of FR phenomena that cannot be explained by the existing direct multiphoton transition scenario. Specifically, it predicts (i) the dominance of high-order FR over above-threshold ionization in the photoelectron energy spectra and (ii) the flat dependence of the FR signal on the laser intensity, both in the nonadiabatic tunneling regime. This Letter experimentally demonstrates these features, corroborating the under-thebarrier-recollision model, and provides intuition into this dynamics, expanding our insights into the control of tunneling dynamics in laser spectroscopy and attosecond physics.

We report our systematic investigation of temperature-dependent Barkhausen volume behavior for CoFeB/Pd with perpendicular magnetic anisotropy by means of magneto-optical Kerr microscopy. In the temperature range where two-dimensional (2D) Ising-like and single-domain features are sustained, hysteresis parameters such as coercivity, hysteresis area, and saturation magnetization are quantitatively analyzed with respect to the temperature. Interestingly it is demonstrated that the Barkhausen length is directly proportional to the domain-wall width via the magnetic anisotropy in this single-domain and 2D Ising-like model system.

The utilization of electromagnetic waves is rapidly advancing into the millimeter-wave frequency range, posing increasingly severe challenges in terms of electromagnetic pollution prevention and radar stealth. However, existing millimeter-wave absorbers are still inadequate in addressing these issues due to their monotonous magnetic resonance pattern. In this work, rare-earth La3+ and non-magnetic Zr4+ ions are simultaneously incorporated into M-type barium ferrite (BaM) to intentionally manipulate the multi-magnetic resonance behavior. By leveraging the contrary impact of La3+ and Zr4+ ions on magnetocrystalline anisotropy field, the restrictive relationship between intensity and frequency of the multi-magnetic resonance is successfully eliminated. The magnetic resonance peak-differentiating and imitating results confirm that significant multi-magnetic resonance phenomenon emerges around 35 GHz due to the reinforced exchange coupling effect between Fe3+ and Fe2+ ions. Additionally, Mössbauer spectra analysis, first-principle calculations, and least square fitting collectively identify that additional La3+ doping leads to a profound rearrangement of Zr4+ occupation and thus makes the portion of polarization/conduction loss increase gradually. As a consequence, the La3+–Zr4+ co-doped BaM achieves an ultra-broad bandwidth of 12.5 + GHz covering from 27.5 to 40 + GHz, which holds remarkable potential for millimeter-wave absorbers around the atmospheric window of 35 GHz.

X-ray free-electron lasers (XFELs) are powerful tools for characterizing and probing the properties of matter at atomic resolution on the ultrafast timescale. However, they have certain limitations such as spectral fluctuation and poor temporal coherence. Atomic X-ray lasers offer the narrow bandwidth, longitudinal coherence, and spectral stability that can overcome these limitations. In this paper, we study the interaction of inner-shell vacancy states with high-intensity XFEL pulses. We show that it is possible to achieve population inversion between K-shell and L-shell vacancy states in calcium and titanium when pumped by high-intensity XFEL pulses. These states can be used to generate atomic X-ray laser emission in the 3-5 keV photon energy range.

We propose the generation of a widely tunable UV-to-IR frequency comb by high-order sideband generation (HSB) spectrum emitted from semiconductors. In our theoretical simulations, we demonstrate the high-order sideband signals of two series (2m Omega(seed) + (2n + 1)omega(driver), and (2m + 1)Omega(seed) + 2 n omega(driver)), where m and n are integers of a seed pulse and a driver laser frequency, respectively. The simulations also reveal the intensity of HSB scale with the driver laser power, both perturbatively and non-perturbatively. We find that the harmonic position and spacing of the high-order sideband emission can be controlled by varying the seed pulse and driver photon energies. In the experiment, we applied a visible ((h) over bar Omega(seed) = 3.1 eV, similar to 400 nm) seed pulse and mid-infrared (MIR, (h) over bar omega(driver) = 0.4 eV, 3.1 mu m) driver pulses to ZnSe target. Our experimental observations confirmed the UV (4.7 eV, 263 nm and 3.9 eV, 317 nm) HSB generation.

Strong-field photoelectron holography is promising for the study of electron dynamics and structure in atoms and molecules, with superior spatiotemporal resolution compared to conventional electron and X-ray diffractometry. However, the application of strong-field photoelectron holography has been hindered by inter-cycle interference from multicycle fields. Here, we address this challenge by employing a near-single-cycle field to suppress the inter-cycle interference. We observed and separated two distinct holographic patterns for the first time. Our measurements allow us not only to identify the Gouy phase effect on electron wavepackets and holographic patterns but also to correctly extract the internuclear separation of the target molecule from the holographic pattern. Our work leads to a leap jump from theory to application in the field of strong-field photoelectron holography-based ultrafast imaging of molecular structures.

Ultrafast control of both electric and spin currents triggered by femtosecond laser pulse has attracted much attention due to future applications for broadband THz emitter as well as high-speed spintronic devices. Optically generated spin current is converted to charge current via multiple spin-charge conversion mechanisms, generating THz wave emission in magnetic multilayers. However, to date, quantitative and comparative investigation of THz emission originating from spin-charge conversion mechanisms has not yet been fully explored. Here, direct and straightforward nondestructive probing to measure THz emission is provided at original Co/Pt and Co/Ta interfaces embedded in Pt/Co/Ta multilayers with polarization analysis of both optical pump and THz emission. These results allow a fundamental understanding of various spin-charge conversion phenomena, which is a key basis for future spintronic THz source development. The first quantitative approach to separate multiple spin-charge conversion mechanisms in magnetic multilayer is reported. The approach is based on THz-based experiment, which can be nondestructive probe for the THz emission at original interfaces embedded in multilayers. The work provides a quantitative analysis of spin-charge conversion phenomena, which can be a key for developing THz source based on magnetic multilayers. image

Anion defect engineering is proven to be an efficient approach to reconstruct the electronic configuration of carbon-based magnetoelectric materials for targeted modulation of electromagnetic (EM) performance. However, traditional mono-anionic doping suffers from low defect concentration and lacks diverse polarization mechanisms. In this work, multi-anions (N/S/F) stepwise-doped carbon/Fe3C magnetoelectric composites are elaborately constructed, wherein the predesigned N defects serve as activated sites for anomalously adopting S anions (Step I) and subsequent F anions (Step II) in non-marginal areas of the carbon layer. It is found that S prefers to replace pyrrolic N defects while F tends to form dangling bonds with the C site adjacent to the pyridinic N. Intriguingly, besides the inherent polarized resonance of N defect at approximate to 15 GHz, customized S and F defects induce new polarization resonances at approximate to 10 GHz and approximate to 15+ GHz, respectively. Under a typical multi-polarization effect with the synergetic magnetic response, the carbon/Fe3C composites with N/S/F defects harvest the broadest bandwidth of 8.28 GHz (9.72-18 GHz) at 2.55 mm, covering a wide frequency range almost from X to Ku bands. This work demonstrates the positive impact of localized multi-defects customization and multi-polarization effect on expanding microwave absorption bandwidth, providing valuable insights for the advanced design of ultra-broadband absorbers. This work constructs the multi-anions (N/S/F) non-edge doped carbon/Fe3C magnetoelectric composites are elaborately to develop synergetic multi-polarization effect covering the frequency range of 9-18 GHz, realizing the broadest bandwidth of 8.28 GHz at 2.55 mm. Therefore, this study offers reliable reference value for developing carbon-based ultra-broadband absorbers by localized multi-defects customization and multi-polarization effect. image

The domain-wall motion in ferromagnetic films exhibits stochastic behavior due to thermal agitation with quenched disorders. The stochasticity is an obstacle in the sense of consistent repeatability of domain-wall position control in magnetic domain-wall devices. In general, the level of stochasticity is expected to decrease as driving force increases. This property suggests that the magnetic domain-wall devices are capable of simultaneously achieving both high operational speed and decrease in the level of stochasticity. However, we report here an observation of stochasticity anomaly, which involves a significant increase in relative speed dispersion near the depinning field. Domain-wall motion measurements were performed in ferromagnetic wires with perpendicular magnetic anisotropy over the same position to measure the relative speed dispersion. The stochasticity in domain-wall motion is intertwined with the number of pinning–depinning events throughout the domain-wall motion. The size of cluster events, which leads to the number of events, reflects the trend in the relative speed dispersion. The observed anomaly is provided by occurrence of large avalanches of cluster events. The anomaly occurs within the tech-relevant speed range of 1–100 m/s, signaling the necessity of material engineering to mitigate its effects.

Despite the abundance of research on the dynamics of magnetic domain walls (DW) on thin-film media, in-depth discussions regarding the behavior around the so-called Walker breakdown phenomenon are still scarce. In this work, propagation dynamics of the Bloch-type DW on CoFeB nanostripes with perpendicular anisotropy driven by a nanosecond magnetic pulse are investigated using micromagnetic simulation. It is found that on smaller nanostripes, the Walker breakdown corresponds with the oscillatory motion of the DW. On larger nanostripes, the formation of a Bloch-line with previously unreported behavior can be observed for magnetic pulses above HWB. Using a nanosecond field pulse instead of a continuous external field may allow for more direct control over the oscillatory DW motion and nucleation of the Bloch-line. A deeper understanding of the dynamics of the Bloch-type DW and its correlation to the applied nanosecond pulse may lead to better control of the DW for future spintronic devices.

High-order harmonics (HH) have drawn attention in the field of condensed matter physics mainly because of the capability of light to encode structural, dynamical, and topological information. In this paper, we address the fundamental question whether HH can map topological information in two-dimensional (2D) quantum materials by studying the interaction between topological materials and an elliptically polarized laser. We use the Haldane model for topological Chern insulators (CIs) and the Kane-Mele model for topological insulators (TIs). In the case of a circularly polarized or nearly circularly polarized driving field in CIs and TIs, the harmonic intensity of the co-rotating orders is increased. This increase in topologically non-trivial materials implies that HH can be used to detect topological transitions in 2D CIs and TIs. Moreover, interference between two spin bands in TIs does not affect the elliptical dependence of co-rotating harmonic orders in the plateau region.

We demonstrate an effective method to generate high-energy narrowband THz pulses by super-Gaussian laser pulses produced from the high-efficiency (over 55%) optical parametric amplifier (OPA).

Demagnetization of ferromagnets initiated by pulsed laser has attracted attention due to ultrafast magnetism quenching within less than a picosecond timescale. The mechanisms for demagnetization have been discussed for a long time and most discussions have focused on secondary processes with angular momentum dissipation [1] and thermal processes [2].

We present a novel nonlinear interferometric autocorrelation technique with unbalanced intensity, which preserves spectral phase information in the autocorrelation signal and enables the complete characterization of ultrafast optical fields without requiring any spectrally resolved measurements.

Very recently, THz emission and its application for THz spintronics has attracted much attention. In this review paper, universally observed THz emission generated by ultrafast demagnetization/remagnetization is introduced for representative 3d transition metal ferromagnets such as Fe, Ni, and Co films. Mostly, THz emission from relatively thick 30-nm films have been investigated to minimize the spin-charge conversion effect existing at the interface between ferromagnetic and nonmagnetic layers, with variation of external magnetic fields, laser pump fluences, and protection layers.

We investigate the interaction between the counter-rotating bicircular field and the trivial and topological insulator with anomalous Hall conductivity (AHC) to show the effect of the asymmetric spin band and topological invariant. We show that the reaction of the system to the counter-rotating bicircular field is classified into the high-field and low-field regimes. In the high-field regime, it is shown that the AHC of the system is controlled by the phase difference between the ω0 and 2ω0 fields. We also show that in the low-field regime, the AHC of the topological insulator is determined by the helicity of the laser, while the AHC is negligible in the trivial insulator. For the spin-orbit coupling (SOC), it is demonstrated that the high SOC increases the required field amplitude for the transition from the low-field to the high-field regime. Also, we show that strong SOC leads to an additional sign change of the AHC in the high-field regime, but with different origins in the trivial and topological insulator.

We develop the theoretical framework of nonequilibrium ultrafast photonics in monolayer quantum spin-Hall insulators supporting a multitude of topological states. In these materials, ubiquitous strong light-matter interactions in the femtosecond scale lead to non-adiabatic quantum dynamics, resulting in topology-dependent nonlinear optoelectronic transport phenomena. We investigate the mechanism driving topological Dirac fermions interacting with strong ultrashort light pulses and uncover various experimentally accessible physical quantities that encode fingerprints of the quantum material's topological electronic state from the high harmonic generated spectrum. Our work sets the theoretical cornerstones to realize the full potential of time-resolved harmonic spectroscopy for identifying topological invariants in two-dimensional quantum spin-Hall solid state systems.

CsPbI3 perovskite quantum dots (CsPbI3-PQDs) have a high potential as semi-transparent photovoltaic absorbers because of the facile control of film thicknesses, size-tunable optical band gaps, and nanometer-scale grain sizes suppressing light scattering. Conventional semi-transparent CsPbI3-PQD solar cells showed low photovoltaic performances due to the low electrical conductivity of the graphene electrodes. Here we report that dielectric/ ultra-thin metal/dielectric (DMD) electrodes with excellent optical transmittance and electrical conductivity deliver superior photovoltaic performances in the semi-transparent CsPbI3-PQD solar cells. Particularly, the asymmetric DMD electrodes composed of MoOx 15 nm/Au 10 nm/MoOx 35 nm (asym-MAM) stacks exhibit higher optical transmittance than that of the symmetric MoOx 15 nm/Au 10 nm/MoOx 15 nm (sym-MAM) stacks. The optical simulation confirms that asym-MAM stacks reduce the parasitic absorption loss in metal interlayers. Therefore, the asym-MAM stacks with a high electrical conductivity show a higher average visible transmittance (AVT) than that of the sym-MAM. Consequently, the semi-transparent CsPbI3-PQD solar cells fabricated using the asym-MAM transparent top electrodes show a power conversion efficiency of 11.3% with a considerable AVT values of 23.4% (400-800 nm) and 20.0% (380-780 nm), respectively, which were calculated from their transmittance spectra. This is the highest-efficiency semi-transparent PQD solar cells that can be used for solar window applications requiring an AVT value of over 20%.