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COVER ARTICLE

  

COVER ARTICLE

High-power high-energy four-channel fiber coherent beam combined system
SHI Zhuo, CHANG Hongxiang, WANG Dongliang, GUO Hongyu, DONG Zikai, DU Zhihang, LIANG Chengbin, LI Can, ZHOU Pu, WEI Zhiyi, CHANG Guoqing
2025, 74 (1): 014205. doi: 10.7498/aps.74.20241476
Abstract +
Ultrafast fiber laser sources with mJ-level pulse energy and kilo-watt average power are of particular importance for various science fields such as attosecond lasers. Currently, several large-scale facilities for attosecond lasers, including ELI-ALPS in Europe, SECUF in China, NeXUS in America and ALFA in Japan are under construction. High-performance femtosecond driven lasers are crucial for attosecond lasers and various ultrafast laser facilities. Fiber lasers have a large surface-to-volume ratio, which enables efficient cooling and is suitable for high average power amplification. However, due to small mode area of optical fibers, detrimental nonlinear optical effects such as self-phase modulation, four-wave mixing, and stimulated Raman scattering limit the peak power of pulse to hundreds of MW, corresponding to pulse energy of hundreds of μJ for femtosecond pulses in large mode area rod-type fibers. In addition, the average power of fiber lasers is limited by transverse mode instability, which reduces the stability and quality of beams above a certain threshold. In rod-type fibers, the threshold is about 250 W. Neither average power nor pulse energy emitted by single fiber meets the requirement for attosecond laser generation.The average power and pulse energy can be further scaled by coherent beam combination, which involves splitting pulses caused by an frontend laser and recombining them after amplification. It is essential for coherent beam combination to maintain the coherence of pulse replicas, which usually involves high speed photodiode detectors, piezo-driven mirrors, and other electronics forming a feedback system to actively control the phase of all replicas. We present a high-energy high-power ultrafast fiber laser system by using filled-aperture coherent combination of four ytterbium-doped rod-type fiber amplifiers. The phase control is achieved by using stochastic parallel gradient descent method. The frontend includes a passively mode-locked Yb-fiber oscillator, a stretcher, a pulse picker, and three fiber pre-amplifiers, which delivers 1 MHz stretched pulses centered at 1032 nm with 700 ps duration and 20 W average power. The pulse is split into four replicas by polarization beam-splitter and half-wave plate pairs, and the replicas pass through delay lines formed by piezo-driven mirrors before amplification. The pulse replicas are equally split and amplified to ensure the same accumulated nonlinear phase, and are combined by thin film polarizer and half-wave plate pairs. A small portion of the combined pulse is split and collected by a photodiode detector after being filtered spectrally and spatially, serving as a signal for controlling phase. The combined pulse is compressed by a compressor using a double-pass diffraction grating pair consisting of two 1739 l/mm gratings.At a repetition rate of 1 MHz, our four-channel Yb-fiber coherent beam combination system generates a combined average power value of 753 W and a combination efficiency of 87%. By utilizing an adjustable pulse stretcher and compressor, a 0.67 mJ, 242 fs near transform-limited pulse can be generated with a compressing efficiency of 89%. The compressed pulse is centered at 1032 nm, and the spectrum width is 8.8-nm. In the 30 min measurement, the root-mean-square of average power is less than 1% , while the residual phase error is less than λ/23, indicating excellent stability on different time scales. The beam quality factor of the 0.67 mJ compressed pulses is 1.17×1.11. At 500 kHz, we obtain pulses of 1.07 mJ and 247 fs with average power of 534 W, exhibiting similar efficiency, long-term stability, and beam quality. The residual phase error decreases below λ/29, indicating better short-term stability. Further scaling power and energy can be achieved by increasing the number of channels. By adding the delay stabilization system and pointing stabilization system, which are currently under development, an eight-channel CBC system can be used to generate 1 kW, 2 mJ pulses.In this work, we implement a four-channel coherent beam combining system based on the SPGD method, and obtain compressed pulses of 673 W, 673 µJ, and 242 fs at 1 MHz and 534 W, 1.07 mJ, and 247 fs at 500 kHz. Both power and energy can be further improved by increasing the channel number, and adding the delay stabilization system and pointing stabilization system which are under construction. By adding coherent pulse stacking amplification technology, the coherent beam combining system ought to generate pulse energy as high as 100 mJ, which constitutes the energy source for applications such as laser wake-field acceleration.

VIEWS AND PERSPECTIVES

  

EDITOR'S SUGGESTION

Recent research progress of ultracold-atom quantum simulation of Fermi-Hubbard model
HE Yuanyao, YANG Bing
2025, 74 (1): 017101. doi: 10.7498/aps.74.20241595
Abstract +
Fermi-Hubbard model is a fundamental lattice model describing correlated electron systems in condensed matter physics and is closely related to high-temperature superconductivity. In recent years, cold-atom quantum simulations have become an important paradigm for studying the Fermi-Hubbard model, and advances in quantum many-body computations have contributed to our understanding of its fundamental properties. Notably, a recent ultracold-atom experiment achieving the well-known antiferromagnetic (AFM) phase transition in the three-dimensional (3D) Hubbard model represents a key step in quantum simulation, laying a foundation for exploring the link between the quantum magnetism and high-temperature superconductivity. In this paper, the experimental and theoretical research progress of Fermi-Hubbard model in 3D systems is reviewed, the development history and present status in this field are discussed, and the future development direction is also prospected.The paper is organized as follows. To begin with, recent progress of observing AFM phase transitions in the 3D Hubbard model is reviewed, focusing on an ultracold-atom experiment conducted by the research group at the University of Science and Technology of China (USTC). Next, a theoretical introduction to the fundamental properties of the 3D Hubbard model is provided, in which prior theoretical studies is summarized, the current research status is outlined, and some unresolved or under-explored problems are discussed. In Section 3, the quantum simulation of the Hubbard model using ultracold atoms in optical lattices is discussed, and the basic principle, historical developments and key challenges are outlined. The USTC team overcame these challenges through innovative techniques such as atom cooling, large-scale uniform box traps, and precise measurements of the AFM structure factor. Their work successfully confirms the AFM phase transition via the critical scaling analysis. Finally, the significance of this achievement is emphasized, and the future research prospects of the 3D Hubbard model are discussed, including experimental studies on the doped regions and related theoretical benchmarks.

SPECIAL TOPIC—Correlated electron materials and scattering spectroscopy

  

EDITOR'S SUGGESTION

Spin excitation spectra of iron pnictide superconductors
LI Zezhong, HONG Wenshan, XIE Tao, LIU Chang, LUO Huiqian
2025, 74 (1): 017401. doi: 10.7498/aps.74.20241534
Abstract +
Spin fluctuations are often considered the most likely candidates for superconducting electron pairing media in unconventional superconductors. The iron-based superconductors provide a wide range of opportunities for studying the mechanism of unconventional superconductivity, as they have many systems with different structures and rich magnetisms. Taking the iron pnictide superconductors for example, this review summarizes the inelastic neutron scattering results of the spin excitation spectrum of iron-based superconductors, especially for their common features.Firstly, we introduce the direct connection between the low-energy spin excitations and superconductivity, which is so called the neutron spin resonance mode. This mode widely exists in the superconducting states of all iron-based superconductors, where the resonance energy ER is linearly proportional to the critical temperature Tc: ER = 4.9kBTc, and it has a universal c-axis preferred characteristic. The in-plane dispersion of spin resonance mode is not limited by the superconducting energy gap, which is in contrast to the traditional spin exciton model. The out-of plane dispersion of spin resonance mode is determined by the Fe-As interplanar distance, indicating that the three-dimensional spin correlation effect cannot be ignored, which may be the key to clarifying the role of spin fluctuations in superconductivity.Secondly, we summarize the energy dispersion, intensity distribution, and total fluctuating moment for high energy spin excitations. Although the Heisenberg model can roughly describe the similar dispersions in different systems based on the anisotropic in-plane nearest neighbor effective exchange couplings and the similar second nearest neighbor effective exchange coupling, the correlated Hubbard model based on itinerant magnetism can more accurately describe the spin wave behavior after degeneracy, thus the spin excitations are more likely to be understood from the perspective of itinerant magnetism. The spin excitation intensity varies greatly with energy in different systems, indicating a competitive relationship between itinerant and localized magnetic interactions. However, the total fluctuating moments are generally the same, indicating that the effective spin S = 1/2. The spin excitation bandwidth is in a range of 100–200 meV, probably is correlated with the height of As away from the Fe-Fe plane.Finally, we make a comprehensive comparison of the spin excitations in iron-based superconductors and copper oxide superconductors. The spin excitation spectra of iron-based superconductors have much richer physics than cuprates, due to the complex physics of multiple orbitals, Fermi surfaces, and energy gaps. These phenomena lead to the diversity of spin excitations, especially the prominent three-dimensional spin correlation effect. This indicates that interlayer pairing and intra layer pairing driven by spin interactions are equally important and must be fully considered in microscopic theories of high-Tc superconductivity.

SPECIAL TOPIC—Correlated electron materials and scattering spectroscopy

  

EDITOR'S SUGGESTION

Inelastic neutron scattering spectrometer and its applications
HU Ze, YUAN Yuan, LI Lisi, REN Qingyong, FENG Yu, SHEN Junying, LUO Wei, TONG Xin
2025, 74 (1): 012501. doi: 10.7498/aps.74.20241412
Abstract +
Inelastic neutron scattering is a pivotal technique in materials science and physics research, revealing the microscopic dynamic properties of materials by observing the changes in energy and momentum of neutrons interacting with matter. This technique provides important information for quantitatively describing the phonon dispersion and magnetic excitation of materials. Inelastic neutron scattering spectrometers can be divided into triple-axis spectrometers and time-of-flight spectrometers, according to the method of selecting monochromatic neutrons. The former has high signal-to-noise ratio, flexibility, and precise tracking capabilities for specific measurement points, while the latter significantly improves experimental efficiency through various measures. The application of inelastic neutron scattering spectrometers is quite extensive, playing an indispensable role in advancing frontier scientific research in the study of mechanisms in various materials such as magnetism, superconductivity, thermoelectrics, and catalysis. The high-energy inelastic spectrometer at the China Spallation Neutron Source is the first time-of-flight neutron inelastic spectrometer in China, achieving high resolution and multi-energy coexistence with its innovative Fermi chopper design. Additionally, the number of available single neutron beams in the experiment of this facility has reached the international leading level.

SPECIAL TOPIC—Correlated electron materials and scattering spectroscopy

  

EDITOR'S SUGGESTION

Neutron scattering studies of complex lattice dynamics in energy materials
REN Qingyong, WANG Jianli, LI Bing, MA Jie, TONG Xin
2025, 74 (1): 012801. doi: 10.7498/aps.74.20241178
Abstract +
Lattice dynamics play a crucial role in understanding the physical mechanisms of cutting-edge energy materials. Many excellent energy materials have complex multiple-sublattice structures, with intricate lattice dynamics, and the underlying mechanisms are difficult to understand. Neutron scattering technologies, which are known for their high energy and momentum resolution, are powerful tools for simultaneously characterizing material structure and complex lattice dynamics. In recent years, neutron scattering techniques have made significant contributions to the study of energy materials, shedding light on their physical mechanisms. Starting from the basic properties of neutrons and double differential scattering cross sections, this review paper provides a detailed introduction to the working principles, spectrometer structures, and functions of several neutron scattering techniques commonly used in energy materials research, including neutron diffraction and neutron total scattering, which characterize material structures, and quasi-elastic neutron scattering and inelastic neutron scattering, which characterize lattice dynamics. Then, this review paper presents significant research progress in the field of energy materials utilizing neutron scattering as a primary characterization method.1) In the case of Ag8SnSe6 superionic thermoelectric materials, single crystal inelastic neutron scattering experiments have revealed that the “liquid-like phonon model” is not the primary contributor to ultra-low lattice thermal conductivity. Instead, extreme phonon anharmonic scattering is identified as a key factor based on the special temperature dependence of phonon linewidth.2) Analysis of quasi-elastic and inelastic neutron scattering spectra reveals the changes in the correlation between framework and Ag+ sublattices during the superionic phase transition of Ag8SnSe6 compounds. Further investigations using neutron diffraction and molecular dynamics simulations reveal a new mechanism of superionic phase transition and ion diffusion, primarily governed by weakly bonded Se atoms.3) Research on NH4I compounds demonstrates a strong coupling between molecular orientation rotation and lattice vibration, and the strengthening of phonon anharmonicity with temperature rising can decouple this interaction and induce plastic phase transition. This phenomenon results in a significant configuration entropy change, showing its potential applications in barocaloric refrigeration.4) In the CsPbBr3 perovskite photovoltaic materials, inelastic neutron scattering uncovers low-energy phonon damping of the [PbBr6] sublattice, influencing electron-phonon coupling and the band edge electronic state. This special anharmonic vibration of the [PbBr6] sublattice prolongs the lifetime of hot carriers, affecting the material's electronic properties.5) In MnCoGe magnetic refrigeration materials, in-situ neutron diffraction experiments highlight the role of valence electron transfer between sublattices in changing crystal structural stability and magnetic interactions. This process triggers a transformation from a ferromagnetic to an incommensurate spiral antiferromagnetic structure, expanding our understanding of magnetic phase transition regulation.These examples underscore the interdependence between lattice dynamics and other degrees of freedom in energy conversion and storage materials, such as sublattices, charge, and spin. Through these typical examples, this review paper can provide a reference for further exploring and understanding the energy materials and lattice dynamics.

SPECIAL TOPIC—Correlated electron materials and scattering spectroscopy

  

EDITOR'S SUGGESTION

Application of neutron scattering to studying low lattice thermal conductivity of Zintl phase compounds
ZHANG Cuiping, ZHU Jinfeng, SHEN Xiaoling, SHU Mingfang, REN Qingyong, MA Jie
2025, 74 (1): 017301. doi: 10.7498/aps.74.20241163
Abstract +
Due to the unique crystal structures and excellent transport properties, the Zintl phase thermoelectric materials have aroused extensive interest in energy storage and conversion. To explore the origins of those excellent performances, a series of experimental and theoretical techniques have been applied, such as neutron scattering, thermal conductivity, and molecular dynamics simulations with machine learning. In this paper, the progress of neutron scattering research on the structure and dynamics of Zintl phase is summarized, for example A14MPn11 compounds with zero-dimensional (0D) substructures, 1D chains-based compounds, 2D layered A2BX2 compounds (including the binary Mg3Sb2) and their structural variants, as well as AB4X3, and ZrBeSi-type compounds. The underlying mechanisms of intrinsically low lattice thermal conductivity in those Zintl phase are discussed in detail. These compounds generally exhibit the following characteristics: 1) strong anharmonicity, which is characterized by strong atomic vibrations and anharmonic phonon-phonon scattering; 2) weak chemical bonding, which usually leads to low sound velocity and interatomic force constants, and corresponding to low-energy phonon branches; 3) intrinsic vacancy defect, which weakens the bond strengths, softens the lattice, and enhances anharmonic phonon-phonon scattering. Neutron diffraction is applied to studying crystal structures, lattice parameters, atomic occupancies, and atomic displacement parameters. Inelastic neutron scattering measures the lattice dynamics, and density of states, which are related to lattice thermal conductivity. Hence, the physical mechanisms of Zintl compounds are analyzed for optimizing material properties and designing new functional materials.

INSTRUMENTATION AND MEASUREMENT

  

EDITOR'S SUGGESTION

Infrared-modulated photoluminescence spectroscopy: From wide-band coverage to micro-area and high-throughput scanning imaging
SHAO Jun, CHEN Xiren, WANG Man, LU Wei
2025, 74 (1): 017801. doi: 10.7498/aps.74.20241491
Abstract +
Photoluminescence (PL) spectroscopy has been widely used in the ultraviolet-near-infrared spectral range for over seventy years since its early reporting in the 1950’s, because it not only reveals the electronic structure information about such as band gap and impurity energy levels of semiconductor materials, but also serves as an efficient tool for analyzing interfacial structures, carrier lifetime, and quantum efficiency. However, in the infrared band beyond about 4 μm, the study of PL spectrum has been limited for decades due to strong thermal background interference, weak PL signals and low detection capability. In this review, a traditional PL method is introduced based on a Fourier transform infrared (FTIR) spectrometer, and a continuous-scan FTIR spectrometer-based double-modulation PL (csFTIR-DMPL) method is briefly described which was proposed in 1989 for breaking through the dilemma of the infrared band, and developed continuously in the later more than 20 years, with its limitations emphasized. Then, a step-scan FTIR spectrometer-based infrared modulated PL (ssFTIR-MPL) method reported in 2006 is analyzed with highlights on its advantages of anti-interference, sensitivity and signal-to-noise ratio. The effectiveness demonstration and application progress of this method in many research groups around the world are listed. Further developments in recent years are then summarized of wide-band, high-throughput scanning imaging and spatial micro-resolution infrared modulated PL spectroscopic experimental systems, and the technological progresses are demonstrated of infrared-modulated PL spectroscopy from 0.56–20 μm visible-far-infrared broadband coverage to >1000 high-throughput spectra imaging and ≤2–3 μm spatial micro-resolution. Typical achievements of collaborative research are enumerated in the visible-far-infrared semiconductor materials of dilute nitrogen/dilute bismuth quantum wells, HgCdTe epitaxial films, and InAs/GaSb superlattices. The results presented demonstrate the advancement of infrared modulated PL spectroscopy and the effectiveness of the experimental systems, and foresee further application and development in the future.

RAPID COMMUNICATION

  

EDITOR'S SUGGESTION

Variation law of micro-void distribution characteristics in early stage of spallation damage
ZHANG Fengguo, WANG Yanjin, WANG Pei, WANG Xinxin
2025, 74 (1): 014601. doi: 10.7498/aps.74.20241338
Abstract +
The development trend of spallation damage mechanics is to construct a physical model that couples information with micro-mesoscale structure of materials, which also promotes the development of numerical calculation methods, experimental techniques and theoretical research. The mechanism responsible for plastic deformation and failure of structural metal materials at high strain rates is complex and ainfluenced by heterogeneities in the micro-mesoscale structure that comprises the distribution of grain boundaries, interfaces, and pre-existing densities voids. The distribution of these mesoscale heterogeneities can provide either strengthening behavior or void nucleation sites and influence spall failure behavior. Due to the lack of evolutionary information of micro-mesoscopic void distribution characteristics, the current spallation damage model is not only restricted in its application in extreme environments with high strain rates, high pressures, and shock, but also does not effectively provide some information about the correlation between material damage and final material fragmentation particle size, which is of very concern in engineering. Therefore, it is urgent to develop a spallation damage model that can reflect the variation law of micro-mesoscopic void distribution characteristics in damaged materials. The probability distribution function of void nucleation based on cosine function is given in this work by analyzing various influencing factors in the process of void nucleation, combining the characteristics of early void growth, and considering the convenience of analytical solution. The analytical calculation results of the new probability function of void nucleation are consistent not only with the results of the variation of void number with time calculated by molecular dynamics, but also with the experimental results of tantalum spallation in the early stage of damage development, that is to say, the new probability function of void nucleation can reflect the variation law of micro-void distribution characteristics in the early stage of spallation damage to a certain extent.

EDITOR'S SUGGESTION

Machine learning identification of fractional-order vortex beam diffraction process
GUO Yan, LYU Heng, DING Chunling, YUAN Chenzhi, JIN Ruibo
2025, 74 (1): 014203. doi: 10.7498/aps.74.20241458
Abstract +
Fractional-order vortex beams possess fractional orbital angular momentum (FOAM) modes, which theoretically have the potential to increase transmission capacity infinitely. Therefore, they have significant application prospects in the fields of measurement, optical communication and microparticle manipulation. However, when fractional-order vortex beams propagate in free space, the discontinuity of the helical phase makes them susceptible to diffraction in practical applications, thereby affecting the accuracy of OAM mode recognition and severely limiting the use of FOAM-based optical communication. Achieving machine learning recognition of fractional-order vortex beams under diffraction conditions is currently an urgent and unreported issue. Based on ResNetA, a deep learning (DL) method of accurately recognizing the propagation distance and topological charge of fractional-order vortex beam diffraction process is proposed in this work. Utilizing both experimentally measured and numerically simulated intensity distributions, a dataset of vortex beam diffraction intensity patterns in atmospheric turbulence environments is created. An improved 101-layer ResNet structure based on transfer learning is employed to achieve accurate and efficient recognition of the FOAM model at different propagation distances. Experimental results show that the proposed method can accurately recognize FOAM modes with a propagation distance of 100 cm, a spacing of 5 cm, and a mode spacing of 0.1 under turbulent conditions, with an accuracy of 99.69%. This method considers the effect of atmospheric turbulence during spatial transmission, allowing the recognition scheme to achieve high accuracy even in special environments. It has the ability to distinguish ultra-fine FOAM modes and propagation distances, which cannot be achieved by traditional methods. This technology can be applied to multidimensional encoding and sensing measurements based on FOAM beam.

EDITOR'S SUGGESTION

Analysis on plasma and electric field property of TM011 mode MPT and its tuning experiment
YANG Juan, SUN Jianghong, WANG Yuxuan, LUO Lingfeng, ZHANG Yan, KANG Xiaolu, JIA Qingqing
2025, 74 (1): 015201. doi: 10.7498/aps.74.20241118
Abstract +
Microwave plasma thruster (MPT) is a kind of electrothermal thruster. Inside its cylindrical cavity, the plasma process, microwave electric field distribution, and TM011 mode resonant state are important factors affecting the performance of MPT seriously. According to previous MPT formed through continuous regulation in the resonant sate of cylindrical cavity, the research is needed on a newly fixed and simple MPT, which will simplify the resonant state regulation and lays an important foundation for further study. Therefore the plasma process is analyzed to find the optimal gas discharge condition, and the microwave electric field intensity and power density distribution inside the cavity running in TM011 resonant sate are calculated to analyse how the parameters are influenced by the cavity dimensions. The resonant state is finely regulated to study how it is influenced by the dimensions of cylindrical cavity and microwave coupling probe with ball and half ball structure. The results of theoretical analysis and calculation show that the discharge power of helium gas is the lowest under the condition of 489 Pa and when the ratio of length to diameter is greater than 1, the microwave electric density distribution inside the cavity is beneficial. Owing to the appropriate length and radius of microwave coupling ball probe, the experiment on resonant state regulation shows that the shortest cylinder cavity is in the optimal resonant sate, with a resonance frequency very close to 2.45 GHz. The helium discharge experiment proves that the cavity and matching ball probe enable high microwave utilization and easy helium gas discharge, and the structure scheme is correct and reliable.
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