To realize the cyclotron resonance at the fundamental harmonic between a 1 THz wave and electrons gyrating in the external magnetic field the magnetic field should be about 38 T. This value is well above what is currently achievable in cryomagnets used in gyrotron experiments (up to 10-15T).The pulsed magnet can generate a high magnetic feld (up to 100T), promoting the development of high-frequency high-power gyrotrons. In this oral presentation, a system of the 40T Flat-top Pulsed Magnet Field for 1THz Gyrotron is introduced.Firstly,a fast cooling and long lifetime 40 T pulsed magnet is developed for a 1 THz gyrotron. Internal cooling channels are introduced to increase the cooling speed without compromising mechanical strength. It is estimated that the lifetime is about 20,000 shots at 40 T. The magnet is capable of realizing a 40 T fat-top pulsed feld (fat-top time 10 ms) with a cool down period of 16 minutes. Secondly,a novel topology, which consists of a sequentially fired pulse forming network (SFPFN) and a coupled inductor active filter, has been developed to generate large current with a high-stability flat-top. Thirdly, a control method based on model predictive algorithm is utilized to compensate the circuit parameters change in a wide range. The thermodynamic models for every circuit state of the topology are built to predict the future value of the state variable. Based on those models, the algorithm of the model predictive control (MPC) is constructed, in which the influence of circuit state transition on the optimal control action is considered. At last, a full-scale system of 10 kV/20 kA is constructed, and the pulsed magnetic field of 40 T/10ms/96 ppm are achieved.This can facilitate the long-pulsed quasi-steady operation of gyrotron operation.

In a conventional gyrotron with a cylindrical cavity, the nonsymmetric modes rotate azimuthally, forming two orthogonal vortex waves with different rotating directions. However, such two waves can be degenerated due to the effect of a misaligned electron beam. The paper investigates the degeneracy and mode competition of two oppositely rotating modes in an THz gyrotron with a misaligned electron beam, using a multimode, time-dependent, self-consistent model. The results show that the beam misalignment induces azimuthal nonuniformity of beam-wave coupling strength, which can lead to the cooperative existence of two oppositely rotating modes. This provides insights into understanding the experimental results where quasi-standing-wave patterns is observed in a THz gyrotron.

Terahertz (THz) radiation possesses unique characteristics that offer enormous application value in frontier scientific research fields, including the high-sensitivity analysis of biomacromolecular structures and various other areas. For advanced spectrometers such as Dynamic Nuclear Polarization Enhanced Nuclear Magnetic Resonance (DNP-NMR) and Electron Spin Resonance (ESR), increasing the power and frequency of THz wave sources is crucial for enhancing the resolution and signal-to-noise ratio of the instruments. However, traditional slow-wave devices and quantum cascade lasers encounter significant challenges when transitioning to the THz frequency range, which often limits their THz power to typically less than ten watts. Fast wave devices, such as gyrotrons based on the Electron Cyclotron Maser (ECM), provide significant advantages in terms of high frequency and power. These devices leverage the efficient interaction between gyrating electrons and fast waves within a resonant cavity, facilitating the generation of high-power radiation in the THz frequency range, with output levels reaching up to megawatt level. However, gyrotrons operating in the THz frequency range require high magnetic field strengths, e.g. approximately 37T is required for 1THz, which exceeds the capability of existing commercial superconducting magnets (usually below 20T). To increase the output frequency of the gyrotron, it is essential to employ high-harmonic operation or pulsed high magnetic field techniques. The Wuhan National High Magnetic Field Center (WHMFC) has developed an 800 GHz second harmonic gyrotron. An experimental research platform has been established, including a 15T superconducting magnet, power supply system, and THz wave measurement system. Experimental results indicate that the stable oscillation of TE_8,5 second harmonic operating mode can be achieved, with a frequency around 799.6 GHz and a maximal power output of 150W^[1]. This gyrotron can satisfies the demands for high-power THz wave sources required for applications such as DNP-NMR and ESR. Currently, a 1THz gyrotron based on a flat-top pulsed magnetic field with duration of 10ms is under development. A 40T long-pulsed magnet has been designed, manufactured and tested successfully^[2]. The 1THz gyrotron tube has been design and fabricated and is under test. In the near future, a series of gyrotrons operating at 140GHz, 400GHz, and other frequencies will be developed to be applied in DNP-NMR and ESR spectrometers for scientific research on the structural analysis of biomacromolecules.

Utilizing visual information perception makes it easier to obtain state-related data, such as color, position, and size. However, in industrial scenarios that require large-scale deployment and in home settings with high replacement frequency, the cost of using cameras to capture visual information can be quite high. We explore how to achieve the desired perception effects through vision-inspired computing. We will share new initiatives in the wine making industry, where vision-like information is used to observe microbial changes that are typically only visible under a microscope. In home scenarios, we investigate how to provide recommendations for oral health using vision-inspired computing.

Terahertz waves (0.1THz~10THz) are electromagnetic waves which is between millimeter waves and infrared light. Terahertz waves due to their excellent performance, are widely applied in the fields of detection, communication, and material testing. Obtaining high-power, wide-bandwidth, high-quality terahertz sources is the basis and prerequisite for system applications. Among various types of terahertz sources, the traveling wave tube (TWT) has a significant advantage in terms of power bandwidth product and power-to-mass ratio compared to another device. This report will present the research work of terahertz traveling wave tubes carried out by National Key Laboratory of Science and Technology on Vacuum Electronics. During the development of the THz TWTs, the key laboratory has made breakthroughs in a series of key technologies. Novel slow-wave structures have been designed to enhance the interaction efficiency between electrons and electromagnetic waves. Concurrently, by employing PCM-focused sheet electron beams, the output power has been increased by more than double. Several TWTs at G-band and 340GHz have been simulated, fabricated and tested, which achieved pulsed output power of 100 W and continuous wave output power of 30 W at 220GHz, and continuous wave output power over 6 W at 340 GHz. The development of 1THz TWT is also underway.

This presentation showcases the channel estimation performance in massive multiple-input multiple-output (MIMO) systems with Ricean fading through considering the relative channel estimation error (RCEE) metric. The closed-form expressions of the probability distribution function (PDF) and cumulative distributionfunction of RCEE are derived, according to which the PDFs of RCEE with Rayleigh fading and pure line-of-sight (LoS) propagation are obtained as special cases. Through analyzing RCEE, it notesthat asthe number of base-station antennasM becomes unbounded, RCEE approaches a given constant. Moreover, the asymptotic analysis proves that, in high M-regime, the increasing rate of the RCEE scales in the order of M^1/2 for Rayleigh fading and pure LoS propagation, while the scaling order with general Ricean fading is capped by M^1/2. All the above mentioned results are verified through Monte-Carlo simulations.

The development of nanophotonics in recent years has provided new opportunities to explore the interaction between free electrons and metamaterials. This advancement helps overcome limitations in THz vacuum electron devices, extending the frequency range, enhancing radiation intensity, controlling phase polarization, manipulating spatial distribution, and even developing entirely new forms of vacuum electron devices. Based on the control and enhancement of free electron radiation through metamaterials, we aim to break through the limitations of vacuum electron devices, creating efficient, tunable vortex waves, frequency combs, and mechanisms for enhancing radiation. This endeavor addresses key challenges in vacuum devices for the terahertz range.

Low-dimensional chiral perovskites combines the chirality of chiral molecules and the outstanding physical properties of perovskites, which rende them promising candidates for high-performance optical, electronic, and spintronic device applications. In particular, carriers in low-dimensional chiral perovskites are highly spin-polarized and thus they can be used as a spin source materials for spintronics and valleytronics. In this talk, I would like to first talk about how we introduce chirality into 2D perovskites and by using chiral 2D perovskite crystals we have demonstrated circularly polarized light emission and detected all polarization states based on chiral 2D perovskite photodetectors. Then, I will also introduce by stacking chiral two-dimensional perovskites and monolayer transition metal chalcogenides to form heterostructures to achieve efficient spin injection without an external magnetic field, thereby achieving valley freedom control. This valley polarization manipulation by using chiral 2D perovskites can also achieve in the interlayer excitons formed between chiral 2D perovskite and different types of monolayer transition metal chalcogenides. In addition, this strategy has been extended to zero-dimensional chiral bismuth-based perovskites, which shows high a high degree of circularly polarized photoluminescence and high spin injection efficiency within perovskite/WSe₂ heterostructure. Finally, I will talk about on-chip filterless full-Stokes polarimeter by using the optical anisotropy between the out-of-plane and in-plane direction of chiral 2D perovskites together with their chirality.

Photovoltaic effect is the key physical mechanism for light-to-electricity conversion and energy harvesting in commercialized solar cells and plays an important role in the field of green technology. Two-dimensional (2D) materials, with layered van der Waals structures, atomically-thin thickness, good compatibility with silicon technology, and rich configurations of heterostructures, offer a unique platform for investigating photovoltaic effect and novel optoelectronic devices. In this talk, I will focus on our recent works on tuning photovoltaic effect in 2D materials and related devices and applications. Firstly, I will show that one-dimensional quantum well structures existing in 2D materials can break the inversion symmetry of their lattice, which supports strong bulk photovoltaic effect (a second order nonlinear optical effect) along the 1D quantum well direction. Secondly, we construct van der Waals twisted structures and systematically investigated their optoelectronic properties. Spontaneous polarization and bulk photovoltaic effect in twisted interfaces are observed. Finally, we nonlinearly and widely tuned the photoresponse profile of 2D materials and their heterostructures under an out-of-plane electric field. This can be attributed to quantum Stark effect, quantum confined Franz-Keldysh effect, and Burstein Moss effect. Based on these effects, we realized an on-chip single-detector-based miniaturized spectrometer which is capable of operating at room temperature with active sensing area of 1500 um² and photodetection range from 1.7 to 3.6 um.

Ferroelectric material featuring its bi-stable states which can be switched by external electric field, has shown potential applications in nonvolatile memory, and neuromorphic computing. For the conventional oxide ferroelectric materials, especially whose thickness comes to a few unit cells, the notable depolarization effect and intrinsic size effect dominate, leading to the vanishing of the ferroelectric polarization. The rising of two-dimensional van der Waals materials provide a new opportunity for ferroelectricity study. Here, we reported new ferroelectricity discovered at the van der Waals interface of 2D materials. We observed anomalous intermediate polarization states in the multilayer 3R MoS2, with a proposed layer-by-layer switching model for polarization swithcing. We also found the extraordinary fatigue-free nature in 3R MoS₂ sliding ferroelectricity. The extraordinary physical properties of sliding ferroelectricity highlight the potential of 2D ferroelectric materials for new concept device applications.

Reference:

1. Peng Meng, Fucai Liu, et al., “Sliding induced multiple polarization states in two-dimensional ferroelectrics”, Nature Commun., 13, 7696 (2022).
2. Renji Bian, Fucai Liu, et al., “High-Performance Sliding Ferroelectric Transistor Based on Schottky Barrier Tuning”, Nano Lett., 23, 4595 (2023).
3. Renji Bian, Fucai Liu, et al., Developing fatigue-resistant ferroelectrics using interlayer sliding switching, Science DOI: 10.1126/science.ado1744, (2024).
4. Fucai Liu, et al., Room-temperature ferroelectricity in atomically thin CuInP₂S_6, Nature Commun. 7, 12357 (2016)