This PDF file contains the front matter associated with SPIE Proceedings Volume 13209, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

In this work, we experimentally demonstrated a high-power ultrafast fiber laser system, constructed in a monolithic fiber configuration. The chirped pulse was boosted in power through a single large-mode-area (LMA) fiber power amplifier. A maximum average power of 1593 W was generated at the central wavelength of 1050 nm. Thanks to the help of a fiber bandpass filter placed ahead, the Stimulated Raman Scattering (SRS) spectra excited at 1100 nm were under good suppression, which exhibited more than 40 dB lower in power. Grating pair compressor were used to de-chirped pulse, and 450-fs pulse width was obtained at the output power of 154 W. We are continuing to work on pulse compression at full power.

It is demonstrated that the inherent structure of Boris pusher may result in the accumulation of errors in numerical integration when estimating the works of electric fields on particles. This, in turn, leads to an incorrect estimations of particle acceleration mechanisms. A method of error correction, reducing error to the acceptable levels of < 10%, is implemented. Numerical integration error in question is most pronounced in scenarios where substantial number of electrons are injected into the accelerating structure through wave breaking of plasma waves i.e. in near-critical density plasma.

The Shanghai soft x-ray free-electron laser facility has two lines of self-amplified spontaneous emission mode and seeding mode, providing electron beams with high charge, high energy and fs length. By utilizing the electron beams with these special characteristics, strong-field THz radiation can be generated through coherent transition radiation. An electron beam with a root-mean-square length of 50 fs compressed by a frequency beating laser pulse can generate broadband THz radiation from 0.1 to 8 THz, with a pulse energy of up to 690 μJ. In addition, an electron bunch train modulated by a frequency beating laser pulse can generate narrowband strong-field THz radiation from 0.1 to 15 THz, and the pulse energy can reach 71 μJ at 15 THz. The proposed method will effectively enhance the characteristics of the THz radiation source at the Shanghai soft x-ray free-electron laser facility, providing an excellent tool for measuring the longitudinal structures of the electron beam and advanced THz experiments. At the same time, under low relativistic electron beam energy, this method also provides a feasible solution for the compact strong-field THz source.

A 1.4-cell photocathode RF gun was developed for the MeV-UED to mitigate the space charge effect during electron emission through a higher acceleration gradient. However, this advancement introduces the risk of field-emitted dark current, leading to a degradation in the quality of the ultrafast electron beam. This paper investigates dark current emission within critical regions of the RF gun cavity. The results show that dark current electrons from the cathode and cathode edge escape from the electron gun, resulting in increased image background noise. The study examines the temporal characteristics of the dark current, including waveform in relation to the emission phase. Additionally, different collimator apertures are analyzed for their suppressive effect on the dark current, aiming to minimize its impact on the ultrafast electron beam.

Recently, the infinite-layer nickelates, which have similar crystal structure as cuprates, was successfully synthesized and exhibited superconductivity. Obviously, this new kind of Ni-based superconductors will create new platform to investigate the complex superconducting mechanisms in the cuprates. In the nickelates, there exist strong Coulomb interactions, which may result in different symmetry-breaking orders, like charge density waves (CDWs), spin density waves (SDWs). These orders may contribute to the superconductivity. Therefore, it’s of importance to find out the possible ordered state in the nickelates. Here, we study the photoinduced change in reflectivity of the parent compound of the nickelate superconductor-NdNiO₂ films. Above 160 K, we observe an additional decay process, which coexist with the decay process at low temperature. At higher temperature, the second decay process dominate the relaxation. This phenomenon may reflect the existence of competing order in the temperature region in the NdNiO₂ film. Further efforts should be made to find out the physical origin of the orders.

Ultrafast Electron Diffraction/Microscopy (UED/UEM) are powerful tools for directly observing ultrafast dynamic processes at the atomic level. The quality of the electron beam is crucial for image resolution, but the space charge effects can degrade parameters such as energy spread and emittance, reducing the spatiotemporal resolution. By reducing the charge per electron bunch and increasing the emission frequency, the space charge effect can be effectively suppressed, ensuring a high signal-to-noise ratio in the images. Superconducting Radio Frequency (SRF) photocathode guns can operate in continuous wave (CW) mode and generate highly stable and bright electron beams, making them promising electron sources for the next generation of UED/UEM. This paper aims to optimize the design of a 1.4-cell SRF gun using Nb₃Sn for UED/UEM. The focus is on minimizing thermal losses in the cavity to enable efficient conduction cooling and ensure stable operation at 4K in the superconducting state. Furthermore, beam dynamics analysis of the electron beam inside the cavity is performed to assess beam quality for different charges and bunch sizes. This enables us to achieve a high-quality electron beam that meets the design requirements.

The paper proposed a type of all-optical switch based on spatial-filter (AOSF), and a rule for this design. An analysis based on the present techniques used with lasers and photorefractive crystals indicated that there are no significant scientific or technical barriers for fabricating an AOSF. The response time of the proposed switch can reach 1ps, and the extinction ratio of the switch can reach 114:1 or higher. The AOSF can be used for measuring laser pulse contrast, taking a short section from a laser pulse, and improving pulse contrast or generating a short pulse from a long pulse.

CsV₃Sb₅ is a recently discovered Kagome metal that exhibits a combination of superconductivity with a critical temperature (T_C) of 2.5 K and charge density wave (CDW) order at T_CDW = 94 K. In this study, we investigate the collective excitations and quasiparticle dynamics in CsV₃Sb₅ using ultrafast optical pump-probe spectroscopy. According to our results, in addition to the CDW phase related electron and coherent phonon dynamics, we found another electronic symmetry breaking by nematic phase transition at T* ( 20 K), which can mediate the relaxation of carriers. At temperature below T*, a gap about 4 meV related to the nematic order is open, which determines the low-temperature dynamics of carriers in the rst 2 picosecond (ps). Moreover, we also observed the abrupt change of electron dynamics caused by the CDW phase transition at T_CDW. Furthermore, we identify the presence of a CDW induced mode at 1.3 THz below T_CDW, as well as two collective modes at 3.1 THz and 3.8 THz below 80 K. It is noteworthy that these two collective modes appear to be associated with the breaking of C₆ rotational symmetry. These observations suggest the rich phase order below T_CDW and emphasize the need for a deeper understanding of CDW and nematicity order in the Kagome metals.

Accurate characterization of the longitudinal profile of ultrafast electron bunches is crucial for observing ultrafast dynamic processes in ultrafast electron diffraction (UED). Real-time monitoring of each ultrafast electron bunch's length would ideally provide a temporal basis for analyzing diffraction patterns. However, accurately and non-destructively measuring the longitudinal profile at the 10 femtoseconds level remains challenging. This paper investigates a method for monitoring the longitudinal profile of bunches using coherent Smith-Purcell radiation (cSPr). Space charge effects cause the elongation of electron bunches during flight, resulting in changes in the Smith-Purcell radiation spectrum. Therefore, the focus is on understanding the impact of space charge effects on the behavior of ultrafast electron bunches and their corresponding cSPr spectrum, with the aim of improving measurement accuracy.