This PDF file contains the front matter associated with SPIE Proceedings Volume 12291, including the Title Page, Copyright information, Table of Contents, and Conference Committee list.

Ultrafast Electron Diffraction (UED) is an indispensable tool that enables the study of ultrafast dynamics on an atomic/molecular scale. Ultrashort high brightness electron beams are needed to capture the critical ultrafast events, particularly for studying the irreversible biochemical processes in the single-shot mode. However, the Coulomb interactions in the space-charge dominated electron beam limit attainable beam length and dilute beam quality during its propagation. The beam emittance increases significantly during propagation due to the severe space charge effect (SCE) because of low energy. It is essential to understand the emittance evolution behavior in detail during its passage for improving the UED performance further. The multi-slit method is selected to eliminate the SCE influence on the measurement by a low sampling rate of the electrons, making it possible to diagnose the emittance. However, the insufficient samplings create challenges in reconstructing the original beam information. This paper introduces an algorithm that can precisely reproduce beam parameters from severely under-sampled data.

The orientation of molecules is essential to study molecular angle-differential properties such as ionization and scattering cross-sections in material physics and chemistry. Ultrafast electron diffraction (UED) facilities offer effective ways to explore the ultrafast dynamics of orientated molecules. Generally, the orientation of molecules is generated by a strong dc-field. However, the presence of a strong field may influence detection outcome. Field-free orientation of molecules is preferable, avoiding the disadvantages of traditional dc-field excitation. This paper proposes a practical and versatile method for field-free molecular orientation using the co-rotating two-color circularly polarized ultrafast laser pulses, and the orientation of the molecules can be controlled by the relative phase of the two-color laser fields. We also performed our simulation in CO molecules with the Born-Oppenheimer and rigid rotor approximations, and the light-molecule interaction Hamiltonian is given by the low-order perturbation theory.

Harmonics from relativistic laser driven plasma surfaces is a prospective high energy attosecond light source in future XUV pump-probe experiments. Among all the schemes, the most efficient and direct way to realize an isolated attosecond pulse is through using a few-cycle laser as the driving pulse. The two goodness criteria: the laser to harmonics energy conversion efficiency and the “purity” of an isolated attosecond pulse are generally determined by a combination of interaction parameters. Through using particle-in-cell simulations and relativistic electron dynamics model analyses, we explain how these two criteria are affected by the laser intensity, incidence angle, carrier-envelope phase, and the plasma scale length. We found that, there exist an optimal plasma scale length and an optimal incidence angle to efficiently generate harmonics and intense attosecond light pulses. When other parameters are fixed, using a moderately intense relativistic laser or using a large incidence angle could result in a higher isolation degree as well as a broader range of controlling parameters to realize an isolated attosecond light pulse.

An all-fiber high-power Mamyshev oscillator (MO) with only one amplification stage was experimentally demonstrated. The achieved maximum output power was 3.4 W with 77 nJ pulse energy and could be compressed to ~100 fs. By adjusting the pump power, the phenomenon of harmonic mode locking is observed in the experiment, and the highest 5th order harmonic can be achieved, which corresponds to the repetition rate of 44.1 MHz. This compact MO ultrafast laser could operate stably several hours and the power fluctuation within 5 h was less than 0.12%. Such a high power ultrafast laser oscillator could apply a promising source for advanced fabrication, biomedical imaging, micromachining and other practical applications.

Bunch trains consisting of ultrashort picosecond-spaced microbunches have potential applications in generating pulsed, tunable, narrow-band radiation sources in the THz region via coherent Smith-Purcell radiation (cSPr). However, the electrons in each microbunch experience longitudinal space-charge field, blurring the periodicity of the bunch train. There has been an increasing interest in manipulating each microbunch individually, and therefore significantly improving radiation intensity and bandwidth. The commonly used RF cavities (with nanosecond working period) cannot match the picosecond bunch spacing and, fail to compress each bunch individually. This paper proposes a novel method to simultaneously compress each microbunch in a picosecond-spaced bunch train using a THz-driven resonator with a customizable working frequency. A multi-pulse drives the THz-driven resonator to compensate for the field decay in the THz-driven resonator and preserve the well-defined periodicity of the bunch train. We demonstrate a resonating field with an amplitude fluctuation within±20%, which can be utilized to compress up to ten microbunches simultaneously.

New accelerator technologies such as laser wakefield accelerators (LWFA) or dielectric laser accelerators (DLA) have pushed the electron bunch length down to femtosecond or sub femtosecond regime. These ultrashort electron bunches find many applications, e.g. seeding for free-electron lasers (FEL), ultrafast electron diffraction (UED) and coherent Smith-Purcell radiation (cSPr) sources etc. The characterization of such ultrashort bunches is becoming a challenging task, especially at low energy regime due to the space charge effects. Usually, the streak cameras based on RF cavities are used to obtain accurate bunch length. However, the phase jitter between the incident beam and the electromagnetic field phase in the cavity set a resolution limit. A bunch length diagnostic based on a self-emission THz driven split-ring resonator (SRR) is proposed to reach the sub-picosecond (ps) or femtosecond (fs) resolution. Since the coherent SmithPurcell radiation from the incident electron beam produces the driving THz pulse, it can essentially eliminate the time jitter between the incident beam and the deflection THz field in the SRR gap. Besides, this THz pulse frequency can be tunable to easily match the SRR resonance frequency. In this paper, we describe the mechanism of the THz generation method and present the simulation results of the novel bunch length measurement based on a THz-driven SRR. The results show that this novel method can successfully measure the bunch length with the temporal resolution of 2-10 fs.

Streak cameras based on THz-driven split-ring resonator (SRR) are recently proposed to achieve electron bunchlengthmeasurement with femtosecond resolution due to the available GV/m level streaking field. However, to apply the SRRtothe streaking experiment, the SRR needs to have a relatively large gap to accommodate the beamto traverse. Alargergap leads to higher electromagnetic power radiation, which requires high exciting THz power to compensate powerradiation to achieve a strong streaking field. The maximum stored energy in the gap is determined by the availableexciting THz power. If a single THz pulse drives the SRR, the achievable streaking field is not enough for highresolution because of the radiation diluting the stored energy. This paper proposes a novel method to illuminate theSRRwith multipulse, which can accumulate the energy stored in the gap to compensate the electromagnetic radiationuntil saturation and consequently enhance the resonance to a much higher peak field. We explore the effects of drivingpulseswith various intervals and obtain an optimal field enhancement factor up to 47 with the THz field strength of 1MV/m. The particle tracking simulation indicates that the multipulse-driven method can achieve the temporal resolution of 0.4fswith the central frequencies of SRR at 0.3 THz.

So far the finite-difference time-domain(FDTD) algorithm is the most effective tool to calculate the Maxwell’s equations directly in time domain. Because of its advantage in describing a specified propagating pulse by considering the phase characteristics, it is essential for the bidirectional investigation of THz radiation generation from micro-air-gas-plasma. However, it shows inconvenience or even impossibility if a general dispersive response of bound electrons within the partially ionized gases is included. Here, an improved FDTD method based on frequency-decomposition(FD) is proposed, and its validity and super-advantage are confirmed by the numerical demonstration of THz radiation generation from ultrafast laser induced gas plasma using a general dispersive response model with coefficients fitted from experimentally measured data in a wide frequency range.