Beam monitoring of relativistic charged particle beams is of great interest for various applications including a high-order harmonic generation-seeded free-electron-laser. In particular, in phenomena with low repeatability/reproducibility such as laser-plasma acceleration experiments, it is important to measure spatio-temporal density profile of accelerated charged particle beams in a single shot. In this research, single-shot ultrafast spatio-temporal density profile measurement of relativistic electron beams in radiofrequency accelerators is conducted via obtaining spatio-temporal electric-field profile around the beams with combination of electro-optic sampling and echelon-based single-shot method. Here, we introduced an analytical model derived by special relativistic electromagnetism to deduce longitudinal and transverse beam sizes by measured spatio-temporal electric-field profiles.
Electro-optic (EO) sampling is a powerful non-destructive technique for measuring terahertz (THz) electric field with high temporal resolution. EO sampling is a popular method for monitoring the electron beams in accelerators because the measurement of the electric field pulse width directly corresponds to the width of the electron beam. Previous researches mainly focused on measuring the temporal profile. In this work, we also measure the spatial (radial) profile that is perpendicular to the propagation (longitudinal/temporal direction). The measurement of the electric field profiles in both time and space paves the way to access the spatio-temporal electron beam profile, which makes this method promising for a high-order harmonic generation-seeded free-electron laser (FEL). In this research, we investigated the electric field strength profile and the pulse broadening in the radial direction based on the spatio-temporal electric field around a picosecond relativistic electron beam with an energy of 35 MeV. Special relativity predicts that the electric field contracts in the propagation direction and becomes like a disk with a uniform thickness. With this postulate, the Gauss’ theorem in cylindrical coordinates can be applied to the experimental results to deduce the electron beam size from the electric field profile.
We demonstrate index-tunable metamaterials working in the terahertz (THz) frequency range based on double-layered closed-ring resonator (CRR) arrays. The double-layered CRR arrays have a narrow-band transmission peak in a relatively wide stop band, and that peak shows spectral shift by slightly shifting relative position of the arrays or changing dielectric constant of the dielectric media inserted between CRR arrays. We show by numerical simulations that the effective refractive index can be widely tuned from un-naturally high positive to near-zero and negative values. Our approach may be utilized to develop THz active devices.
Since Chen et al. reported on the photonic nanojet (PNJ), many researches have been carried out from various viewpoints
such as fundamental physics and device applications. We have numerically analyzed, based on the finite-difference
time-domain (FDTD) method, generation of PNJ from microcylinders incorporating the liquid crystals (LCs) with radial
hedgehog and tangential alignments, in which the director of LC molecules is perpendicular or parallel to the LC/matrix
interface. For the radial hedgehog alignment of LC molecules, the PNJ from LC microcylinders is separated into two
beams. For the tangential alignment of LC molecules, we show that the PNJ from LC microcylinders are uniquely
polarized reflecting birefringence of LCs, which cannot be obtained using optically isotropic microdroplets or
microcylinders. By using the LC micro-systems, we may obtain a rich variety of PNJ with electrical tunability.
The liquid crystals (LCs) form various types of nano- and micro- structures in a self-organized manner. In recent years, numerous studies have been carried out to develop novel types of optical functional materials and devices utilizing such self-organizing characteristics of the LCs. Based on the finite-difference time-domain (FDTD) method or its extended version, auxiliary differential equation FDTD (ADE-FDTD) method, we have been numerically studying on the optical characteristics and functionalities of the self-organized LCs such as: (1) lasing from the cholesteric LCs (CLCs) and (2) photonic nanojet (PNJ) from LC micro-systems. Based on the ADE-FDTD method incorporating the equation of motion of the macroscopic polarization and the rate equations at the four level energy structures, we have successfully reproduced circularly polarized lasing from CLC at the edge energy of the stop band. It has also been clarified that the introduction of the defect is effective to lower the lasing threshold. Our technique can be utilized to design the CLC laser devise architecture for much lowered lasing threshold. The PNJ from LC micro-systems are uniquely polarized reflecting birefringence of LCs, which cannot be obtained using optically isotropic microdroplets or microcylinders. A small degree of birefringence drastically changes the optical characteristics of the obtained PNJ. Our findings may open the way for the development of the novel optical functional materials and devices.
Since Urata et al. reported on the Smith Purcell superradiance, numerous studies have been carried out to develop novel
type of terahertz free electron lasers. The particle-in-cell finite-difference time-domain (PIC-FDTD) method has been
widely employed to numerically study the process. We show our studies on the parallel computing based on the general
purpose computation on the graphic processing unit (GPGPU) to accelerate our homemade PIC-FDTD simulation. We
have succeeded in reducing the computational time to the quarter of that required for the same simulation using only
CPU.
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