We have numerically analyzed, based on a simplified particle-in-cell finite-difference time-domain (PIC-FDTD) method, an electron-beam (e-beam) induced terahertz (THz) radiation from metallic grating structures with graded depths (graded grating). Upon exciting with e-beam, directional THz radiations with wide-band spectrum containing several sharp peaks are obtained only from the one of the edge of the grating, which cannot be expected from the conventional theory of Smith-Purcell radiation. It was clarified that each modes originate from different locations on the graded grating reflecting different dispersion characteristics of spoof surface plasmon polariton (spoof SPP) at each locations, and they can propagate toward only the shallower groove as a surface wave due to the cutoff at each locations, and all of these modes eventually emitted from the one of the edge of the graded grating. These directional radiations can be directed toward either backward or forward by making the groove depth deeper or shallower. The lowest and the highest frequency of the radiation can be chosen by appropriately designing the deepest and the shallowest groove depths, respectively. These unique radiations cannot be obtained from the uniformly grooved grating. Our findings may open the way for a development of novel THz radiation source based on the spoof SPP on the wide variety of metallic grating structures or metasurfaces.

The photonic nanojet (PNJ) from a microcylinder with liquid crystals (LCs) showing tangential molecular alignment inside the microcylinder has been numerically analyzed on the basis of the finite-difference time-domain method. By introducing a small degree of birefringence, the characteristics of the PNJ, such as propagation length and polarization state, can be drastically changed. The azimuth angle and the ellipticity of the elliptically polarized PNJ obtained from the LC microcylinder changes within the propagation lengths in the micrometer range even in the isotropic matrix, which might be attributed to the jet like spatial profile of the PNJ. By using LC microcylinders or microspheres, we may obtain a rich variety of PNJs with unique polarization characteristics, which might open a new avenue for the development of novel optical devices with electrical tunability.

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The compact terahertz (THz) radiation source based on Smith-Purcell (SP) effect has been expected to be the nextgeneration table-top THz free electron laser (THz-FEL). We have performed a numerical simulation of the generation of SP radiation at THz frequency under the condition of using the electron beam (e-beam) with high repetition THz frequency by simplified particle-in-cell finite difference-time domain (PIC-FDTD) method, and clarified that frequency of SP radiation can be tuned with pre-bunch frequency of e-beam. Our results can lead to the development of more tunable table-top THz-SP-FEL.

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.