Analysis of Smith-Purcell radiation in optical region.

Smith-Purcell radiation (SPR), emitted when an electron beam is traveling above a metallic grating, has attracted a lot of attention as a source of electromagnetic (EM) radiation in the millimeter to visible spectrum. We conducted a theoretical investigation of SPR in the optical region using a two-dimensional finite-difference time-domain (FDTD) method. The permittivity of metal was represented using the Drude model. During the simulation, we observed three types of EM radiations when an electron bunch passes above a metal grating. We think these three types of EM radiation were basic SPR, original surface plasmon polariton (SPP), and mimic-SPP, caused by the periodic grating structure. Our observations were in accordance with analytical models of original SPP and mimic-SPP EM radiation.

A grating-bicoupled plasma-wave photomixer with resonant-cavity enhanced structure.

A novel terahertz plasma-wave photomixer that can improve the conversion gain and terahertz radiation power is proposed and evaluated. The photomixer is based on a high-electron mobility transistor and incorporates doubly interdigitated grating strips for the gate electrodes that periodically localize the 2D plasmons in 100-nm regions with a micron-order interval. A vertical cavity structure is formed in between the top metal grating and a terahertz mirror placed at the backside. The device features electronic tuning of plasmon characteristic frequencies, providing continuously-tunable operation below 1 THz to beyond 10 THz. Frequency-dependent finite-differential time-domain analysis demonstrates that the grating-bicoupled plasmonic structure acts as a broadband terahertz photomixer and antenna and that the vertical cavity structure effectively enhances the conversion gain and radiation power.

Effect of a thin dielectric layer on terahertz transmission characteristics for metal hole arrays.

We studied the role of surface-plasmon polaritons (SPPs) in a bandpass transmission property of two-dimensional metal hole arrays (2D-MHAs) by investigating the effect of thin dielectric layers on the 2D-MHA surfaces. We measured zero-order transmission spectra of the 2D-MHAs by changing the thickness of the dielectric layer and found that the bandpass transmission peak shifted to the lower-frequency side with increasing layer thickness, owing to the change of the resonant frequency of the SPP. This result shows that SPPs play a crucial role in the transmission property of 2D-MHAs in the terahertz region.