We report on the terahertz transmission properties through a single slit in a thin metallic film. The properties are studied by comparing the transmissions of TE- and TM-polarized electromagnetic waves over a broad spectral range from the geometrical regime to the subwavelength limit. In the geometrical regime, the remarkable terahertz transmission due to guided modes is observed even without the contribution of surface waves. Whereas in the subwavelength limit, the surface charge oscillations associated with the TM-polarized guided mode give rise to strong transmission enhancement. The nature of the mechanisms for the terahertz transmission is elucidated using theoretical simulations of the near-field distributions and electromagnetic energy flow.

We report that asymmetric c-shaped holes show mode switching between single- and double-resonance frequencies in the terahertz region, when the polarization direction of the incident light is controlled. The number of resonant frequencies is attributed to fundamental shape modes such as circle-, coaxial-, and rectangular-shaped holes activated under each polarization condition. All of these resonant peaks also show a transmission of over 90% with the assistance of the periodically arranged holes. These experimental results are in close agreement with theoretical simulations.

The polarization dependence of the optical properties of individual subwavelength holes in a thin metallic film is studied using terahertz time-domain spectroscopy. We show that for parallel polarization of the incident light, the coupling is predominantly to short-range bonding film plasmons while for perpendicular polarization the incident light couples more efficiently to long-range antibonding film plasmons. These results represent a direct observation of antisymmetric hybridized plasmons and clarify the nature of plasmonic excitations in metallic structures with subwavelength-scale geometrical features. They show that polarization can be used as a means for selective excitation of film plasmon modes.Plasmonics in metallic subwavelength structures has been the inspiration for much research in the study of fundamental light-matter interactions as well as in developments of new photonic technologies. 1,2 In particular, the ability to accumulate and control the electromagnetic field at subwavelength scales is critical for the development of active optical systems such as plasmon-based solar cells, 3 optical antennas, 4-6 and photonic and optoelectronic devices for biological and chemical sensing. 7 Despite intense studies of surface plasmons at metal-dielectric interfaces 8,9 and of the optical properties of metallic surfaces with subwavelength structural defects, 10-15 many fundamental questions remain unanswered. For instance, for parallel polarization, i.e., the inplane polarization component, of the incident light toward a single circular hole in a thin metallic film, the appearance of resonant behavior has been explained as the excitation of a localized hole plasmon with a dipolar charge oscillation in the plane of the aperture 10 or, alternatively, as the effect of guided modes. 11-15 However, no studies have been performed for light polarized perpendicularly to the surface.The hole plasmon resonance ͑HPR͒ is not only responsible for the induced dipole moment around the hole but is also coupled to the plasmon modes of the extended metal film. 16,17 Using the plasmon hybridization method, it has been shown that the dispersion relations of the plasmon modes of a thin metallic film with a hole are the same as those of a continuous film. In the presence of a hole, among many plasmon modes, the particular bonding film plasmon modes for which half of their spatial wavelength is equal to the hole diameter induce a dipole moment across the hole. Thus the HPR is revealed when the incident light frequency is resonant to a film plasmon mode of half wavelength equal to the hole diameter. This theoretical explanation was experimentally confirmed by showing that the HPR exhibit strong redshift with increasing hole diameter or decreasing film thickness.In the case of a thin film, the surface-plasmon modes on the two film surfaces interact electromagnetically and give rise to hybridized film plasmon modes. The lower-energy bonding mode has a symmetric alignment of surface charges on the two surfaces of the film and the hi...

We report the terahertz (THz) transmission characteristics of subwavelength slit arrays with a variety of structures ranging from periodic to random. THz electromagnetic wave transmission experiments clarify the relative contributions of the periodicity and the geometric shape effects to transmission enhancement. In the spectral region slightly below the first Rayleigh minimum, strong transmission enhancement is achieved through the contributions of both effects. In the long-wavelength region, however, enhancement is achieved even in the random arrays of slits because the slit shape has its fundamental resonance. The understanding of the mechanisms of resonant THz transmission is elucidated by using a theoretical approach that is based on a perfect conductor model.