Antibonding plasmon mode coupling of an individual hole in a thin metallic film

Abstract

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...

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