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Strong resonant transmission of microwave radiation through a very narrow (much less than the radiation wavelength) metallic slit is recorded. The results show that a set of resonant self-coupled surface plasmons are excited within the small gap, giving a Fabry-Pérot-like behavior in accord with analytical results published earlier [Y. Takakura, Phys. Rev. Lett. 86, 5601 (2001)]. The metallic slit, formed by two thick metal plates spaced apart by tens of microns, is inserted in a wavelength-sized aperture. On resonance the transmissivity through the metal slit is more than 2 orders of magnitude greater than the radiation impinging on the slit area. DOI: 10.1103/PhysRevLett.89.063901 PACS numbers: 41.20.Jb Very recently, remarkable transmission of light via 2D periodic arrays of subwavelength metallic holes in otherwise fully reflecting screens has been reported [1]. The transmitted spectral response exhibits much more transmission than expected for an equivalent set of single holes in an opaque aperture as predicted by standard aperture theory [2]. Apart from its fundamental interest, this extraordinary transmission effect has applications potential which has consequentially stimulated much new work. While discussion still continues about the detailed interpretation [3] of this effect, most agree that the enhanced transmission is related to surface plasmon excitation. There is first diffractive coupling into some form of localized mode at the grating surface, followed by evanescent tunneling through the small holes, followed again by diffractive coupling back to free radiation. While the study by Ebbesen et al. [1] was strictly limited to holes, some model calculations have been undertaken for slits in metal screens [4][5][6][7][8]. In this instance, high transmission is also predicted for a periodic array of subwavelength slits. However, because slits have a very different symmetry to holes, the mechanism for this enhanced transmittance is different, involving propagating, rather than evanescent, modes in the slits. These propagating modes in the narrow slits, for radiation polarized with its E field perpendicular to the slit axis, may be regarded as either waveguide modes or coupled surface plasmons. In order to give a clear physical explanation of the origin of the extraordinary transmission from a metallic slit grating, Takakura [9] analyzed the interaction of TM-polarized waves with a single subwavelength metallic slit. The results show, for a thick enough conducting plate, a series of resonant transmission peaks as a function of incident wavelength. These resonances arise through the setting up of standing waves between the two ends of the slit, Fabry-Pérot-like modes. The ''extraordinary transmission'' observed in such metallic slit gratings is then the result of excitation of these Fabry-Pérot-like resonances localized in each slit cavity.In this Letter we experimentally realize transmission through a single narrow metallic slit in a wavelengthwidth plate. The results confirm the predictions from [9]...

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