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We investigate the properties of the modes that are supported by 3-D subwavelength plasmonic slot waveguides. We first show that the fundamental mode that is supported by a symmetric plasmonic slot waveguide, which is composed of a subwavelength slot in a thin metallic film embedded in an infinite homogeneous dielectric, is always a bound mode for any combination of operating wavelength and waveguide parameters. Its modal fields are highly confined over a wavelength range extending from zero frequency to the ultraviolet. We then show that for an asymmetric plasmonic slot waveguide, in which the surrounding dielectric media above and below the metal film are different, there may exist a cutoff slot width and/or a cutoff metal film thickness above which the mode becomes leaky, and there always exists a cutoff wavelength above which the mode becomes leaky. We investigate in detail the effect of …