We present the first experimental observation of Anderson localization in the terahertz frequency range using plasmonic structures. To accomplish this goal, we designed THz waveguides consisting of a one-dimensional array of rectangular apertures that were fabricated in a freestanding metal foil. Disorder is introduced into the waveguide by offsetting the position of each aperture by a random distance within a prescribed range. For example, for a waveguide with apertures spaced by 250 μm in a periodic waveguide, 10% disorder would correspond to the apertures being shifted by a random value between ±25 μm along the waveguide axis. We find that for disorder levels below 25%, there is only an increase in the propagation loss along the device. However, for two specific waveguides with 25% disorder, we observe a spatially localized mode that lies just within the stop band of the device and exhibits a double-sided exponential spatial decay away from the maximum.
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