In the past decades, many efforts have been devoted to the temporalmanipulation of waves, especially focusing on slowing down their propagation.In electromagnetism, from microwave to optics, as well as in acoustics or forelastic waves, slow wave propagation indeed largely benefits both applied andfundamental physics. It is for instance essential in analog signal computingthrough the design of components such as delay lines and buffers, and it is oneof the prerequisite for increased wave/matter interactions. Despite theinterest of a broad community, researches have mostly been conducted in opticsalong with the development of wavelength scaled structured composite media,that appear promising candidates for compact slow light components. Yet theirminimum structural scale prevents them from being transposed to lowerfrequencies where wavelengths range from sub-millimeter to meters. In thisarticle, we propose to overcome this limitation thanks to the deepsub-wavelength scale of locally resonant metamaterials. In our approach,implemented here in the microwave regime, we show that introducing coupledresonant defects in such composite media allows the creation of deepsub-wavelength waveguides. We experimentally demonstrate that waves, whilepropagating in such waveguides, exhibit largely reduced group velocities. Wequalitatively explain the mechanism underlying this slow wave propagation andfirst experimentally demonstrate, then numerically verify, how it can be takenadvantage of to tune the velocity, achieving group indices ng as high as 227over relatively large bandwidths. We conclude by highlighting the threebeneficial consequences of our line defect slow wave waveguides in locallyresonant metamaterials: the deep sub-wavelength scale, the very large groupindices and the fact that slow wave propagation does not occur at the expenseof drastic bandwidth reductions.
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