Passive metamaterials, while still able to create many exotic effects, are limited by losses and constrained bandwidth due to their constitutive structures. By changing these inclusions from passive to active, the limitations created by passivity can be overcome and new effects can also be discovered. Prof. Steven Cummer's team has shown that through the use of embedded active circuits and devices, active metamaterials can be created. Examples of these active metamaterials are metamaterial structures with zero loss [Y. Yuan, B-I. Popa, S. A. Cummer, "Zero loss magnetic metamaterials using powered active unit cells," Optics Express, vol. 18, no. 19, 2009] and "smarter" metamaterials that can self-tune [J. P. Barrett, S. A. Cummer, "Roadmap to electrically self-tuning metamaterials: Design and experimental validation," ICEAA, 2014]. Recently, this approach has been extended to acoustic metamaterials in order to implement material functionality hard to access in passive media. For example, non-linear and highly non-reciprocal media have been demonstrated in the audible range of frequencies at low sound levels [Popa, B.-I. and S. A. Cummer, "Non-reciprocal and highly nonlinear active acoustic metamaterials", Nature Communications 5, 3398 (2014)]. Currently, through the inclusion and implementation of various exotic transistor-based circuits and devices, the Duke team is working on expanding the design space of active metamaterials to explore and create new effects.
