A Lossless Sink Based on Complex Frequency Excitations.

The creation of a sink in a lossless wave-bearing medium is achieved using complex frequency signals-harmonic excitations that exponentially grow in time. The wave sink, where incident waves are confined to a point, has attracted interest for imaging and sensing since it may lead to arbitrarily small hotspots that surpass the diffraction limit. However, most methods of creating sinks require careful tuning, such as by impedance matching the sink to free space through the inclusion of loss, which imposes constraints on emerging applications.

Non-Foster acoustic radiation from an active piezoelectric transducer.

The quality factor of a passive, linear, small acoustic radiator is fundamentally limited by its volume normalized to the emitted wavelength, imposing severe constraints on the bandwidth and efficiency of compact acoustic sources and of metamaterials composed of arrangements of small acoustic resonators. We demonstrate that these bounds can be overcome by loading a piezoelectric transducer with a non-Foster active circuit, showing that its radiation bandwidth and efficiency can be largely extended beyond what is possible in passive radiators, fundamentally limited only by stability considerations. Based on these principles, we experimentally observe a threefold bandwidth enhancement compared to its passive counterpart, paving the way toward non-Foster acoustic radiation and more broadly active metamaterials that overcome the bandwidth constraints hindering passive systems.