Abstract
Sensing an incoming signal is typically associated with absorbing a portion of its energy, inherently perturbing the measurement and creating reflections and shadows. Here, in contrast, we demonstrate a non-invasive, shadow-free, invisible sensor for airborne sound waves at audible frequencies, which fully absorbs the impinging signal, without at the same time perturbing its own measurement or creating a shadow. This unique sensing device is based on the unusual scattering properties of a parity-time (PT) symmetric metamaterial device formed by a pair of electro-acoustic resonators loaded with suitably tailored non-Foster electrical circuits, constituting the acoustic equivalent of a coherent perfect absorber coupled to a coherent laser. Beyond the specific application to non-invasive sensing, our work broadly demonstrates the unique relevance of PT-symmetric metamaterials for acoustics, loss compensation and extraordinary wave manipulation.
Highlights
Sensing an incoming signal is typically associated with absorbing a portion of its energy, inherently perturbing the measurement and creating reflections and shadows
Translated to optics, it has been theoretically argued that PT-symmetric systems, obtained with properly balanced distributions of absorbing and gain media, may produce lossless propagation[7,8,9,10,11], remarkably opening paths to loss compensation[12,13,14,15,16,17,18] and, under special conditions, unidirectional invisibility in one-dimensional systems[17,18], a phenomenon known as anisotropic transmission resonance (ATR)[14]
Experimental investigations of these exotic scattering phenomena have been so far quite limited, conducted in the temporal domain[19,20,21], or in the spatial domain for passive[22,23] and active[11,24,25] spatial distributions of refractive index. We observe these properties for sound waves, and use PT symmetry to realize a non-invasive, shadow-free, fully invisible acoustic sensor, based on a PT-symmetric distribution of balanced gain and loss
Summary
Sensing an incoming signal is typically associated with absorbing a portion of its energy, inherently perturbing the measurement and creating reflections and shadows. Translated to optics, it has been theoretically argued that PT-symmetric systems, obtained with properly balanced distributions of absorbing and gain media, may produce lossless propagation[7,8,9,10,11], remarkably opening paths to loss compensation[12,13,14,15,16,17,18] and, under special conditions, unidirectional invisibility in one-dimensional systems[17,18], a phenomenon known as anisotropic transmission resonance (ATR)[14] Experimental investigations of these exotic scattering phenomena have been so far quite limited, conducted in the temporal domain[19,20,21], or in the spatial domain for passive (that is, nonfully-PT-symmetric)[22,23] and active[11,24,25] spatial distributions of refractive index. Our experiment constitutes direct experimental evidence of PT-symmetric unidirectional invisibility in acoustics, and highlights its remarkable implications for noninvasive sensing
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