Negative Transient Flux in the Near Field of a Subwavelength Source

The emission of waves by a small source is a generic wave problem that has long been thought to be well studied. Here, we report the experimental observation that the energy flux of an outgoing sound wave emitted by a deep-subwavelength-sized source can become negative, which indicates the backward flow of energy. Such a negative transient flux exists, even in a homogenous medium, but is only observable in the time domain and in the extreme near field of the source. By wave-impedance analysis, we show that such a phenomenon is fundamentally rooted in the geometry of the wavefield itself and, hence, is generic. Our findings have implications in the time-dependent emission, absorption, and scattering of waves.