Transient total absorption for water waves

Wave absorption has been studied, theoretically and experimentally, in different domains of wave, like in optics, in acoustics or in water waves. In those examples, the absorption is obtained with dissipation processes in an harmonic regime (real frequencies). Baranov et al. introduced recently, in optics, the concept of coherent virtual absorption in a lossless case, involving scattering zeros at complex frequencies (sine wave with an exponential envelop). This leads to the capacity to store energy in a cavity and release it. We explore the same scenario in water waves, using a waveguide with a plunger-type wavemaker at one end, and a cavity at the other end, playing the role of a resonator. The waves are measured with the Fourier Transform Profilometry (FTP) technique. Studying the reflection, we show that it is possible to excite a complex frequency in the upper half-plane of the complex frequency map (corresponding to a exponential divergence in time), to obtain a transient total absorption with zero reflexion. In water waves, it is impossible to have a lossless system due to inevitable linear losses, but the transient total absorption is still robust in such lossy systems. The type of device described can be applied to the control of wave energy (absorption, storage).