Subwavelength Focussing in Metamaterials Using Far Field Time Reversal

Time reversalSubwavelength is a physical concept that allows focussing of wavesWaves both spatially and temporally regardless of the complexity of the propagation medium. Time reversal mirrorsTime reversal mirror have been demonstrated first in acousticsAcoustics, then with electromagneticElectromagnetics wavesWaves, and are being intensively studied in many fields ranging from underwater communications to sensing. In this chapter we review the principles of time reversalTime reversal and in particular its ability to focus wavesWaves in complex media. We show that this focussing effect depends on the complexity of the propagation medium rather than on the time reversal mirrorTime reversal mirror itself. A modal approach is utilized to explain the results and grasp the physical mechanisms underlying the concept. A particular focus is given to the possibility of overcoming the diffraction barrier from the far fieldFar-field using time reversalTime reversal. With this aim, we return to the first proof of concept of this original approach. Those results are explained in terms of the coherent excitation of subwavelengthSubwavelength modes. In particular, we show that a finite size medium consisting of coupled subwavelengthSubwavelength resonatorsResonator, which we call a resonantResonant metalensMetalens, supports modes which radiate spatial information of the near fieldNear-field of a source efficiently in the far fieldFar-field. We show that such a process, due to reversibility, enables us to beat the diffraction limitDiffraction limit using far fieldFar-field time reversalTime reversal, and especially that this result occurs due to the inherent broadbandBroadband nature of time reversalTime reversal. We then generalize the concept to other types of media, and finally show experimentally that it is also valid for acousticAcoustics wavesWaves, demonstrating deep subwavelengthSubwavelength focal spots obtained within an arrayArray of soda cans.