Phononic band gap crystals are periodic structures, analogous to photonic crystals for electromagnetic waves, that forbid propagation of acoustic waves for a certain range of frequencies. We present here time-reversal experiments performed through 2D phononic crystals. Inside the stop band, no propagating mode exists but tunneling and evanescent waves are involved. Thus using a receiving transducer array with the same central frequency as the gap, it is possible to measure a transmitted wave which comes from the conversion of evanescent waves into propagating modes at the output of the sample. This wave is then time-reversed and back-propagated. Both time compression and spatial focusing are quantified.
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Document PDF [166791 octets]: http://www.loa.espci.fr/~vdb/publis/000342.pdf