Topological gallery of non-Hermitian whispers
I always wanted to have something equivalent to an acoustic laser at hand. Lasing or gain medium for sound waves? Yes, this is exciting. Topics in optical metamaterials are entirely dedicated to that, like “loss-compensated metamaterials” or “active plasmonics”, let alone “laser physics”!! My old friend Ying had this fantastic idea of using thermoacoustics to generate sound in conductors when an AC current is properly applied. Yes, we would not generate quanta of sound, like photons, but it would be a neat way to incorporate this effect in lattices to explore topological lasers…sorry, I mean, topological sasers! “S” is for sound my friends! Ying suggested using carbon nanotube films as they have already shown in the past to be efficient sound generators. I don’t have to explain the effort (time and money) behind the construction that consisted in coating really many crystal rods by those CNT films. Each coated rod needed electrical leads, a connection to a circuit, and all arranged in a honeycomb lattice! A truly active acoustic graphene-like material! Now that you are here, you have probably heard of a band gap. With the right geometrical arrangement you can make it topological non-trivial, but in its active construction, the resulting edge states appear with enhanced amplitude. To my amazement, we were able to out-couple those guys into highly collimated, and topologically protected acoustic SASER beams! I hope you enjoy reading the paper!
Nature 597, 655 (2021).
Link to the paper: https://doi.org/10.1038/s41586-021-03833-4
Summary published in Science & Vie (monthly science magazine published in France).