Propagation of electromagnetic waves play an important role in modern Physics (microwaves, acoustics and optics). One important aspect in this context is the propagation of waves in waveguides. Waveguides are structures, where waves can propagate, e.g., optical fibers or coax cables.
The propagation of the waves in such a guide depend on parameters determined by the waveguide itself. Such a parameter can be the width of the waveguide or its intrinsic loss, for instance. When the system parameters are tuned in such a way that two propagating waves, one likes to speak of modes in the context of waveguides, coalesce an exceptional point is appears. Coalescing means, that the two modes have the same characeteristic eigenfrequency and the same “shape”. The exceptional point condition can only be met if there is loss in the waveguide and if one has very precise control about the loss and the other system parameters.
Microwave waveguides are extremely useful to study this kind of phenomena as the waveguide parameters can be tuned very precisely. The wavelength of microwaves is typical in the cm-regime, so that one can easily fabricate metallic waveguides with a sub-wavelength precision.
The idea of our waveguide experiment is to dynamically drive two propagating modes (mode 1 and mode 2) around the exceptional point condition. The wavy waveguide which we fabricated is shown below.
It demonstrated that in such a case, where we encircle an exceptional point, a peculiar phenomenon occurs. If we inject a mode from one side (left side) the result on the other side (right side) is always the same, no matter which mode we inject in the waveguide. The outgoing mode is always mode 1. Astonishingly, when we inject something from the right side, the outgoing modes is always mode 2. This asymmetric switching behaviour is the result of the dynamical transport around the exceptional point and it is illustrated in the picture below.
This special switching behaviour was predicted theoretically and we confirmed it experimentally by investigating the modal transport of our specially shaped lossy microwave waveguide.
Our realisation opens up a door to study exceptional point physics experimentally and our results can be used to build such asymmetric switching devices in any kind of wave-based waveguide system.
This article was highlighted in Nature Physics.
"Dynamically encircling an exceptional point for asymmetric mode switching", Jörg Doppler, Alexei A. Mailybaev, Julian Böhm, Ulrich Kuhl, Adrian Girschik, Florian Libisch, Thomas J. Milburn, Peter Rabl, Nimrod Moiseyev & Stefan Rotter Nature 537, 76–79 (01 September 2016) doi:10.1038/nature18605
Fluides & Matériaux Complexes, Physique Mésoscopique
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