Exceptional points, coherent perfect absorption and disorder scattering in non-Hermitian media*

In my talk I will discuss the physics of exceptional points (EPs) and how encircling them creates a robust and asymmetric switch between the two resonant modes that meet at such an EP. In particular, I will focus on the topological aspects of this state transfer protocol that has meanwhile been implemented in a number of different experimental setups [1-3]. I will then point out that EPs can not only be obtained by merging two resonant states, but also by the coalescence of two S-matrix zeros giving rise to a perfect chiral absorber [4]. By studying the movement of such perfectly absorbing S-matrix zeros in a disordered system, we recently achieved the first experimental demonstration of a "random anti-laser" - a disordered system that perfectly absorbs a suitably shaped incoming wave state [5]. If time permits, I will also report on scattering states in disordered media with constant intensity and perfect transmission [6]. Implementing these states using several loudspeakers with gain and loss allows us to steer an incoming sound wave across a strongly disordered waveguide without any reflection or variation in its pressure [7].

[1] J. Doppler, A.A. Mailybaev, J. Böhm, U. Kuhl, A. Girschik, F. Libisch, T.J. Milburn, P. Rabl, N. Moiseyev, and S. Rotter, Nature 537, 76 (2016).
[2] H. Xu, D. Mason, L. Jiang, and J. G. E. Harris, Nature 537, 80 (2016).
[3] J. W. Yoon et al., Nature 562, 86 (2018).
[4] W. R. Sweeney, C. W. Hsu, S. Rotter, and A. D. Stone, arXiv:1807.08805.
[5] K. Pichler, M. Kühmayer, J. Böhm, A. Brandstötter, P. Ambichl, U. Kuhl, and S. Rotter (in preparation).
[6] K. G. Makris, A. Brandstötter, P. Ambichl, Z. H. Musslimani, and S. Rotter, Light Sci. Appl. 6, e17035 (2017).
[7] E. Rivet, A. Brandstötter, K. G. Makris, H. Lissek, S. Rotter, and R. Fleury, Nature Physics 14, 942 (2018).