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M. Liertzer, P. Ambichl, M. Brandstetter, K. Makris, T. Hisch, L. Ge, Y. Chong, A.D. Stone, H.E. Tureci, C. Deutsch, K. Unterrainer, S. Rotter:
"Exceptional points in resonant structures with gain and loss";
Talk: IARD/PT Meeting 2014, Storrs, Connecticut, USA; 06-09-2014 - 06-13-2014.

In this talk I will speak about our recent theoretical and
experimental advances in understanding and controlling the effects of
exceptional points in optical structures with gain and loss. The
first part of my talk will focus on PT-symmetric scattering systems
which can undergo a symmetry-breaking transition in the corresponding
scattering matrix eigenvalues. We relate the PT-symmetry-breaking
points of such unbounded scattering systems to those of the underlying
bounded systems [1]. Based on this relation, we show that the PT
transitions in the scattering matrix are, under very general
conditions, entirely insensitive to a variable coupling strength
between the bounded region and the unbounded asymptotic region - a
result that can be tested experimentally and visualized using the
concept of Smith charts. The second part of my talk will be dedicated
to our experimental realization of a pump-induced exceptional point
with two evanescently coupled quantum-cascade lasers [2]. Following
our recent prediction [3], we observed very counter-intuitive behavior
in the parametric vicinity of such an exceptional point. In
particular, our experiment exhibits a decrease of the emitted laser
light intensity for increasing pump strength and a characteristic
shift of the lasing frequency close to such a non-hermitian
degeneracy. If time permits, I will also report on how to control the
angular emission profile of a random laser with a feedback-controlled
pump-grating [4]. Using this concept we were able to overcome the
lack of control over a random laser's irregular emission pattern and
to achieve highly directional emission.

[1] P. Ambichl, K. G. Makris, L. Ge, Y. Chong, A. D. Stone, and S. Rotter,
Phys. Rev. X 3, 041030 (2013).
[2] M. Brandstetter, M. Liertzer, C. Deutsch, P. Klang, J. Schöberl, H. E.
Türeci, G. Strasser, K. Unterrainer, and S. Rotter (under review, 2014).
[3] M. Liertzer, L. Ge, A. Cerjan, A. D. Stone, H. Türeci, and S. Rotter,
Phys. Rev. Lett. 108, 173901 (2012).
[4] T. Hisch, M. Liertzer, D. Pogany, F. Mintert, and S. Rotter, Phys.
Rev. Lett. 111, 023902 (2013). (PRL editors' suggestion)