Light Stops at Exceptional Points

Abstract

Almost twenty years ago, light was slowed down to less than 10^{-7} of its vacuum speed in a cloud of ultracold atoms of sodium. Upon a sudden turn-off of the coupling laser, a slow light pulse can be imprinted on cold atoms such that it can be read out and converted into a photon again. In this process, the light is stopped by absorbing it and storing its shape within the atomic ensemble. Alternatively, the light can be stopped at the band edge in photonic-crystal waveguides, where the group speed vanishes. Here, we extend the phenomenon of stopped light to the new field of parity-time (PT) symmetric systems. We show that zero group speed in PT symmetric optical waveguides can be achieved if the system is prepared at an exceptional point, where two optical modes coalesce. This effect can be tuned for optical pulses in a wide range of frequencies and bandwidths, as we demonstrate in a system of coupled waveguides with gain and loss.

Anomalous energy transport in laminates with exceptional points

“…Firstly, we show that the spectrum of in-plane waves in laminates exhibits exceptional points, accessible in a purely elastic setting. Exceptional points are states of a system at which two (or more) of its normal modes coalesce, together with their natural frequencies (Moiseyev andFriedland, 1980, Ding et al, 2015), and are the source of counterintuitive phenomena such as enhanced sensitivity, wave stopping and asymmetric transmission (Hodaei et al, 2017, Achilleos et al, 2017, Goldzak et al, 2018, Merkel et al, 2018. Exceptional points occur only in non-Hermitian systems (Moiseyev, 2011), a property which usually describes systems that interact with the environment.…”

PT -symmetric non-Hermitian Dirac semimetals

“…Though experimentally challenging at first, it was later discovered that optic and photonic systems offer an ideal platform for the PT -symmetric quantum theory, where non-Hermitian effects of terms are naturally introduced by optical radiation and loss/gain of particles [20,[22][23][24][25]. Studies on PT -symmetric quantum theory not only deepen our understanding on fundamental physics [26][27][28][29], but also lead to novel applications [30][31][32][33][34].…”

PT symmetry breaking in multilayers with resonant loss and gain locks light propagation direction

“…Third, PT symmetry breaking plays an important role in laser physics, offering new types of lasers [26][27][28] and antilasers [29] based on the effect of coherent perfect absorption [30,31]. Finally, the possibility of light stopping at the exceptional point was recently reported [32].…”