Light stops at exceptional points

Light travels in vacuum with a constant speed of 300,000,000m/sec, almost twenty years ago the 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 photon again. Alternatively, the light can be stopped at the band edge in photonic-crystal waveguides. 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 two coupled waveguides with gain and loss. We also investigate how the structure of multiple coupled wave guides affect the dispersion relation of the two coalesced modes.

Tamar Goldzak, Alexei A. Mailybaev, and Nimrod Moiseyev, Phys. Rev. Lett. 120, 013901 (2018)