  The parity-time symmetry (PT symmetry) breaking phenomenon is investigated in
a coupled nanobeam cavity system. An exceptional point is observed during the
tuning of the relation of the gain/loss and coupling strength of the closely
placed nanobeam pairs. The PT symmetry concept can be applied to realize
unidirectional light propagation and single mode operation lasers, which may
allow for a new way to harness the optical signal in photonic integrated
circuits. Otherwise, operating at this particular exceptional point,
sensitivity of tiny perturbation detection can be enhanced greatly compared
with conventional sensors.

Optics