  Non-Hermitian photonic systems are known to exhibit unique phenomena, where
non-Hermiticity is typically introduced by material loss or gain. Here, we
propose and experimentally demonstrate unidirectional phenomena solely based on
radiation. Our design is on the basis of a double-lattice photonic crystal that
has a linear dispersion with a single exceptional point, where the magnitudes
of Hermitian and non-Hermitian couplings are cancelled out in one direction.
Based on this concept, we realize a unidirectional waveguide which shows
perfect radiation when light is incident from one side, and shows perfect
reflection when light is incident from the other side. Our results will open up
a new route toward harnessing non-Hermiticity.
