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 based on 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 canceled out in one direction. Using this concept, we realize a unidirectional waveguide that 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 an alternative route toward harnessing non-Hermiticity.

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• Light propagation, transmission & absorption
• Photonic crystal waveguide
• Photonics
• Lattices
• Non-Hermitian systems
• PT-symmetry