Broadband unidirectional transmission realized by properties of the Dirac cone formed in photonic crystals

An excellent unidirectional transmission device is proposed, which consists of a two-dimensional photonic crystal and a dielectric grating. The photonic crystal is designed to have a Dirac cone at the Γ point which allows the normal incident light to transmit while reflecting the oblique incidence in a broad frequency band. The aim of the dielectric grating is to diffract the normal incident light into other directions. Thus, when the incident light first passes through the grating and then incidents upon the photonic crystal, it will be blocked completely. While when the sequence is reversed it will be transmitted. This sequence selectivity of the transmission makes the photonic crystal/grating structure present an excellent unidirectional transmission effect. The simulation results show that in a broad frequency region, T ₁ and T ₂, the transmittances in forward and backward directions, satisfy T ₁– T ₂ > 80%, T ₁/ T ₂ > 1000, and 10 log(1/ T ₂) > 20 simultaneously. (paper)