Tailored unidirectional spectral responses generated by an asymmetric plasmonic cavity

Unidirectional spectral responses generated by a plasmonic cavity formed by two asymmetric metasurfaces are reported and interpreted using interference theory and a transmission line (TL) model. Illuminated by a p-polarized plane wave from the front or back side, the plasmonic cavity functions as either an absorber or reflector according to the illumination side at a certain wavelength under Fabry-Perot cavity resonance. Based on interference theory, it is revealed that the phase shift of reflection at each metasurface plays a key role in the formation of the unidirectional spectral response, while the TL model demonstrates that the unidirection comes from the difference of characteristic impedance between the two metasurfaces. This work provides profound understanding of the underlying physics for unidirectional spectral response generated by a plasmonic cavity and benefits diverse applications such as direction-involved active plasmonic devices and unidirectional thermal emissions.