Surface plasmon induced transparency in coupled microcavities assisted by slits

Abstract

The coupled waveguide-microcavity structure has a wide range of applications in optical filters and optical modulators. The optical transmission properties of structure are mostly determined by the coupling strength of the modes. In the conventional waveguide-microcavity structure, the mode coupling is finished by the form of evanescent field, which is usually achieved by controlling the geometric spacing between waveguide and microcavity. Surface plasmon polaritons are the excitations of the electromagnetic waves coupled to collective oscillations of free electrons in metal. Since the electromagnetic waves are attenuated sharply in the metal, this requires precise control of the spacing between the waveguide and the metal microcavity, and poses a great challenge for controlling the coupling of modes in the metal waveguide-cavity structure. In this paper, we proposed a scheme of using a metal-dielectric-metal waveguide side coupling metal microcavities to overcome this limit. Based on the resonant characteristics of the Fabry–Pérot mode in the metal microcavity, a slit is introduced to connect the waveguide and microcavities. By adjusting the width and the offset location of slits, the leakage rate and coupling strength of the mode in metal microcavity can be controlled. The finite difference frequency domain (FDFD) method was used to numerically simulate the electromagnetic properties of structure. First, we have studied the transmission behaviors of surface plasmon polaritons in the system consisted by metal waveguide and single microcavity. As other microcavity is introduced to the structure and connected the original microcavity by slit, the electromagnetically induced transparency phenomena based on surface plasmon polaritons are demonstrated in the coupled metal waveguide and double microcavities structure. As the width of slit connected the microcavity is increased, the transmission peak of structure and the full width at half maximum of the transparency window also increase accordingly. The change of the geometric parameters of slit will modulate the resonance characteristics of structure, and the corresponding physical mechanism is explained by the temporal coupled mode theory. In our works, the metal waveguide and microcavities are coupled by the energy leakage of microcavities assisted by slits, which breaks the limit of separation distance between metal waveguide and microcavity, and contributes to the manufacture of devices. The results of the paper will have applications in designing the compact photonic devices based on surface plasmon polaritons.