Evaluation of the coupling characteristics of a sandwiched plasmonic waveguide between two dielectric waveguides using air-gap couplers to achieve high-performance optical interconnects

Abstract

To achieve ultrahigh-density nanoplasmonic integrated circuits, low-loss conventional dielectric waveguides (CDWs) are used to transfer light into and out of the metal–dielectric–metal (MDM) plasmonic waveguides. We show that sandwiching the MDM plasmonic waveguide between two CDWs forms a Fabry–Perot cavity-like structure, which causes oscillations in the transmission coupling efficiency (TCE) into the output CDW as the length of the MDM waveguide is increased. Three types of compact air-gap couplers (AGCs) were used at the interface between those two types of waveguides to enhance the coupling efficiency between them. Our simulation results indicate that tapering CDW before it is connected to AGC not only enhances TCE into the output CDW by 9% over a wide spectrum range but also reduces the need for high-precision fabrication techniques to align AGC at the interface. Moreover, we achieved a TCE into the output CDW of 77% optimized for the optical communications wavelength of 1550 nm when the length of the MDM plasmonic waveguide is 600 nm. In addition, we showed that our proposed design has large fabrication tolerance by investigating the change in its spectrum response as various parameters of the design dimensions differ from that of the targeted optimum dimensions. We found that increasing the dimensions of AGC reduces the width of the spectrum, whereas increasing the width of CDW with its tapered part shifts the spectrum to the right by 100 nm per 40 nm increase in the width. We also found that increasing the refractive index of the MDM plasmonic waveguide with AGC controls the cutoff wavelength, in which TCE is almost zero at wavelengths shorter than the cutoff wavelength, and consequently provides a unique advantage in using our proposed design in sensing applications.