Nanoantennas Inversely Designed to Couple Free Space and a Metal-Insulator-Metal Waveguide.

Yeming Han,Jan G Korvink,Wei Ma,Yu Lin,Huigao Duan,Yongbo Deng

doi:10.3390/nano11123219

Abstract

The metal–insulator–metal (MIM) waveguide, which can directly couple free space photons, acts as an important interface between conventional optics and subwavelength photoelectrons. The reason for the difficulty of this optical coupling is the mismatch between the large wave vector of the MIM plasmon mode and photons. With the increase in the wave vector, there is an increase in the field and Ohmic losses of the metal layer, and the strength of the MIM mode decreases accordingly. To solve those problems, this paper reports on inversely designed nanoantennas that can couple the free space and MIM waveguide and efficiently excite the MIM plasmon modes at multiple wavelengths and under oblique angles. This was achieved by implementing an inverse design procedure using a topology optimization approach. Simulation analysis shows that the coupling efficiency is enhanced 9.47-fold by the nanoantenna at the incident wavelength of 1338 nm. The topology optimization problem of the nanoantennas was analyzed by using a continuous adjoint method. The nanoantennas can be inversely designed with decreased dependence on the wavelength and oblique angle of the incident waves. A nanostructured interface on the subwavelength scale can be configured in order to control the refraction of a photonic wave, where the periodic unit of the interface is composed of two inversely designed nanoantennas that are decoupled and connected by an MIM waveguide.

Highlights

Plasmonic devices have been intensely studied in the recent years from both fundamental and applied perspectives [1,2,3,4]
It has been shown that an MIM structure with a dielectric regional thickness of ∼ 100 nm can support a propagating mode with a nanoscale modal size at a wavelength range extending from deep centimeter (DC) to visible [11]
This paper has developed a topology optimization-based inverse design method for nanoantennas that are localized at the interface between free space and the metal–insulator

Summary

Introduction

Plasmonic devices have been intensely studied in the recent years from both fundamental and applied perspectives [1,2,3,4]. It has been shown that an MIM structure with a dielectric regional thickness of ∼ 100 nm can support a propagating mode with a nanoscale modal size at a wavelength range extending from deep centimeter (DC) to visible [11]. Such a waveguide acts as an important interface between conventional optics and subwavelength photoelectrons; it has attracted attention for applications ranging from subwavelength optical routing and control [12,13,14] to molecular spectroscopy [15] and even as a negative index waveguide [16]

Methods

Results

Conclusion