Asymmetric photonic mode conversion in waveguides and metasurfaces

Abstract

In this dissertation, asymmetric characteristics of photonic mode conversion structures in waveguides and metasurfaces have been discussed. More specifically, I propose design schemes for i) adjustment of mode conversion asymmetry in tri-mode waveguide system, ii) compact unidirectional mode converter in plasmonic waveguide and iii) unidirectional scattering of polarization-converted wave from bilayer metasurface. Firstly, a Lorentz reciprocal mode conversion asymmetry in reflectionless tri-mode waveguide system with weak waveguide gratings is discussed. In particular, the dark-mode which is the photonic analogue of atomic dark-state has been exploited for independent design of forward and backward direction characteristics. Due to the stationary property of the dark-mode, the mode conversion characteristics in one propagation direction could be fixed regardless of the length of the grating that defines the dark-mode. By carefully selecting the dark-mode and the length of the waveguide grating, the mode conversion asymmetry could be controlled. Secondly, a compact spatial plasmonic mode converter with unidirectional mode conversion characteristics is proposed. By combining mode-selective blockers with simple stub mode converter, unidirectional mode conversion characteristics could be achieved. Furthermore, it was found that the redundant scattering and the backward reflection can be completely eliminated by mode filtering and destructive interference, respectively. An application of the design strategy using the mode-selective blockers is also presented for the problem of near-complete out-coupling from subwavelength nanoslits. Lastly, a bilayer metasurface which transmits polarization converted signal only to the forward direction is proposed. The bilayer metasurface was designed by assembling two identical thin metasurfaces, the property of which is well-known. After numerical design of the bilayer metasurface, the designed structure was fabricated and its transmission and reflection characteristics were measured. It was found that the reflectance of the fabricated structure is successfully suppressed. The issue of amplitude distortion and its compensation is discussed and experimentally verified. The results on the dark-mode based asymmetric conversion device offer a method to control the transmission asymmetry and this capability can pave a way to actively tunable asymmetry of optical systems. Furthermore, by using mode selective blockers, asymmetric mode converters can be constructed in a compact form which is suitable for nanophotonic applications. The bilayer metasurface can be easily extended to the reflection-type and the multiplexing of transmitted signal and reflected signal can be made possible by making a supercell of a transmission-type cell and a reflection-type cell. This opens a new way of metasurface function multiplexing.