Date of Award

August 2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

George W Hanson

Committee Members

Brian S R Armstrong, Chiu T Law, Michael T Weinert, Prasenjit Guptasarma

Keywords

Electromagnetics, Graphene, Magnetized semiconductors, Nonreciprocity, Plasmonic, Unidirectional SPPs

Abstract

In plasmonic systems, interaction of light and surface plasmons leads to excitation of surface plasmon polaritons (SPPs) carrying energy on the surface. In an isotropic plasmonic system, the SPPs optical response is reciprocal, which means that the forward and backward surface waves have identical propagation behaviors and SPPs refract when they encounter a discontinuity on the surface. In order to excite SPPs resilient to the surface disorders, the system reciprocity needs to be broken by different techniques such as applying an external magnetic bias. In this case, the plasmonic system becomes a gyrotropic medium. Recently, it has been shown that magnetized continuous plasmonic systems such as semiconductors and graphene support unidirectional SPPs, the surface waves that are propagating only in one direction and are robust to the surface impurities. This topic has attracted the attention of many researchers, including our group. In this work, we study the properties of unidirectional SPPs in different plasmonic configurations. Our findings set a solid foundation for future active nonreciprocal plasmonic devices based on unidirectional SPPs.

First, we study SPPs in the well-known topological Voigt configuration. Since indium antimonide (InSb) crystal is often cited as a suitable magneto-optics platform that supports unidirectional SPPs, we evaluate the functionality of this crystal as a topological platform by considering realistic conditions. So, using the far-field time-domain THz spectroscopy measurement, our group, along with colleagues at the University of West Virginia, examine the magneto-optical effects of the undoped InSb crystal at different temperatures varied from 5K to 300K. We apply a multi-carrier material model to consider the effect of both electrons and holes charge carriers. Then, using the measured data we examine the unidirectional SPPs and discuss the constrains that limit applications. We design a grating metallic coupler on the surface of the magnetized InSb to launch unidirectional SPPs. The measured reflection data reveals strongly nonreciprocal SPPs that are tunable by temperature and magnetic field intensity. The patterned InSb sample is tilted to examine topological behavior. The measured data are consistent with the theoretical predictions.

Next, via simulation we study unidirectional SPPs on the surfaces of a magnetized plasma slab coated by a dielectric material below the plasma frequency. The equi-frequency contours are extracted from dispersion surface which follows by obtaining the group velocity vectors to estimate the SPP propagation behaviors at different operation frequencies. We mainly focus on a frequency window wherein there exists narrow-beam unidirectional SPPs. We present a Green's function model for a gyrotropic slab to examine the effect of thickness on the narrow SPP beams. We observe that when the slab is thin, in addition of two excited narrow beams at the top interface, two other narrow beams form at the bottom interface due to energy coupling. We characterize them by an asymptotic dispersion relation derived from a quasi-static approach.

Then, we study the nonlocality effects and Chern numbers in a continuous plasma medium. Topological SPPs are characterized by integer Chern invariants. When a continuous plasma systems is model by the (overly simplistic, but often used) local Drude model, there is a dispersion band that is ill-behaved at large wavenumbers and assigned by a non-integer Chern number. In this case, the number of unidirectional edge modes cannot be determined using the bulk-edge correspondence principle. This problem has been previously solved by introducing an ad hoc material model which includes a spatial cutoff wavenumber in the model. However, the proposed nonlocal model leads to some difficulties such as non-realistic material response at large wavenumbers and the need to interpolate the interfaced materials so that the Chern numbers sum to zero as they must. To overcome this issue, we instead suggest applying the hydrodynamic material model which is a more realistic, physical, nonlocal model. In this case, we evaluate the Chern numbers and dispersion bands. We show that this model form a complete, self consistent model that clarifies the topological physics of plasma continua.

In the next work, we propose a new plasmonic configuration to excite nonreciprocal curved SPPs. We demonstrate that by applying radial bias in a plasma system, one-way SPPs travel on a circular path, unlike in an axially-biased system which supports SPPs with linear trajectory. We derive a Green's function model for a radially-biased plasma system to examine curved SPPs. A nonreciprocal circular junction is proposed to effectively guide SPPs on the curvature.

Finally, we examine the unidirectional SPPs in two-dimensional plasmonic platform. It has been previously shown that graphene monolayer biased by external magnets supports unidirectional edge modes. Here, we evaluate the magneto-optical effects of graphene/chromium triiodide (CrI3) heterostructure. The exchange field between layers provides an effective out of magnetic field. The optical conductivity is a tensor with non-zero off-diagonal elements which manifest the nonreciprocal response. We obtain one-way edge modes and Faraday rotation in this multi-layer structure. However, we argue that the nonreciprocal response of this heterostructure is weaker than the isolated graphene biased by external magnets. Therefore, CrI3 magnetic monolayer does not work as an alternative magnetic source that causes strong non-reciprocity.

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