Hyperbolic plasmon\u2013phonon dispersion on group velocity reversal and tunable spontaneous emission in graphene\u2013ferroelectric substrate

Desalegn T Debu,Faezeh Tork Ladani,David French,Stephen J Bauman,Joseph B Herzog

doi:10.1038/s41699-019-0112-8

Desalegn T Debu, Faezeh Tork Ladani + Show 3 more

Open Access

https://doi.org/10.1038/s41699-019-0112-8

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Abstract

This work presents a new substrate platform, which provides tunability of the group velocity and spontaneous emission of a dipolar scatterer graphene–ferroelectric slab hybrid system in the terahertz ranges. We use analytical models to determine the hybridization of graphene surface plasmon and ferroelectric LiNbO3 type I and type II reststrahlen hyperbolic phonon–polariton. The variation of the chemical potential of graphene and the thickness of the ferroelectric layer results in several distinct features. Flipping the group velocity, strongly coupled hybrid hyperbolic surface plasmon–polaritons, and surface plasmon–polariton mode exists for the same momentum at different frequencies. The group velocity sign reversal for both a single-graphene- and double-graphene-integrated system depends on the thickness of the hyperbolic layer and the chemical potential of graphene. Comparative analysis of Purcell radiation is presented for a quantum emitter positioned at different locations between ferroelectric and graphene-integrated ferroelectric layers, revealing that this system can support strong spontaneous emission that can be modulated with the graphene chemical potential. Changing the chemical potential through selective voltage biasing demonstrates a substantial increase or decrease in the decay rate for spontaneous emission. Further analysis of the emission phenomenon shows a dependence on factors, such as the relative radiating source position and the thickness of the ferroelectric film. These characteristics make graphene–ferroelectric materials promising candidates to modify the light–matter interaction at the low terahertz ranges.

Highlights

For the purpose of our investigation of the hybridization of surface plasmon–polaritons (SPPs) of graphene hyperbolic phonon–polaritons (HPPs) of a ferroelectric, we computed the dispersion of the air/graphene/LiNbO3/Si heterostructure, shown in Fig. 1a, using computations outlined in the “Methods” section
One can compare these results to freestanding graphene, which are plotted in Supplementary Fig. 1 and a ferroelectric (LiNbO3) on a substrate, plotted in Supplementary Fig. 2a–e
These dispersion modes are not purely SPP nor HPP modes but rather a combination effect regarded as hybridized surface plasmon–phonon–polariton (HSPP) modes

Summary

Introduction

The discovery of two-dimensional (2D) materials, such as graphene,[1,2] has drawn immense scientific attention, both experimentally and theoretically for advanced electrical and optoelectronic applications.[3,4] Light, an external time-varying electric field, can interact with graphene’s conduction electrons, causing collective charge oscillations known as surface plasmon–polaritons (SPPs).[5,6] In graphene, the oscillating surface electrons are extremely well confined with a relatively low level of losses, offering much-better-quality plasmonic properties than traditional noble metals, which primarily operate in the optical spectrum.[7,8] Conventional metals used for plasmonics, such as Au, Ag, and Al, behave as nearly perfect electrical conductors mainly at longer wavelengths, while around near-infrared wavelengths, they suffer losses.[9]. On the other hand, are unbounded by the phonon limits and can be controlled through several mechanisms, such as optimization of geometry,[11] number of layers,[11] mechanical strain,[12] magnetic bias,[13] or dynamic tuning by chemical doping or gate voltage from the infrared to the THz range.[11]

Objectives

Methods

Results