Waveguide-Plasmon Polariton Quasiparticles with Exceptional Point Characteristics

TL;DR

This work investigates non-Hermitian photonics in hybrid plasmonic-dielectric waveguides to realize exceptional point degeneracies between coupled plasmonic and dielectric modes. By engineering a composite hybrid plasmonic waveguide (CHPW) and modeling it as a two-level non-Hermitian system, the authors demonstrate EP when the real parts of the modal indices match and the loss-difference equals the inter-modal coupling, i.e., $n_1=n_2=n_0$ and $Δγ = κ$, enabling coalescence of eigenvalues and eigenmodes without material gain. The results show tunable strong/weak coupling and maximal mode overlap at the EP, offering ultracompact, low-power modulators and reconfigurable on-chip components. The findings establish a pathway to harness non-Hermitian degeneracies in integrated nanophotonics, with potential use of ENZ and nonlinear materials to reconfigure EP dynamically.

Abstract

The growing complexity of integrated photonics necessitates compact, low-power devices that transcend traditional, material-centric design approaches. In this study, we harness non-Hermitian physics to uncover novel properties of coupled plasmonic waveguide modes exhibiting exceptional point (EP) degeneracy. Our hybrid plasmonic waveguide architecture, capable of supporting both strong and weak coupling regimes between plasmonic and dielectric waveguide modes, is precisely engineered to reach an EP where eigenmodes coalesce. This strategic tuning not only enhances the modal contrast between minimized-loss and highly dissipative states but also enables unprecedented control over device characteristics. Our findings introduce a new paradigm in integrated photonics, paving the way for ultracompact modulators and highly tunable on-chip communication systems with reduced power consumption.

Figures (4)

• Figure 1: Observation of exceptional point in hybrid plasmonic waveguide. The short-range and long-range modes have the characteristics of plasmonic and waveguide modes, respectively. The confined modes can be considered as confined energy levels in a potential well as in photonic molecules. The modes inherently do not overlap with each other. However, with judicious tuning of space parameters, the short-range and long-range modes can overlap, and in extreme cases, these two modes coalesce (EP).
• Figure 2: (a) A layered structure enables the overlapping between waveguide and plasmonic modes through changes in the width of the layers. (b) A plasmonic waveguide with an $Al$ film between two Si layers. (c) Effective refractive indices and (d) propagation loss of the supermodes. (e) Structure with an $SiO_2$ thin layer beneath an $Al$ thin layer. (f) $n_{eff}$ and (g) propagation loss of the modes in the structure for different values of the refractive index of the thin layer beneath the metal layer. (h) Achieving degenerate modes in the structure in parameter space. (i) Quantum mechanical approach to model the evolution of $TM_{LR0}$ ($\phi_1$) and $TM_{SR2}$ ($\phi_2$) modes in the structure.
• Figure 3: (a) By tuning the thickness of the top Si layer, the mode evolution in the structure reveals (b) weak coupling regime, (c) exceptional point, and (d) strong coupling regime.
• Figure 4: Overlap between the modes vs the height of the top Si layer, for the width of layer $598$ nm.