Plasmonic time domain effects at the epsilon near-zero

Abstract

Epsilon near-zero (ENZ) materials have attracted considerable attention owing to the strong optical nonlinearities mediated by these materials both in bulk form and as constituent materials in plasmonic systems. These optical nonlinearities are underpinned by the near-zero refractive index, field-enhancement and reduced group velocity manifested at the ENZ condition. A reduction in group velocity would enable an efficient nonlinear response as a consequence of the increased time-scale of the wave-matter interaction. However, the group velocity definition for bulk ENZ samples has not been conclusively determined in the literature. Consequently, chapter 2 of this thesis presents our findings of finite-domain time-difference (FDTD) computations of the pulse reshaping for varying material and input radiation parameters. The role of FDTD discretisation and pulse temporal duration are also elucidated. Alongside two analytical group velocity definitions, we also applied a saddle point treatment to characterise the time-domain properties of the wave-matter interaction. Furthermore, chapter 3 discusses, for the first time, surface plasmon polariton modes arising at the ENZ condition in the presence of a DC electric bias. Such modes contain a near-flat dispersion profile and exhibit tremendously high field-enhancement characteristics. Similarly, we consider a hybridised, subwavelength ENZ (ITO and SiC) nanolayer decorated with nanoantennae which hybridise the nanoantennae cavity resonance with an ENZ mode. The result is a strongly coupled plasmonic system and we used FDTD simulations to characterise the pulse reshaping and field-enhancement throughout the coupling regime. In summary, in chapter 2 we concluded that only the real refractive index contributes to the group velocity and indeed the saddle point treatment offers additional insight into wave-matter interaction considering it accounts for the role of pulse duration. However, all group velocity models considered showed deviation from the FDTD results when determining the group index at the ENZ condition of low-loss materials. Further theoretical study is necessary to determine the correct analytical formulation of the group velocity for at a low-loss ENZ condition. In chapter 3, we also established, for the first time, that the presence of current brings into existence quasi-confined (QC) modes at the ENZ condition which exhibit tremendous field-enhancement properties, along with enabling nonreciprocal propagation. Toward that end, we can enable field-enhanced nonreciprocity using high mobility materials that exist in bulk, such as the InAs that we considered as our representative example. In chapter 4, our strongly coupled ENZ plasmonic system demonstrates that the time-domain effects and field-enhancement are particularly pronounced within the strong coupling region, and we highlight the different polarisation responses of the ITO-based case from the SiC one. The latter, owing to lower losses, shows extreme reshaping of resonant radiation close to the ENZ region and is, therefore, a potential contender for slow-light-enhanced effects.

Date of Award	Jul 2023
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Fumin Huang (Supervisor) & Matteo Clerici (Supervisor)