Photon tunneling mechanism of internal plasmon polaritons between two thick slabs under near-field radiation excitation

Internal plasmon polariton (IPP), featuring localization of electromagnetic fields in the interior of material, is recently revealed to be supported in slabs with gradient carrier density distribution under near-field thermal excitation. However, the dispersion relation and photon tunneling mechanism of IPP have not been fully elucidated. To address the issues, in this paper, we investigate the photon tunneling mechanism of IPPs both in the symmetric and asymmetric gradient systems of two thick slabs at the near-field configuration with thermal excitation. It is revealed that infinite modes of IPP are supported in the gradient slabs. The frequencies of these IPP modes are highly space-dependent. They poorly couple to each other and collectively contribute to the photon tunneling with respective trade-off wavevectors and tunneling probabilities. For asymmetric systems where the slab at one side is replaced by that of uniform carrier density, we find the resonance locking effect of IPP. Moreover, the photon tunneling spectrum can be tailored by the gradient layer and the vacuum gap sizes. The mechanism outlined here deepens our understanding of IPPs as well as offers guidance for the design of nanoscale devices with tunable carrier density.