  The hybridization of hyperbolic Polaritonics with THz Plasmonics has
attracted immense attention due to its fascinating applications in this region.
However, to effectively enhance the performance of these coupled modes, one way
is the usage of smart materials in the heterostructures. Here, we introduce a
symmetric graphene-based structure containing hexagonal Boron Nitride (hBN)
layers, where two nonlinear layers have been utilized as claddings to improve
the confinement of Hybrid Surface Phonon-Plasmon-Polaritons. A new analytical
model is derived. A set of nonlinear equations are solved numerically and the
obtained results are reported. Harnessing a nonlinear medium together with
hybrid graphene-hBN layers allows one to tune the propagation properties of the
structure by changing the chemical potential, the relaxation time, and the
nonlinear factor. The study has been done for two frequency regions: the upper
Reststrahlen and the lower Reststrahlen bands. The numerical results show a
long propagation length (Lprop=300 {\mu}m) and a large Figure of Merit (FOM=98)
for HSP3 propagating in the upper Reststrahlen band at the frequency of 45 THz.
The proposed structure and its analytical model can open new insight into the
design of tunable devices in mid-infrared plasmonics.

Optics