Dispersion properties of van der Waals phonon polaritons modulated by Weyl semimetals

Abstract

Surface phonon polaritons (SPhP) as an alternative constituent for mid-infrared (MIR) nanophotonic applications have attracted extensive attention and they maybe solve the intrinsic loss problem of plasmonics. SPhP arise in polar dielectrics due to IR-active phonon resonances, leading to negative permittivity within the Reststrahlen band. Although SPhP have great potential in enhancing the interaction between light and matter in the infrared region, it is still limited to enhance optical fields and fixed resonance band because of the existing Reststrahlen band. Moreover, active manipulating of phonon polaritons in MIR range remains elusive. The significant research progress of natural van der Waals (vdW) crystal and heterostructures have been made, which are characterized by an anisotropic polaritonic response, leading to elliptical, hyperbolic, or biaxial polaritonic dispersions. Among these structures, SPhP with hyperbolicity in <i>α</i>-MoO<sub>3</sub> are of particular interest, due to not only the strong field confinement, low losses, and long lifetimes, but also the natural in-plane anisotropic dispersion. A heterostructure composed of a biaxial vdW material (<i>α</i>-MoO<sub>3</sub>) and a Weyl semimetal (WSM) is proposed to study the active tunability of anisotropic SPhP. The control of polaritons can show more degrees of freedom, which has not yet been addressed. Under the incident condition of transverse magnetic incident wave, the reflection coefficient and field distribution in the heterogeneous system are accurately solved by the 4×4 transfer matrix method, and the dispersion properties of anisotropic SPhP are described in detail. Variation of dispersion spectrum with azimuthal angle and <i>α</i>-MoO<sub>3</sub> thickness is presented. The research results indicate that mode hybridization and dispersion manipulation can be realized by controlling the azimuth angle and the thickness of <i>α</i>-MoO<sub>3</sub>. More importantly, the Fermi level of WSM enable the adjustment of dynamic dispersion curve, which depends on the change of external temperature. Isofrequency curves of hybridized SPhP at different Fermi levels are also demonstrated. By chemically changing the Femi level of <i>α</i>-MoO<sub>3</sub>, the topology of polariton isofrequency surfaces transforms from open shape to closed shape as a result of polariton hybridization. Therefore, our research is helpful in further optimizing and designing active optoelectronic devices based on vdW materials, which have good application prospects in infrared heat radiation and biosensing.