The transition from very weak to ultrastrong plasmon-polariton-phonon coupling in (surface) doped ${\mathrm{HfS}}_2$ multilayers has been investigated. A plasmon-polariton (PP) in a single-layer ${\mathrm{HfS}}_2$ interacts weakly with polar phonons, however in ${\mathrm{HfS}}_2$ multilayers ($N>25$) the PP-phonon splitting becomes ${\mathrm{\Omega }}_{\text{PP-Ph}}\ge {\omega }_{\text{LOx}}/3$, indicating ultrastrong PP-phonon coupling. A particularly interesting electrodynamic effect appears for $d>10\text{−}\mu \text{m}$-thick ${\mathrm{HfS}}_2$ slabs. Then the absorption spectrum is dominated by a series of infrared active cavity resonances that spatially and by frequency resemble ${\mathrm{HfS}}_2$ cavity modes; they are standing waves confined to the ${\mathrm{HfS}}_2$ film but also radiate into the surrounding vacuum. The intensity of cavity resonances is comparable with the intensity of evanescent surface plasmon polaritons, while their number and frequency can be tuned by changing the ${\mathrm{HfS}}_2$ surface doping and/or thickness $d$. These are also very robust modes, so they may be very useful for the development of multifunctional nanoscale and mesoscale photonic devices.

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• Phonons
• Photonics
• Plasmonics
• Semiconductors
• Transition metal dichalcogenides
• Density functional theory
• Many-body techniques