“…The strong light–matter interaction leading to an efficient quantum-level system can be provided by phonon polaritons based on multifunctional materials. − In particular, metal halide perovskites have attracted considerable attention owing to their significant physical properties, such as large diffusion length, high absorption coefficient, and high carrier mobility. − The crystallinity of metal halide perovskites is a vital factor for the transport properties in optoelectronic devices, which can be accomplished by a low-cost solution-processed spin-coating method followed by annealing of the films. Owing to their advantageous optical properties, they are considered a novel platform for optoelectronic device applications, such as solar cells, , transistor, , light emitting diodes (LEDs), , and photodetectors. , More recently, metal halide perovskites have emerged as novel materials for THz devices as they have representative phonon modes in the THz frequency region. − Importantly, phonon states in metal halide perovskite materials have been introduced as an excellent platform for polaritonic states that are strongly coupled with the electric fields in metamaterials. , Phonon-polaritons with large Rabi splitting have been observed in highly crystallized films, reaching a strong coupling regime.…”