The strong light–matter interaction between the exciton of atomically thin transition metal dichalcogenides (TMDCs) and photonic nanocavities leads to the formation of unique hybrid light-matter quasiparticles known as exciton-polaritons. The newly formed mixed state has the advantages of the photonic part such as rapid propagation and low effective mass and the highly desirable optical properties of TMDC’s exciton, including the interparticle strong interactions nonlinearity and spin-valley polarization. These joint properties make such systems an ideal platform for studying many compelling physics phenomena and open the possibility of designing novel optoelectronic devices. This work reviews recent progress of strong coupling between exciton in TMDC and different resonant photonic structures, such as optical microcavities, plasmonic and all-dielectric nanocavities. Furthermore, we discussed the unique valleytronic and nonlinear properties of TMDC monolayers in the strong coupling regime. Finally, we highlighted some of the challenges and potential future research opportunities in this field.

Exciton-polariton arising from strong light-matter interaction between exciton and optical cavity has attracted considerable attention due to its potential applications in Bose-Einstein condensation, low-threshold lasing, and slow light. In recent years, two-dimensional lead halide perovskite has emerged as an ideal candidate for realizing exciton polariton at room temperature because it has large exciton binding energy and quantum yield. Here, we demonstrate that strong coupling could be enabled with a perovskite metasurface that supports multipolar Mie resonance, including magnetic quadrupole dominant, anapole, and toroidal resonances. For an array of perovskite nanodisks, the strong coupling behavior between these resonances and exciton is confirmed by the anticrossing features in absorption spectra mapping, while the Rabi splitting is increased from 230.7 meV in magnetic quadrupole-exciton strong coupling to 253 meV in both anapole-exciton and toroidal-exciton strong coupling. The enhanced Rabi splitting is attributed to the stronger field localization within the perovskite instead of within the air gap. In addition, we find that the Rabi splitting depends on the oscillatory strength of the exciton mode and can then be boosted to 362 meV in anapole-exciton strong coupling. Our results provide promising ways to improve the performance of optoelectronic devices such as low-threshold lasers and slow-light devices.

In recent years, the formation of exciton-polaritons in semiconductor materials has attracted major interest because of their exotic optical properties, allowing the study of many interesting physical phenomena, such as superfluids and Bose-Einstein condensation. Here, we demonstrate a double strong coupling regime in a perovskite and tungsten disulfide $\left(\mathrm{W}{\mathrm{S}}_2\right)$ monolayer using single and dual quasibound states in the continuum (QBIC). Except for supporting exciton resonance, bi(phenethylammonium)tetraiodoplumbate perovskite thin film is patterned as a grating to act as a single symmetry-protected bound state in the continuum (BIC) or dual BICs by harnessing its high index. A $\mathrm{W}{\mathrm{S}}_2$ monolayer is sandwiched inside the perovskite nanowire so that a single BIC can be strongly coupled to both exciton resonances of perovskite and $\mathrm{W}{\mathrm{S}}_2$ monolayers with a Rabi splitting of 380 and 42.2 meV. We found that the $Q$ factor and thickness of perovskite film play a central role in governing the coupling strength. In addition, we demonstrate that double strong coupling can be satisfied by using dual QBICs, where each QBIC is individually coupled to the exciton of either a perovskite or a $\mathrm{W}{\mathrm{S}}_2$ monolayer. The design approach achieves a Rabi splitting as high as 353.6 and 31.80 meV in the perovskite and the $\mathrm{W}{\mathrm{S}}_2$ monolayer, respectively. This is a demonstration of a double strong coupling regime using a single structure based on perovskite and $\mathrm{W}{\mathrm{S}}_2$ monolayers. The realization of double strong coupling paves the way for the emergence of polaritonic devices with advanced functionalities, such as dual wavelength polariton nanolasers.

Exciton polaritons in metallic nanocavities and transition-metal dichalcogenide monolayers has led to striking discoveries, ranging from Bose-Einstein condensation to slowing light. Although plasmonic nanocavities offer small mode volumes, the intrinsic losses of plasmonic nanocavities remain an open challenge in exciton-polariton devices. Consequently, dielectric nanocavities are used as an alternative candidate due to their low intrinsic losses. However, large mode volumes are a central bottleneck in dielectric nanocavities. Here, we theoretically propose to use a hybrid dielectric-metallic nanocavity to enhance light-matter interactions between the excitons of the ${\mathrm{WS}}_2$ monolayer and the hybrid nanocavity. Such a hybrid nanoresonator inherits the advantages of both dielectric and metallic nanocavities, including ultrasmall mode volume, ultralow losses, and reasonably-high-Q factor. It is demonstrated that the thickness and material of the central gap film together with the thickness of the metallic substrate play vital roles in governing the coupling strength between 1L-${\mathrm{WS}}_2$ excitons and the cavity. After optimizing the geometry and material parameters, the Rabi splitting is increased to 113 meV, almost twice that in dielectric metasystems. The significant improvement can be attributed to the greatly enhanced near field and the ultrasmall mode volume. Furthermore, we show that Rabi splitting can be further boosted to 151 meV by increasing the number of layers of ${\mathrm{WS}}_2$ and h-$\mathrm{BN}$ film in the nanocavity.