Strongly bound excitons in atomically thin transition metal dichalcogenides offer many opportunities to reveal the underlying physics of basic quasiparticles and many-body effects in the two-dimensional (2D) limit. Comprehensive reflection investigation on band-edge exciton transitions is essential to exploring wealthy light–matter interactions in the emerging 2D semiconductors, whereas angle-resolved reflection (ARR) characteristics of intralayer and interlayer excitons in 2D MoS₂ layers remain unclear. Herein, we report ARR spectroscopic features of A, B, and interlayer excitons in monolayer (ML) and bilayer (BL) MoS₂ on three kinds of photonic substrates, involving distinct exciton–photon interactions. In a BL MoS₂ on a protected silver mirror, the interlayer exciton with one-third amplitude of A exciton appears at 0.05 eV above the A exciton energy, exhibiting an angle-insensitive energy dispersion. When ML and BL MoS₂ lie on a SiO₂-covered silicon, the broad trapped-photon mode weakly couples with the reflection bands of A and B excitons by the Fano resonance effect, causing the asymmetric lineshapes and the redshifted energies. After transferring MoS₂ layers onto a one-dimensional photonic crystal, two high-lying branches of B-exciton polaritons are formed by the interactions between B excitons and Bragg photons, beyond the weak-coupling regime. This work provides ARR spectral benchmarks of A, B, and interlayer excitons in ML and BL MoS₂, gaining insights into the interpretation of light–matter interactions in 2D semiconductors and the design of their devices for practical photonic applications.

Light-matter interactions in two-dimensional transition metal dichalcogenides (TMDs) are sensitive to the surrounding dielectric environment. Depending on the interacting strength, weak and strong exciton–photon coupling effects can occur when the exciton energy is resonant with the one of photon. Here we report angle-resolved spectroscopic signatures of monolayer tungsten disulfide (1L-WS₂) in weak and strong exciton–photon coupling environments. Inherent optical response of 1L-WS₂ in the momentum space is uncovered by employing a dielectric mirror as substrate, where the energy dispersion is angle-independent while the amplitudes increase at high detection angles. When 1L-WS₂ sits on top of a dielectric layer on silicon, the resonant trapped photon weakly couples with the exciton, in which the minimum of reflection dip shifts at both sides of the crossing angle while the emitted exciton energy remains unchanged. The unusual shift of reflection dip is attributed to the presence of Fano resonance under white-light illumination. By embedding 1L-WS₂ into a dielectric microcavity, strong exciton–photon coupling results in the formation of lower and upper polariton branches with an appreciable Rabi splitting of 34 meV at room temperature, where the observed blueshift of the lower polariton branch is indicative of the enhanced polariton-polariton scattering. Our findings highlight the effect of dielectric environment on angle-resolved optical response of exciton–photon interactions in a two-dimensional semiconductor, which is helpful to develop practical TMD-based architectures for photonic and polaritonic applications.

Strong light-matter interactions involved with photons and quasiparticles are fundamentally interesting to access the wealthy many-body physics in quantum mechanics. The emerging two-dimensional (2D) semiconductors with large exciton binding energies and strong quantum confinement allow to investigate exciton-photon coupling at elevated temperatures. Here we report room- temperature formation of Bragg polaritons in monolayer semiconductor on a dielectric mirror through the exciton-Bragg photon coupling. With the negative detuning energy of ~ 30 meV, angle-resolved reflection signals reveal anti-crossing behaviors of lower and upper polariton branches at ±18° together with the Rabi splitting of 10 meV. Meanwhile, the strengthened photoluminescence appears in the lower polariton branch right below the anti-crossing angles, indicating the presence of the characteristic bottleneck effect caused by the slowing exciton-polariton energy relaxation towards the band minimum. The extracted coupling strength is between the ones of weak and distinct strong coupling regimes, where the eigenenergy splitting induced by the moderate coupling is resolvable but not large enough to fully separate two polaritonic components. Our work develops a simplified strategy to generate exciton-polaritons in 2D semiconductors and can be further extended to probe the intriguing bosonic characteristics of these quasiparticles, such as Bose-Einstein condensation, polariton lasing and superfluidity, directly at the material surfaces.

Twisted van der Waals homo- and hetero-structures have aroused great attentions due to their unique physical properties, providing a new platform to explore the novel two-dimensional (2D) condensed matter physics. The robust dependence of phonon vibrations and electronic band structures on the twist angle has been intensively observed in transition metal dichalcogenide (TMD) homo-structures. However, the effects of twist angle on the lattice vibrational properties in the TMD heterostructures have not caused enough attention. Here, we report the distinct evolutions of Raman scattering and the underlying interlayer interactions in the twisted WS₂/MoS₂ heterostructures. The shifts and linewidths of E_2g(Γ) and A_1g(Γ) phonon modes are found to be twist angle dependent. In particular, analogous to that of the twisted TMD homostructures, the frequency separations between E_2g(Γ) and A_1g(Γ) modes of MoS₂ and WS₂ in the twisted heterostructures varying with twist angle correlate with the interlayer mechanical coupling, essentially originating from the spacing-related repulsion between sulfur atoms. Moreover, the opposite shift behaviors and broadening of A_1g(Γ) modes caused by charge transfer are also observed in the twisted heterostructures. The calculated interlayer distances and band alignment of twisted WS₂/MoS₂ through density functional theory further evidence our interpretations on the roles of the interlayer mechanical coupling and charge transfer in variations of Raman features. Such understanding and controlling of interlayer interaction through the stacking orientation are significant for future optoelectronic device design based on the newly emerged 2D heterostructures.