Brillouin-zone-edge phonons are of great significance to many-body interactions and intervalley dynamics in two-dimensional (2D) materials. However, probing these elusive modes typically relies on destructive defect engineering for relaxing fundamental momentum conservation. Here, we establish a non-destructive and tunable paradigm by constructing a van der Waals heterostructure of few-layer MnPS₃ and monolayer WSe₂, which induces the emergence of giant anomalous Raman responses from large-momentum phonons from the intact WSe₂ lattice. Based on comprehensive spectroscopic investigations, we propose a phenomenological framework suggesting that this phenomenon is driven by a synergistic structural-electronic mechanism. We postulate that the massive lattice mismatch between few-layer MnPS₃ and monolayer WSe₂ creates a highly incommensurate interface, which provides necessary momentum compensation. Meanwhile, we suppose that the highly localized Mn 3d flat band acts as an energy-matched resonant amplifier. Crucially, we demonstrate the potential tunability of this system through the h-BN spacer insertion and elemental substitutions (FePS₃ and NiPS₃). These manipulations corroborate our hypothesis, allowing modulation of the anomalous responses from giant amplification to complete suppression via deliberate energy detuning. The ability to probe and modulate these fundamental mediators of many-body interactions in a non-destructively way establishes a robust framework for developing advanced 2D quantum phononic architectures.

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.