Activation and Detection of Forbidden Low‐Frequency Raman Modes in MoS ₂ by Near‐Field Gradient Illumination

Interlayer coupling in two‐dimensional (2D) materials is governed by weak van der Waals forces, resulting in low‐frequency (LF) vibrations in Raman scattering. These LF‐phonons provide direct insights into interlayer interactions, stacking configurations, and defects; however, many of these modes are inaccessible in conventional Raman spectroscopy due to symmetry‐based selection rules, leading to a loss of critical structural information. Here, we demonstrate that plasmonic near‐field gradients can relax Raman selection rules and enable the activation of forbidden LF phonons. Using silver‐nanoparticle‐decorated 3‐layer MoS 2 as a model system, we experimentally activated and detected forbidden LF‐phonons, namely the E'' and A 2 '' modes. Combining Raman imaging with density functional theory and electromagnetic simulations, we demonstrate that activation is mainly governed by the local near‐field gradient, and it occurs only when the near‐field gradient aligns with the phonon vibration direction. This work reveals a near‐field‐gradient mechanism for accessing Raman‐forbidden interlayer phonons and offers a framework for probing hidden vibrations, symmetry breaking, and interlayer interactions in 2D materials.