Chemical Imaging of Two-Dimensional Dielectric Crystals by Stimulated Raman Scattering

Conventional Raman imaging of two-dimensional (2D) dielectric materials is hindered by their intrinsically small Raman cross-sections and atomic thickness. Here, we demonstrate label-free chemical imaging of monolayer dielectrics using stimulated Raman scattering (SRS). Thin hexagonal boron nitride (hBN) and calcium hydroxide, Ca(OH)₂, were used as 2D model systems. SRS resolved the E₂g vibration of hBN at 1366 cm⁻¹ and the OH stretching mode of Ca(OH)₂ at 3650 cm⁻¹. While six layers of hBN represented the detection limit in the epi-detection mode, monolayers could be resolved in the forward-detection mode. A direct comparison between the two detection geometries elucidated the role of cross-phase modulation in generating SRS contrast. SRS images reproduced the material contrast observed in topographic and optical micrographs, confirming its chemical specificity. These results establish SRS as a sensitive platform for chemical mapping of dielectric 2D crystals and suggest instrumental strategies for efficient monolayer detection.