Quantitative Analytical Spheroid Model for Scattering‐Type Scanning Near‐Field Optical Spectroscopy

Abstract Scattering‐type scanning near‐field optical microscopy (s‐SNOM) is a versatile technique in nanooptics, enabling local probing of optical responses beyond the diffraction limit from vis to THz frequencies. Its theoretical modeling based on tip‐sample interactions typically relies on computationally intensive numerical methods or phenomenological models with empirically fitted parameters, complicating spectral analysis and interpretation. Developing a rigorous quantitative analytical model remains a significant challenge in near‐field microscopy. Here, an accurate analytical solution is introduced for the prolate spheroid model of s‐SNOM in the quasi‐electrostatic limit. The solution is validated through comparisons with numerical simulations and experimental spectra. Due to its higher computational efficiency compared to numerical simulation and higher accuracy compared to phenomenological solutions, the solution for the spheroid model facilitates spectrum prediction and interpretation for homogeneous bulk samples, enables systematic exploration of parameter effects, and supports data generation for machine learning applications. Furthermore, the generality of the approach allows straightforward extension to more complex nanostructures.