Clarifying modelling ambiguities in high-NA surface plasmon microscopy

High-numerical-aperture (High-NA) objectives are used for surface plasmon excitation, yet modelling focal fields in surface plasmon microscopy (SPM) remains ambiguous. Previous works predominantly focused on the excitation conditions often overlooking the high-NA diffraction effects rigorously described introduced by high-NA optics–a phenomenon rigorously addressed by Richards-Wolf theory. This partial treatment causes two common ambiguities in high-NA SPM modelling: (i) the omission of the amplitude apodization term (cos^1/2θ) and (ii) misinterpretations of polarization-wavevector orthogonality. By integrating Richards-Wolf theory with Fresnel analysis, we propose the present framework for high-NA SPM and validate its impact on focal field calculations. To the best of our knowledge, this is the first work to systematically resolve these modelling ambiguities in high-NA SPM.