Sensitivity-guided illumination engineering for coherent Fourier ellipsometry

Coherent Fourier ellipsometry (CFE) enables highly flexible thin-film metrology by combining high-numerical-aperture imaging with computational reconstruction. In this Letter, we present a sensitivity-analysis framework for polarization engineering in CFE. A vectorial forward model based on high-NA Fourier-field propagation is employed to analyze the sensitivity of focused light fields. Absolute and normalized sensitivity matrices are introduced to evaluate parameter sensitivity under different illumination polarization states, with which the incident polarization is treated as an optimization variable. Numerical results show that elliptically polarized illumination improves the joint parameter sensitivity and separability compared with conventional linear polarization. The proposed framework provides a practical strategy for adaptive illumination engineering in coherent Fourier-domain thin-film metrology.