Far-field extraction of the dielectric function of exfoliated flakes near phonon resonances

The lateral dimensions of mechanically exfoliated flakes of emerging van der Waals (vdW) materials rarely exceed a few tens of micrometers. This limits the experimental determination of their complex dielectric function (ϵ(ω)) at mid-infrared (IR) frequencies, as the IR beam in conventional angle-resolved spectroscopic ellipsometry is typically larger than a flake. To overcome this, previous methods mapped the dispersion of surface phonon polaritons using near-field probes, but these require costly instrumentation, are sensitive to external factors, demand extensive numerical fitting, and become cumbersome for anisotropic or dispersive materials. Here, we introduce a simple empirical method to extract the in-plane dielectric tensor components of small flakes using conventional Fourier Transform Infrared (FTIR) micro-spectrometry. By identifying reflectance minima near phonon resonances, we determine ϵ per frequency without model fitting. Applying this approach to flakes of varying thicknesses enables reconstruction of ϵ(ω) over a broad spectral range.