  High-quality low-dimensional layered and van der Waals materials are
typically exfoliated, with sample cross sectional areas on the order of tens to
hundreds of microns. The small size of flakes makes the experimental
characterization of their dielectric properties unsuitable with conventional
spectroscopic ellipsometry, due to beam-sample size mismatch and
non-uniformities of the crystal axes. Previously, the experimental measurement
of the dielectrirc permittivity of such microcrystals was carried out with
near-field tip-based scanning probes. These measurements are sensitive to
external conditions like vibrations and temperature, and require
non-deterministic numerical fitting to some a priori known model. We present an
alternative method to extract the in-plane dielectric permittivity of van der
Waals microcrystals, based on identifying reflectance minima in spectroscopic
measurements. Our method does not require complex fitting algorithms nor near
field tip-based measurements and accommodates for small-area samples. We
demonstrate the robustness of our method using hexagonal boron nitride and
{\alpha}-MoO3, and recover their dielectric permittivities that are close to
literature values.
