  Bidimensional materials are ideally viewed as having no thickness, as their
name suggests. Their optical response have been previously modelled by a purely
bidimensional surface current or by a very thin film with some contradictory
results. The advent of multilayer stacks of bidimensional materials and
combinations of different materials in vertical van der Waals heterostructures
highlights however that these materials have a finite thickness. In this
article, we propose a new model that reconciles both approaches and we show how
volume properties of stacked bidimensional layers can be calculated from the
bidimensional response of each individual layer, and conversely. In our
approach, each bidimensionnal layers is surrounded by vacuum and described as a
kind of transfer matrix with intrinsic parameters that do not depend on the
external medium. This provides a link between continuous thin films and
discrete layers. We show how to model heterostructures of bidimensional
materials and identify the parameters of the current sheet that represents the
bidimensional material in the zero-thickness limit, namely the in-plane surface
susceptibility and the out-of-plane displacement susceptibility.
  We show that our unified model is perfectly compatible with existing
ellipsometric data with the same reliability as the existing interface model
but with different values of the surface susceptibility or bulk dielectric
function. We discuss in details the origin of the discrepancies and show that
our approach allows to determine intrinsic properties of the bidimensional
materials with the advantage that multilayer and monolayer systems are
described in a same framework.
