  Graphene/hBN heterostructures can be considered as one of the basic building
blocks for the next-generation optoelectronics mostly owing to the record-high
electron mobilities. However, currently, the studies of the intrinsic optical
properties of graphene are limited to the standard substrates (SiO2/Si, glass,
quartz) despite the growing interest in graphene/hBN heterostructures. This can
be attributed to a challenging task of the determination of hBN's strongly
anisotropic dielectric tensor in the total optical response. In this study, we
overcome this issue through imaging spectroscopic ellipsometry utilizing
simultaneous analysis of hBN's optical response with and without graphene
monolayers. Our technique allowed us to retrieve the optical constants of
graphene from graphene/hBN heterostructures in a broad spectral range of
250-950 nm. Our results suggest that graphene's absorption on hBN may exceed
the one of graphene on SiO2/Si by about 60 %.
