  Both the intrinsic anisotropic optical materials and fullerene-assembled 2D
materials have attracted a lot of interests in fundamental science and
potential applications. The synthesis of a monolayer (ML) fullerene makes the
combination of these two features plausible. In this work, using
first-principles calculations, we systematically study the electronic
structure, optical properties of quasi-hexagonal phase (qHP) ML and
quasi-tetragonal phase (qTP) ML fullerenes. The calculations of qHP ML show
that it is a semi-conductor with small anisotropic optical absorption, which
agrees with the recent experimental measurements. However, the results for qTP
ML reveal that it is a semimetal with highly in-plane anisotropic absorption.
The dichroic ratio, namely the absorption ratio of $x$- and $y$-polarized light
$\alpha$$_x$$_x$/$\alpha$$_y$$_y$, is around 12 at photon energy of 0.29 eV.
This anisotropy is much more pronounced when the photon energy is between 0.7
and 1.4 eV, where $\alpha$$_x$$_x$ becomes nearly zero while $\alpha$$_y$$_y$
is more than two orders of magnitude larger. This indicates qTP ML as a
candidate for long-pursuit lossless metal and a potential material for
atomically thin polarizer. We hope this will stimulate further experimental
efforts in the study of qTP ML and other fullerene-assembled 2D materials.
