  The low crystal symmetry of rhenium disulphide (ReS2) leads to the emergence
of dichroic optical and optoelectronic response, absent in other layered
transition metal dichalcogenides, which could be exploited for device
applications requiring polarization resolution. To date, spectroscopy studies
on the optical response of ReS2 have relied almost exclusively in
characterization techniques involving optical detection, such as
photoluminescence, absorbance, or reflectance spectroscopy. However, to realize
the full potential of this material, it is necessary to develop knowledge on
its optoelectronic response with spectral resolution. In this work, we study
the polarization-dependent photocurrent spectra of few-layer ReS2
photodetectors, both in room conditions and at cryogenic temperature. Our
spectral measurements reveal two main exciton lines at energies matching those
reported for optical spectroscopy measurements, as well as their excited
states. Moreover, we also observe an additional exciton-like spectral feature
with a photoresponse intensity comparable to the two main exciton lines. We
attribute this feature, not observed in earlier photoluminescence measurements,
to a non-radiative exciton transition. The intensities of the three main
exciton features, as well as their excited states, modulate with linear
polarization of light, each one acquiring maximal strength at a different
polarization angle. We have performed first-principles exciton calculations
employing the Bethe-Salpeter formalism, which corroborate our experimental
findings. Our results bring new perspectives for the development of ReS2-based
nanodevices.
