  We conduct polarization-dependent ultrafast spectroscopy to study the
dynamics of singlet fission in tetracene single crystals. The spectrotemporal
species for singlet and triplet excitons in transient absorption spectra are
found to be strongly dependent on probe polarization. By carefully analyzing
the polarization dependence, the signals contributed by different transitions
related to singlet excitons have been disentangled, which is further applied to
construct the correlation between dynamics of singlet and triplet excitons. The
anisotropy of exciton dynamics provides an alternative approach to tackle the
long-standing challenge in understanding the mechanism of singlet fission in
organic semiconductors.
