  During the last years, giant optical anisotropy demonstrated its paramount
importance for light manipulation which resulted in numerous applications
ranging from subdiffraction light guiding to switchable nanolasers. In spite of
recent advances in the field, achieving continuous tunability of optical
anisotropy remains an outstanding challenge. Here, we present a solution to the
problem through chemical alteration of the ratio of halogen atoms (X = Br or
Cl) in single-crystal CsPbX$_3$ halide perovskites. It turns out that the
anisotropy originates from an excitonic resonance in the perovskite, which
spectral position and strength are determined by the halogens composition. As a
result, we manage to continually modify the optical anisotropy by 0.14. We also
discover that the halide perovskite can demonstrate optical anisotropy up to
0.6 in the visible range -- the largest value among non-van der Waals
materials. Moreover, our results reveal that this anisotropy could be in-plane
and out-of-plane, depending on perovskite shape -- rectangular and square.
Hence, it can serve as an additional degree of freedom for anisotropy
manipulation. As a practical demonstration, we created perovskite anisotropic
nanowaveguides and show a significant impact of anisotropy on high-order
guiding modes. These findings pave the way for halide perovskites as a
next-generation platform for tunable anisotropic photonics.
