  Violet phosphorus (VP), the most stable phosphorus allotrope, is a van der
Waals semiconductor that can be used to construct $\textit{p}$-type
nanodevices. Recently, high-quality VP crystals have been synthesized while a
deep insight into their excitonic properties and bandgap tailoring approaches,
which are crucial for their optoelectronic device applications, is still
lacking. Here, we study the optical properties of ultrathin VP by second
harmonic generation, photoluminescence, and optical absorption spectroscopy. We
observed strong bound exciton emission that is 0.48 eV away from the free
exciton emission, which is among the largest in 2D materials. In addition, the
bandgaps of VP are highly sensitive to the number of layers and external
strain, which provides convenient approaches for bandgap engineering. The
strong bound exciton emission and tunable bandgaps make VP a promising material
in optoelectronic devices.
