  Monolayer and few-layer phosphorene are anisotropic quasi-two-dimensional
(quasi-2D) van der Waals (vdW) semiconductors with a linear-dichroic
light-matter interaction and a widely-tunable direct-band gap in the infrared
frequency range. Despite recent theoretical predictions of strongly-bound
excitons with unique properties, it remains experimentally challenging to probe
the excitonic quasiparticles due to the severe oxidation during device
fabrication. In this study, we report observation of strongly-bound excitons
and trions with highly-anisotropic optical properties in intrinsic bilayer
phosphorene, which are protected from oxidation by encapsulation with hexagonal
boron nitride (hBN), in a field-effect transistor (FET) geometry. Reflection
contrast and photoluminescence spectroscopy clearly reveal the linear-dichroic
optical spectra from anisotropic excitons and trions in the hBN-encapsulated
bilayer phosphorene. The optical resonances from the exciton Rydberg series
indicate that the neutral exciton binding energy is over 100 meV even with the
dielectric screening from hBN. The electrostatic injection of free holes
enables an additional optical resonance from a positive trion (charged exciton)
~ 30 meV below the optical bandgap of the charge-neutral system. Our work shows
exciting possibilities for monolayer and few-layer phosphorene as a platform to
explore many-body physics and novel photonics and optoelectronics based on
strongly-bound excitons with two-fold anisotropy.
