  Anisotropy describes the directional dependence of a material's properties
such as transport and optical response. In conventional bulk materials,
anisotropy is intrinsically related to the crystal structure, and thus not
tunable by the gating techniques used in modern electronics. Here we show that,
in bilayer black phosphorus with an interlayer twist angle of 90{\deg}, the
anisotropy of its electronic structure and optical transitions is tunable by
gating. Using first-principles calculations, we predict that a
laboratory-accessible gate voltage can induce a hole effective mass that is 30
times larger along one Cartesian axis than along the other axis, and the two
axes can be exchanged by flipping the sign of the gate voltage. This
gate-controllable band structure also leads to a switchable optical linear
dichroism, where the polarization of the lowest-energy optical transitions
(absorption or luminescence) is tunable by gating. Thus, anisotropy is a
tunable degree of freedom in twisted bilayer black phosphorus.
