Observation of a partially rotating superfluid of exciton-polariton

Rotation of a container holding a viscous fluid forms a vortex which grows with increasing angular velocity. A superfluid, however, is intrinsically different from these normal fluids because its rotation is quantized. Even if a container of superfluid is rotating, the fluid itself remains still until a critical velocity is reached. Beyond the critical velocity, all the particles conspire to suddenly pick up an angular momentum of $\hbar$ each and forms a quantized vortex. As a result, a superfluid is known to increase its rotation by a total angular momentum of $N\hbar$. In this letter, we show that exciton-polariton superfluid can split into an irrotational part and a rotational part. The relative ratio between the two states can be controlled by either pump beam's power or spot size. Consequently, angular momentum of exciton-polariton superfluid can be tuned from zero to $N\hbar$ continuously. This striking observation sets the stage for studying non-equilibrium properties of a superfluid with exciton-polaritons.