  We have theoretically studied the collective response properties of the
two-dimensional chiral electron gas in bilayer graphene within the random phase
approximation. The cooperation of external controlling factors like
perpendicular electric bias, temperature, doping, and substrate background
provides great freedom to manipulate the dynamic dielectric function and the
low-energy plasmon dispersion of the system. Intriguing situations with
potential application are systematically explored and discussed. Extra undamped
plasmon modes might emerge under electric bias. They have almost zero group
velocities and are easy to manipulate.
