  We investigate the group velocity of edge magnetoplasmons (EMPs) in the
quantum Hall regime by means of time-of-flight measurement. The EMPs are
injected from an Ohmic contact by applying a voltage pulse, and detected at a
quantum point contact by applying another voltage pulse to its gate. We find
that the group velocity of the EMPs traveling along the edge channel defined by
a metallic gate electrode strongly depends on the voltage applied to the gate.
The observed variation of the velocity can be understood to reflect the degree
of screening caused by the metallic gate, which damps the in-plane electric
field and hence reduces the velocity. The degree of screening can be controlled
by changing the distance between the gate and the edge channel with the gate
voltage.
