  Observations of terahertz loss to gain switching in transmission through
gates on biased graphene sandwiches are explained by response under electron
streaming in graphene. Basing on this consideration the terahertz source is
proposed consisting of silicon resonator placed on the sandwich with current
flow in graphene. It should provide tunable by bias emission in wide terahertz
band. The source is direct terahertz analog of tunable microwave laser
demonstrated.
