  Backward terahertz radiation can be produced by a high-intensity laser
normally incident upon an underdense plasma. It is found that terahertz
radiation is generated by electrons refluxing along the bubble shell. These
shell electrons have similar dynamic trajectories and emit backward radiations
to vacuum. This scheme has been proved through electron dynamic calculations as
well as by using an ionic sphere model. In addition, the bubble shape is found
to influence the radiation frequency, and this scheme can be implemented in
both uniform and up-ramp density gradient plasma targets. The terahertz
radiation may be used for diagnosing the electron bubble shape in the
interaction between an intense laser and plasma. All results are presented via
2.5 dimensional particle-in-cell simulations.
