Fig. 1. (a) Schematic for the generation of a high-power, collimated, narrow-band and center-frequency-tunable THz pulse. An intense femtosecond laser pulse irradiates on the left-hand side of a block-shaped near-critical density plasma. Hot electrons generated by laser ponderomotive force can be separated into two groups: the electrons in group A moving forward leaving the plasma and the electrons in group B reciprocating under the sheath fields ; here the transverse sheath fields are induced when electrons pass through the plasma transverse interfaces. Under the action of , electrons in group B could be pulled back into the plasma and pass through the transverse interface on the other side. Such wiggler-like motions of these electrons can emit the desired THz pulse. (b) The electron accelerating in the plasma. (c) The trajectories of the two groups of electrons (blue and red) in the surface charge separation field.

Fig. 2. In the case of the electron penetrating with different (the angle at which the electron enters ), the relation between the electron threshold kinetic energy and the transverse location where the electron could be pulled back into the plasma.

Fig. 3. The angular-spectra distribution of the hot electrons. The electrons from a plasma length of (a) collected by a screen with a radius of in the first 270 fs of the simulation could be classified to group A, and (b) the electrons behind group A can be assigned to group B. The electrons from a plasma length of (c) in group A collected by a screen whose radius is in the first 770 fs and (d) the electrons in group B.

Fig. 4. Simulation results from the plasma of different lengths , while the thickness was fixed to . The angular-spectra distribution of the THz pulses from (a) , (b) and (c) . (d) The radiation field before filtering (blue line) and the field of the THz pulse (red line) collected at from the simulation of .

Fig. 5. (a) The center frequency of the THz source and the laser–THz energy conversion efficiency from the simulations with plasmas of different lengths from 50 to , while was fixed to . (b) The center frequency of the THz source from the simulations (blue line) and the theoretical model of Equation (4) for the plasma thickness changing from 20 to (light black shadow).