Fig. 1. Experimental setup

Fig. 2. Four kinds of step phase plates and longitudinal distribution of filament array. (a) Semicircular phase plate (SCPP); (b) quarter-circle phase plate (QCPP); (c) six-octant phase plate (SOPP); (d) eight-octant phase plate (EOPP); (e) longitudinal distribution of filament array when QCPP is inserted

Fig. 3. THz signals under different incident laser energies. (a) 0-6 mJ; (b) local magnification of Fig. 3(a)

Fig. 4. Experimental results when the phase plate is covered by a black paper. (a) Complete laser facula; (b)-(e) phase plate covered by a black paper; (f) THz signal ratio for covered and uncovered phase plates; (g) THz signal versus incident laser energy when the phase plate is covered by a black paper

Fig. 5. Experimental data and theoretical fitting results