Fig. 1. Schematic diagram for the principle of terahertz field modulated FEL induced photoelectron time and energy distribution

Fig. 2. Simulation results with delays of -60 fs, 0 fs, and 60 fs. (a)-(c) Correlation in-between the terahertz streaked photoelectron final energy and initial energy at different time delays; (d)-(f) photoelectron final energy spectrum distribution

Fig. 3. Photoelectron spectrum of FEL pulses with different lengths at the zero point of the terahertz vector potential. (a)-(c) Pulse length is 35 fs, 20 fs, and 10 fs respectively

Fig. 4. Schematic diagram for two arbitary FEL pulses with various mutual delays modulated by terahertz field near to the zero-crossing of its vector potential, where the pulse lengths are 50 fs and 35 fs. (a) Schematic diagram of double-pulse modulation by terahertz vector potential, when the double pulses are relative to the zero of terahertz vector potentials are -40 fs and 70 fs; (b) variation of center enegy of photoelectron spectrum with delays of double-pulse in Fig. (a); (c)(d) schematic diagram

Fig. 5. Relative errors between retrieval result and original FEL pulse in different lengths, deviated from the zero of terahertz vector potential from -100 fs to 100 fs

Fig. 6. Offset errors for center energy of photonelectron spectrum at different delays under an identical terahertz streaking field for FEL pulses with different pulse lengths. (a) 20 fs; (b) 10 fs