Fig. 1. Schematic diagram of ECOPS terahertz probe system using piecewise voltage functions

Fig. 2. Control principles of ECOPS. (a) Repetition frequency variations of lasers under ECOPS and ASOPS; (b) time delay under ECOPS and ASOPS; (c) time axis acquisition setup for ECOPS; (d) schematic diagram of piecewise voltage functions

Fig. 3. Time axis extraction method for terahertz signals under piecewise voltage function (all signals are results averaged over 5000 times). (a) Schematic diagram of time delay curves corresponding to steady-state voltage of 0.9 V and modulation frequency of 500 Hz; (b) terahertz time-domain signal of a 1 mm thickness silicon wafer under ASOPS; (c) forward-sampled terahertz time-domain signal of 1 mm thickness silicon wafer under ECOPS; (d) backward-sampled terahertz time-domain signal of 1 mm thickness high-resistivity silicon wafer under ECOPS

Fig. 4. Terahertz time-domain signal and frequency-domain amplitude spectra under ECOPS. (a) Forward-sampled terahertz time-domain signal; (b) backward-sampled terahertz time-domain signal; (c) terahertz frequency-domain amplitude spectra

Fig. 5. Influence of different piecewise voltage functions on terahertz signals under ECOPS. (a) Terahertz time-domain signals corresponding to steady-state voltages of 0.7, 0.8, and 0.9 V when modulation frequency is 500 Hz; (b) enlarged view of time-domain signals in Fig. 5(a); (c) corresponding terahertz frequency-domain amplitude spectra of Fig. 5(a); (d) terahertz time-domain signals corresponding to modulation frequencies of 250, 400, and 500 Hz when steady-state voltage is 0.9 V; (e) enlarged view of the time-domain signals in Fig. 5(d); (f) corresponding terahertz frequency-domain amplitude spectra of Fig. 5(d)

Fig. 6. Comparison of ASOPS and ECOPS signals within 10 s sampling time. (a) Time-domain signals comparison; (b) frequency-domain amplitude spectra comparison

Fig. 7. Two-dimensional distribution of terahertz spots. (a) Intensity distribution at 0.55 THz; (b) phase distribution at 0.55 THz; (c) intensity distribution at 0.60 THz; (d) phase distribution at 0.60 THz

Fig. 8. Terahertz imaging of metallic cross structure. (a) Physical image of orthogonal metallic cross structure; (b) intensity distributions of orthogonal metallic cross structure at 0.55 THz and 0.60 THz; (c) phase distributions of orthogonal metallic cross structure at 0.55 THz and 0.60 THz; (d) physical image of tilted metallic cross structure; (e) intensity distribution of tilted metallic cross structure at 0.55 THz and 0.60 THz; (f) phase distributions of tilted metallic cross structure at 0.55 THz and 0.60 THz