Fig. 1. The experimental setup of the THz frequency upconversion detection based on RTP crystal

Fig. 2. The transmission curve of the 1 064 nm long pass edge filter in the range of 1 000~1 200 nm. Inset：the transmission curve in the range of 1 064~1 070 nm

Fig. 3. The non-collinear phase matching relationship in SPS process

Fig. 4. The dependences of χT2(2) and α_T of the ω₁=271 cm^-1 strong vibration mode on the THz frequency, the eigenfrequencies of multiple weak vibration modes and the frequency ranges of generated THz waves

Fig. 5. Propagating directions and phase-matching relation of the pumping，Stokes，and terahertz waves in RTP1

Fig. 6. Angle-tuning characteristics of the RTP crystal and several very weak A1 vibration modes

Fig. 7. Propagating directions and phase-matching relation of the pumping，Stokes，and terahertz waves in RTP2

Fig. 8. Simulation on the upconverted signal energy with the input THz energy

Fig. 9. The generated THz wave pulses measured by the Golay cell

Fig. 10. Spectra of the upconverted Stokes signal waves

Fig. 11. Temporal profile of the detected upconverted signal

Fig. 12. Upconverted signal equivalent voltages as functions of the input THz pulse energy at 3.84 THz and 5.58 THz，respectively