Fig. 1. Schematic of working principle of terahertz quantum-cascade laser[7]

Fig. 2. Output power stability of a 4.3 THz QCL in a liquid nitrogen dewar (LND)

Fig. 3. Schematic conduction band edge profile of the THz QWP[18]. (a) Zero bias; (b) finite bias

Fig. 4. Comparison of the normalized emission spectra of three THz QCLs and the normalized photocurrent spectrum of a THz QWP

Fig. 5. Emission spectra of the THz QCL, under different drive currents, measured by THz QWP (dash line) and DTGS-PE (solid line), respectively[33]

Fig. 6. Schematic of the setup measuring the peak pulse power of the THz QCL by using a THz QWP

Fig. 7. Current responsivity with the bias under different temperatures of a standard THz QWP

Fig. 8. Comparison of the emission spectrum of the THz QCL and the photocurrent spectrum of the THz QWP

Fig. 9. Modulation signal waveform applied to the THz QCL (lower) and response signal of the THz QWP (upper)

Fig. 10. Transmittance spectrum of the 3 mm-thick HDPE window fixed on the cryostat used for THz QWP

Fig. 11. Schematic of the setup for calibrating the responsivityof THz QWP based on a single frequency laser

Fig. 12. Comparison of the emission spectrum of a single frequency THz QCL (solid line) and the photocurrent spectrum of a THz QWP (dash line)

Fig. 13. 2D profile of the converging spot in the area equal to the sensitive facet of the THz QWP

Fig. 14. Peak current responsivity calibration curves of the 3.22 THz QWP under different temperature conditions

Fig. 15. Photograph of the THz QWP with transmission line packaging

Fig. 16. Schematic of high speed modulation and fast direct detection setup based on THz QWP and THz QCL[38]

Fig. 17. Waveform comparison of the drive signal of THz QCL (lower) and the response signal of THz QWP (upper)[38]. (a) 300 MHz; (b) 500 MHz

Fig. 18. Schematic of scanning reflection imaging setup based on THz QWP and THz QCL[42]

Fig. 19. Optical photograph (upper) and the THz reflected image (lower) of logo on the surface of a U-disk[42]

Fig. 20. (a) Photograph of the fast THz imaging system and (b) pattern of the converged beam spot

Fig. 21. Images of a printed letter ‘F’ under visible and THz radiation. (a) Photograph under visible light; (b) result of visible scanning imaging; (c) result of THz scanning imaging

Fig. 22. Schematic of a mini-type OAP coupled setup and the output THz beam

Fig. 23. Two-dimensional beam pattern of the THz light at a certain distance from the THz QCL end facet. (a) The distance is 150 mm, no aperture; (b) the distance is 150 mm, with aperture; (c) the distance is 200 mm, with aperture

Fig. 24. Amplitude curves and the corresponding Gauss fitting curve of the quasi-Gauss distributed THz beam spot at X=0 pixel and Y=0 pixel (inset: 2D profile of the THz beam spot)

Fig. 25. Photographs of the THz QWP and the coupling Winston cone fixed in the liquid He dewar, and schematic of gathering THz light

Fig. 26. Comparison between the inner diameter of entrance of a Winston cone (circle) and the size of the THz QWP (square)