# Terahertz Spiral Spatial Filtering Imaging
## 4 Experimental Results  
To realize THz spiral spatial filtering imaging, we adopted a compact THz-QCL as the light source, and constructed a QCL-based setup with a 4\(f\) imaging scheme, as illustrated in Figure 4. The tested objects were illuminated by the QCL prototype, which was designed by the Institute of Semiconductors, Chinese Academy of Sciences, emitting 0.3 mW at 3.7 THz. A Golay cell with higher sensitivity and SNR was selected as the detector in the proposed THz SSF imaging system. The diameter of the entrance cone of the Golay cell was 11 mm, which is too large to act as a receiving aperture. A diaphragm was installed at the entrance cone of the Golay cell, which restricts the diameter of the receiving aperture of the Golay cell. In the experiments, the diaphragm was adjusted to the smallest aperture of 0.5 mm. The received THz signal was converted to the voltage signal in the Golay cell, and the voltage signal was digitized and recorded by a Stanford Research Systems Model SR830 DSP lock-in amplifier. To obtain THz images with a large imaging range, the Golay cell was mounted on a lifting platform, which was controlled by a scanning controller. The lifting platform can move 100 mm both in the \(x\) direction and \(y\) direction. As the optimum modulation frequency of the Golay cell is 15 \(\pm\) 5 Hz, we input the modulation current of 10 Hz to the THz-QCL. In the experimental setup, an SPP designed from HDPE at 3.7 THz was placed in the Fourier plane and the focal length of the two polymethylpentene (TPX) lenses in the 4\(f\) imaging system was 100 mm. The transmitted THz beam from the THz-QCL was reshaped by a small hole with 2 mm diameter, and the irregular THz beam was converted to a quasi-elliptical Gaussian beam. Then, the THz Gaussian beam was collimated by a TPX lens with a 200 mm focal length. After passing through the tested objects, the target information was modulated on the THz Gaussian beam. Finally, the beam enters the 4\(f\) imaging system and is detected by the Golay cell. Figure 4: Experimental setup of the THz SSF imaging. Figure 3: THz spiral spatial filtering imaging results of the simulated letters object of “THz”. (**a**,**c**,**e**) The letter objects with different image contrasts of 53.85, 25 and 5.26%. (**b**,**d**,**f**) The corresponding edge detection results of the outline profiles. As typical high-contrast targets, the metal objects were selected to verify the THz SSF imaging. The cardboard wrapped in copper foil was used to make the targets. The letters \"T\", \"H\" and \"Z\" were handmade using cardboard wrapped in copper foil. The letters have been cut off and can completely penetrate the THz wave, and other areas completely block the THz wave. The letters objects are shown in Figure 5a,d,g. In the experiments, to compare with the THz SSF images, the THz bright-field (BF) images are acquired by the THz SSF imaging system without the SPP. Figure 5b,e,h show the normalized THz BF images of the letters objects, indicating that the energies are concentrated inside the letters, and there are nearly no energies in other areas. The intensity distributions in the letters are nonuniform, which is caused by the quasi-elliptic Gaussian distribution of the THz beam. From the corresponding normalized THz SSF images shown in Figure 5c,f,i, the edges of the letters objects have been extracted perfectly. Meanwhile, it is clearly that the energies at the edges of \"T', \"H' and \"Z' are enhanced and the energies in the other region are suppressed. There might be two reasons to cause the uneven intensity distributions on the edges. First, the intensity distributions in the letters are not uniform. Second, there are certain phase errors due to the slight surface errors of the THz SPP. Although the intensities of the edges in experiments are not the same as the uniform edges in simulations, the experimental results are in general consistent with that in simulations, which validates the effectiveness of the SSF imaging in the THz frequency.