Fig. 1. Schematic of the 3D-printed meta-device used as a reconfigurable terahertz beam splitter. Four different focal spot arrays can be switched by rotation.

Fig. 2. Flowchart of the inverse design method based on gradient-descent optimization. The phase profiles are optimized by minimizing the loss function. In each step, the forward propagation is calculated using Eq. (1), while the backward propagation is automatically calculated according to the gradient of the loss. After some iterations, we can get the final updated phase profiles.

Fig. 3. Design principle of the meta-device. (a), (b) Optimized phase profiles of the supercells for the two metasurfaces (a), and corresponding far-field intensities at four different rotation angles of 0°, 90°, 180°, and 270° (b). (c), (d) The beam-splitting phase profiles of the two metasurfaces (c), and corresponding far-field intensities at the four different rotation angles (d). (e) The beam-focusing phase profile encoded in the second metasurface. (f), (g) Fabricated samples of the first metasurface (f) and the second metasurface (g) using the 3D printing technique. (h) Schematic of the meta-atoms composing the meta-device.

Fig. 4. Experimental demonstration of the meta-device. (a) The diagram of the THz measurement setup used in this work. (b) Numerically simulated (upper row) and experimentally measured (lower row) field distributions (intensities) on the focal plane at four different rotation angles of 0°, 90°, 180°, and 270°.

Fig. 5. Simulated field intensities on the focal plane with different distances (Δz) between the two metasurfaces. All the sub-figures share the same spatial range of 50mm×50mm.

Fig. 6. Simulated field intensities on the focal plane with different repetitive numbers (Nrep) of the two metasurfaces. We adopt Nrep=3 in this work. Note that all the sub-figures here share the same spatial range of 50mm×50mm.

Fig. 7. Simulated and measured efficiencies of every focal area on the focal plane in working mode 1 (a), (b); 2 (c), (d); 3 (e), (f); and 4 (g), (h).

Fig. 8. Simulated field intensities on the focal plane at three different frequencies of 0.27 THz, 0.30 THz, and 0.33 THz. All the sub-figures share the same spatial range of 50mm×50mm.

Fig. 9. The convergence plot of the optimization process.

Fig. 10. Photograph of the THz measurement setup used in this work.