Fig. 1. Functional schematic of designed metasurface. (a) Phase and amplitude manipulation for LCP incidence. The generated multiplex amplitude E1(x,y)exp[iφ1(x,y)] is the superposition of POVs |POVL,la=1⟩ and |POVL,lb=−1⟩. (b) Phase and amplitude manipulation for RCP incidence. The generated multiplex amplitude E2(x,y)exp[iφ2(x,y)] is the superposition of POVs |POVR,lp=2⟩ and |POVR,lq=−2⟩. For all the cases, only the flipped handedness to the incidence is considered in the output, and E1(x,y)exp[iφ1(x,y)] is independent of E2(x,y)exp[iφ2(x,y)].

Fig. 2. Design of the meta-molecules of the metasurface. (a) Left, schematic diagram and optical microscope image of Metasurface-I made up of silicon pillars. Scale bar, 300 μm. Right, a typical meta-molecule including two diagonal silicon pillars M and two anti-diagonal silicon pillars S. (b) Twenty-four levels of discrete phase in the full range [0, 2π] for exact phase level δx and δy (described by blue and yellow bars, respectively) and simulated phase level Px and Py (described by triangles and pentagons symbols, respectively). Each selected silicon pillar is an effective half-wave plate. (c) Calculated polarization conversion efficiency at 0.6 THz for varied dimensions (Dx and Dy) ranging from 40 μm to 200 μm. The selected silicon pillars are indicated as black dots.

Fig. 3. Design flow of the metasurfaces. (a) Phase profiles of four POVs with TCs of 1, −1, 2, and −2 from top to bottom. (b) Target amplitude and phase profiles of superposed POVs, which are encoded on the metasurface. (c) Silicon pillars M and S, which depend on the setting parameters (δM,x, δM,y, δS,x, δS,y, θM, and θS). (d) Top, vortex mode purity of the superposition of |POVR,la=1⟩ and |POVR,lb=−1⟩. Middle, arrangement of the silicon pillars on the designed metasurface. Bottom, vortex mode purity of the superposition of |POVL,lp=2⟩ and |POVL,lq=−2⟩.

Fig. 4. Theoretical and experimental demonstration of Metasurface-I generating superposed POVs with different TCs and having an identical ring radius for LCP and RCP incidences. (a) Schematic diagram under LCP incidence. (b) Calculated and measured intensity and phase distributions under LCP incidence at 0.6 THz. The outer and inner radii of the ring pattern are r and r0, respectively. (c) Calculated and measured purity of OAM modes for LCP incidence. The sampling radius in corresponding intensity profiles is 4.0 mm. (d) Schematic diagram under RCP input. (e) Calculated and measured intensity and phase distribution under RCP input states at 0.6 THz. (f) Calculated and measured purity of the OAM mode for RCP incidence. The sampling radius in the corresponding intensity profile is 4.6 mm.

Fig. 5. Theoretical and experimental demonstration of Metasurface-II transforming LCP and RCP incidence into the superposition of two POVs with completely different TCs and radii. (a) Schematic diagram under LCP incidence. (b) Calculated and measured intensity and phase distributions under LCP incidence at 0.6 THz. The outer and inner radii of the ring pattern are r and r0, respectively. (c) Calculated and measured purity of OAM mode for LCP incidence. The sampling radius in corresponding intensity profile is 3.7 mm. (d) Schematic diagram under RCP incidence. (e) Calculated and measured intensity and phase distributions under RCP incidence at 0.6 THz. (f) Calculated and measured purity of OAM mode for RCP incidence. The sampling radius in corresponding intensity profile is 3.7 mm.

Fig. 6. Sample processing flow.

Fig. 7. Schematic of the near-field scanning terahertz time-domain spectroscopy system. Transmitter, terahertz photoconductive antenna; L1, terahertz lens; P1, terahertz linear polarizer 1; P2, terahertz linear polarizer 2; probe, terahertz near-field probe with ≥20  μm resolution.

Fig. 8. Calculated polarization conversion efficiency of 24 silicon pillars in the range of 0.5 THz to 0.7 THz.

Fig. 9. Measured intensity and phase distribution corresponding to the output light from Metasurface-I at the frequencies of 0.50 THz, 0.55 THz, 0.65 THz, and 0.7 THz. (a) Experimental intensity and phase distribution for the incidence with LCP. (b) Experimental intensity and phase distribution for the incidence with RCP.

Fig. 10. Measured intensity and phase distribution corresponding to the output light from metasurface-II at the frequencies of 0.50 THz, 0.55 THz, 0.65 THz, and 0.7 THz. (a) Experimental intensity and phase distribution for the incidence with LCP. (b) Experimental intensity and phase distribution for the incidence with RCP.

Fig. 11. Schematic of the dielectric antireflection silicon pillar array.

Table 1. Setting Parameters of Superposed POVs Provided by Metasurface-I and Metasuface-II^a