**3. Results**

The simulated transmitted electric field distributions at a transverse plane 250 mm away from the metasurface are depicted in Figure 7. Figure 7a–d,e–h show the transmitted electric field distributions with incidences propagating along −*z* direction with a *y*-polarization and along *z* direction with a *x*-polarization, respectively. Figure 7a,c,e,g show the normalized transmitted electric field amplitude distributions at 5.14 GHz with design schemes in Figure 5a,b, respectively. Amplitude nulls can be observed due to the phase singularity at the center of OAM carrying beams, and along with the donut-shaped field distribution verified the characteristic of the vortex beams.

The transmitted phase distributions are shown in Figure 7b,d,f,h. The phase accumulations along a full circular path around the beam null in Figure 7b,d are 2 *π* and <sup>−</sup>2*π*, which indicates OAM orders of +1 and −1, respectively. Figure 7f,h depict 4 *π* and −4*π* phase accumulations along a full circular path and therefore indicate OAM orders of +2 and −2. Thus, by using the proposed structure, the designed metasurfaces can simultaneously convert the polarization of the incident wave and generate vortex beams carrying OAM of four different orders, which has grea<sup>t</sup> potentials for radar imaging applications.

**Figure 7.** Simulated cross-polarized electric field distributions of the transmitted OAM carrying beams at a transverse plane 250 mm away from the metasurface: (**a**) Amplitude and (**b**) phase distributions for OAM order of *l* = +1. (**c**) Amplitude and (**d**) phase distributions for OAM order of *l* = +2. (**e**) Amplitude and (**f**) phase distributions for OAM order of *l* = −1. (**g**) Amplitude and (**h**) phase distributions for OAM order of *l* = −2.

The proposed metasurface was fabricated using PCB processing as shown in Figure 8. The overall size of the fabricated sample is 326.72 mm × 326.72 mm with a thickness of 4.35 mm. Vias connecting the middle layer to the top and bottom layers are fabricated by back drilling leaving two holes on the top and bottom layers of each unit cell. The back drill holes and prepregs have been considered in the simulations and have little influences on the metasurfaces performance.

The fabricated metasurfaces were measured using a vector network analyzer Agilent E8363b (Keysight Technologies, California, United States) and a two-dimensional near field scanning measurement system. The metasurface was placed between a lens horn antenna (used as the excitation) and a WR-229 open-ended rectangular waveguide probe (used for receiving the OAM carrying beams). The measurement was conducted in the anechoic chamber. The response calibration was used to eliminate the effect of external noise during the measurement. The polarization conversion was confirmed by receiving and analyzing the co-polarization and cross-polarization components of the EM waves, which was realized by rotating the open-ended rectangular waveguide probe. A schema of the measurement devices and settings is depicted in Figure 9.

**Figure 8.** Photos of the fabricated metasurface: (**a**) Front view. (**b**) Back view.

**Figure 9.** Schema depicting the measurement devices and settings.

The measured amplitude and phase distributions of the cross-polarized transmitted electric field at a transverse plane 250 mm away from the metasurface are shown in Figure 10. Figure 10a–d show the amplitudes and phases of the *x*-polarized transmitted wave with a *y*-polarized excitation propagating along −*z* direction. Figure 10a,c depict an amplitude null at the field center. Figure 10b,d show phase accumulations of 2*π* and 4*π* along a full circular path, indicating OAM orders of +1 and +2, respectively. Figure 10e–h show the amplitudes and phases of the *y*-polarized transmitted wave with an *x*-polarized excitation propagating along *z* direction. The amplitude distributions shown in Figure 10e,g show amplitude nulls level at 0.01 compared to the maximum value. Due to the deviations in fabrication and measurement, the perfect offset of amplitude at the center may be compromised. Still, −20 dB nulls level is satisfying [29]. The phase distributions in Figure 10f,h show −2*π* and −4*π* phase accumulations along a full circular path, indicating OAM order of −1 and −2, respectively.

**Figure 10.** Measured cross-polarized electric field distributions of the transmitted OAM carrying beams at a transverse plane 250 mm away from the metasurface: (**a**) Amplitude and (**b**) phase distributions for OAM order of *l* = +1. (**c**) Amplitude and (**d**) phase distributions for OAM order of *l* = +2. (**e**) Amplitude and (**f**) phase distributions for OAM order of *l* = −1. (**g**) Amplitude and (**h**) phase distributions for OAM order of *l* = −2.

The simulated and measured amplitudes of the co-polarized transmitted wave are shown in Figure 11a,b, respectively. For each condition, the co-polarized transmissions are randomly distributed with a low amplitude. In the simulation results, the co-polarized electric fields amplitude level is lower than 0.06, while the measured results show co-polarization level lower than 0.15. The discrepancy between simulation and measurement comes from fabrication deviations and the slight amount of diffracted EM waves. However, compared with the cross-polarization level, the co-polarization level is low and does not affect the generated cross-polarized vortex beams. The co-polarization level can be enhanced if absorbers are placed around the metasurface.

**Figure 11.** Simulated co-polarized electric fields amplitude distributions for different OAM orders: (**a**) *l* = +1. (**b**) *l* = +2. (**c**) *l* = −1. (**d**) *l* = −2. Measured co-polarized electric fields amplitude distributions for different OAM orders: (**e**) *l* = +1. (**f**) *l* = +2. (**g**) *l* = −1. (**h**) *l* = −2.
