Fig. 1. Schematic of the proposed moiré meta-device for order-variable Bessel beams generation. The moiré meta-device is cascaded by two metasurfaces with different phase profiles. Bessel beams can be obtained after a THz beam passes through the moiré meta-device. The order of the Bessel beam can be changed by mutual rotating with a step of 20° between the two metasurfaces.

Fig. 2. Design principle of the proposed moiré meta-device to generate order-variable Bessel beams. (a) Schematic of the combination of a flat spiral and a flat axicon. (b) Design principle of the proposed moiré meta-device to generate order-variable Bessel beams. Φ1 and Φ2 are the phase profiles of metasurface 1 and metasurface 2, respectively. Φjoint is the phase profile of the functional device to be realized. α0 is the mutual rotation angle between two metasurfaces.

Fig. 3. Schematic of designed processes of the metasurfaces. (a) Basic elements of the metasurface are high-resistivity silicon pillars with a height h of 300 μm and variable diameters d ranging from 30 to 180 μm on a silicon substrate with a period P of 200 μm and height H of 700 μm. (b) Transmission and phase shift of silicon pillars with different diameters at the operation frequency of 0.6 THz. (c), (d) SEM images of fabricated metasurface 1 and metasurface 2, respectively. The scale bar is 200 μm.

Fig. 4. Experimental results. (a) Schematic of experimental setup. PM, parabolic mirror; HWP, half-wave plate; P, polarizer; BS, beam splitter; QWP, quarter-wave plate; WP, Wollaston prism; and CCD, charge-coupled device. (b) Experimental and (c) simulated results for the zeroth-order Bessel beam when the mutual rotation of two metasurfaces is 0°. Two columns on the left are the normalized amplitudes on the y–z plane and the x–y plane, and on the right on the y–z plane and the x–y plane are the corresponding phase profiles. (d) Transverse amplitude and phase profiles along the horizontal white dotted cut lines across the centers of experiment and simulation results.

Fig. 5. Experimental and simulated results for order-variable Bessel beam generation based on the moiré meta-device with the mutual rotation of two metasurfaces changing from 0° to 80° with a step of 20°. (a)–(b) Normalized amplitude and phase profiles measured in the experiment. (c)–(d) Normalized amplitude and phase profiles obtained in the simulation.

Fig. 6. Simulated results for nondiffraction region variable Bessel beam generation based on the moiré meta-device with the mutual rotation of two metasurfaces changing from 15° to 60° with a step of 15°. (a) Near-field phase distribution of entire 12 mm diameter cascaded metasurfaces on the plane that is 1 mm away from the device obtained using the FDTD solver and field distribution of the device along the direction of propagation on the y–z plane. (b) Cut lines across the centers of field distribution illustrated in (a).

Fig. 7. Diagram of alignment method for two metasurfaces.

Fig. 8. Simulation results for nondiffraction region variable higher-order Bessel beams generation based on the moiré meta-device with the mutual rotation of two metasurfaces changing from 15° to 60° with a step of 15°.