Generation of structured light beams with polarization variation along arbitrary spatial trajectories using tri-layer metasurfaces

Conventionally, the spatially structured light beams produced by metasurfaces primarily highlight the polarization modulation of the beams propagating along the optical axis or the beams' spatial transmission trajectory. In particular, along the optical axis, the polarization state is either constant or varies continuously in each output plane. Here, we develop innovative spatially structured light beams with continually changing polarization along any arbitrary spatial transmission trajectories. With tri-layer metallic metasurfaces, the geometric characteristics of each layer structure can be adjusted to modulate the phase and polarization state of the incident terahertz (THz) wave. The beam will converge to the predefined trajectory along several paths to generate a Bessel-like beam with longitudinal polarization changes. We demonstrate the versatility of the approach by designing two THz-band structured light beams with varying polarization states along the spatial helical transmission trajectory. Continuous linear polarization changes and linear polarization to right circular polarization (RCP) and back to linear polarization changes are realized respectively. The experimental results are basically consistent with the simulated results. Our proposal for arbitrary trajectory structured light beams with longitudinally varying polarization offers a practical method for continuously regulating the characteristics of spatial structured light beams with non-axial transmission. This technique has potential uses in optical encryption, particle manipulation, and biomedical imaging.