Spin decoupling-based broadband terahertz encoded metalens with multi-focal points

Metasurfaces constructed using artificial sub-wavelength structure arrays have successfully acheived precise manipulation of electromagnetic wavefronts. Despite this, the precise control of electromagnetic waves with different spin-states remains a challenge. To address this, based on the new principle of spin-decoupling, we proposed two innovative metalenses with multiple focal points. Specifically, the first metalens can generate four foci with uniform intensity under 0.6 THz linearly polarized (LP) waves. When illuminated by left (LCP) or right circularly polarized (RCP) waves, it can form two foci that are symmetrically distributed about the center. For the second metalens, when LP waves are incident, it produces three weak foci and one strong focal point. When LCP waves are incident, the metalens generates only one strong focal point. When RCP waves are incident, they produce three weaker foci. The foci intensity and focal region for both types of metalens can be flexibly adjusted by modulating the ellipticity of the incident wave. In addition, they are capable of achieving spin-decoupled multi-focal points within a wide bandwidth range of 0.55 to 0.7 THz. This technology enables simultaneous control of orthogonal spin-state electromagnetic waves and precise regulation of spatial energy distribution, thereby offering new possibilities for the design and integration of multifunctional devices.