Dual-band achromatic wavefront control of spintronic terahertz via cascaded metasurfaces

In the past decades, terahertz (THz) technology has made significant progress. However, multifunctional THz devices are still lacking, particularly low-cost and convenient ones. In this paper, we combine a multifunctional metasurface with the unique forward and backward emission characteristics of THz waves from a thin spintronic THz emitter (STEs), carefully designing two types of dual-band achromatic metasurfaces (DBAMs). In these two DBAMs, THz waves at 0.85 THz and 1 THz generated by the STE are focused on the same focal point. More importantly, one of the THz waves features a conventional focused wavefront, while the other exhibits a focused helical wavefront. Our design significantly expands the capabilities of metasurfaces and greatly advances the development of state-of-the-art multifunctional THz devices.