Continuous-zoom bifocal metalens at visible wavelength

Metasurfaces and metalenses have drawn great attentions since they can manipulate wavefront versatilely with a miniaturized and ultrathin configuration. Here we propose and numerically verify a tunable bifocal metalens with two continuous-zoom foci. This device utilizes two cascaded and circle layers of metasurfaces with different phase distributions for incidences of opposite helicities imparted on each layer by the combination of geometric phase and propagation phase. By relative rotation of both layers, focal lengths of both foci can be tuned continuously with the zoom range for each focus designed deliberately, and the relative intensity of both foci can be adjusted by changing the polarization state of incidence. The proposed device is anticipated to be applied in polarization imaging, depth estimation, multi-plane imaging, optical data storage, and so on.