High numerical aperture bifocal metalens with regulatory focusing intensity

The metalenses with focusing and imaging based on metasurfaces, which can manipulate the amplitude, phase, and polarization of light waves, have attracted enormous attentions. A high numerical aperture bifocal metalens with regulatory focusing intensity was designed, and both theoretical analysis and simulation verification were demonstrated. The simulation results reveal that the designed metalens can focus circularly polarized incident light efficiently to a spot of full width at half-maximum as small as ~0.44λ, and the corresponding numerical aperture reaches up to 0.95. Besides, the relative intensity of two focal points can be adjusted flexibly through changing the polarization states of the incident light, which is unlike previous bifocal metalenses with repatterned intensity. More importantly, when the circularly polarized light is incident, the focusing efficiency is both up to 65%, and it is available for a relatively broad frequency range from 0.8 to 1.2 THz and a wide incident angles of 0°-20°. This work provides an important idea for designing the multi-focal metalenses, and it will also have high application value in many fields such as multi-imaging system, optical tomography.