UV-Transparent Bifocal Meta-Lens for Spin-Space Multiplexing with High-Quality Aluminum Nitride Buffer

We designed and fabricated the first bifocal meta-lens with a high-quality AlN buffer on a sapphire substrate. According to the ellipsometry measurement, the refractive index of AlN is above 2.2, and the extinction coefficient is below 0.002 in the whole ultraviolet (UV) spectral region. The meta-atom library consists of nanofins with near-unity half-wavelength plate efficiency with a full 2π coverage in the effective propagation phase of the converted spin. Two independent lens profiles for right-circularly polarized (RCP) and left-circularly polarized (LCP) spins are constructed within a single closely packed metasurface by simultaneous modulation of the geometric phase and the effective propagation phase of the converted spin. The fabricated 100 μm × 100 μm bifocal meta-lens showed a ∼1.3 μm full width at half-maximum at both foci at 365 nm, which is close to the theoretical prediction. When the incident light is pure LCP or RCP spin, the signal-to-noise ratio between the two foci is above 40. The bifocal meta-lens can be applied to spin demultiplexing in proximity and space-division multiplexing in the UV spectral region.