High-numerical-aperture high-reflectivity focusing reflectors using concentric circular high-contrast gratings.

We present a concise approach to design focusing reflectors using concentric circular subwavelength high-contrast gratings (CCSW-HCGs). With this method, we design a high-numerical-aperture, high-reflectivity CCSW-HCG focusing reflector. By designing the CCSW-HCG structure, the grating can give the target phase front of reflected light while maintaining a high reflectivity. The properties of focusing and reflection are numerically studied with the finite element method. Simulation results show that, when the incident wavelength is 1550 nm, the reflectivity is 0.8538, and the numerical aperture is up to 0.9213. At the reflection focal plane, the full width at half-maximum of the field distribution is 0.9845 μm. Furthermore, the CCSW-HCG can simultaneously focus the reflected and transmitted light and has a strong robustness.