Beam focusing with a wide range of focal length from a single slit surrounded by Ag–SiO2–Ag multilayer gratings

A silver (Ag)–SiO2–Ag multilayer grating structure with a single slit for beam focusing has been proposed. Compared with the other grating-based focusing structure, the focusing properties of our designed structure can be enhanced on the range of focal length and the transmission at the focal point. Numerical simulations using the finite different time domain (FDTD) method verify that the focal length can be easily varied from ∼1 to above 15 μm by changing the period number of the grating, and the transmission is over 65%.