Figure 1. Schematic diagram of a 4Pi focusing system consisting of two confocal high-NA objective lenses.

A line antenna (denoted by solid black line) with a cosine-squared taper (denoted by dashed blue curve) centered at the foci of two high-NA objectives is aligned along the optical axis. The radiation field from the line antenna (denoted by black arrows) is collected and inversely propagated to the focus region (denoted by red arrows). ê_θ and ê_ϕ are perpendicular to each other on Ω.