(color online) (a) Coordinate system of the simulation. r_p, φ_p, and z_p are the radial, azimuthal, and axial components in the cylindrical coordinates, respectively. θ denotes the angle between the incident light and the optical axis. AL represents an achromatic lens. z is the axial direction of the incident beam. x and y are the axes perpendicular to the optical axis. (b) Illustration of the excitation beam (green), STED beam (red), and the resulting I eff x (yellow). Here, experimental case 2 is plotted for reference.