(a) A radially polarized collimated beam is tightly focused by a microscope objective (NA=0.9) onto a sub-wavelength silicon antenna sitting on a glass substrate (see inset). The light emitted into the angular regime with NA∈[0.95, 1.3] is collected by an oil-immersion type microscope objective (NA=1.3). The back-focal plane is imaged onto a CCD camera. (b) Acquired far-field intensity distribution I(kx, ky) for the antenna placed on the optical axis and (c) for the antenna displaced laterally by x≈40 nm (off-axis). The dashed black lines and the magnified insets indicate four regions with averaged intensities I1, I2, I3 and I4. (d) Directivity parameter Dx versus the antenna’s position along the x axis; the slope of the curve (0.01 nm−1) defines the sensitivity of the measurement to the antenna displacement. (e) Far-field intensity difference images for four antenna positions along the x axis (left to right: Δx≈40, 20, 10 and 5 nm away from the optical axis). The left image corresponds to the intensity difference ΔI(kx, ky) between b,c.