Figure 4.16 — Focal fields in the presence of spherical aberrations at an interface Figure 4.16 — Focal fields in the presence of spherical aberrations at an interface. Intensity distribution at the focus of a 1.30 NA oil-immersion objective with aperture radius of 4 mm for a x-polarised Gaussian beam with beam waist w0 = 4 mm in the presence of a glass (nt = 1.55)–water (ns = 1.33) interface placed at zi = −5 μm, −10 μm and −50 μm for (a), (b) and (c), respectively. As (the absolute value of ) zi increases, the quality of the focus deteriorates due to the presence of increasingly strong spherical aberrations. The beam has wavelength λ0 = 632 nm and the full laser power after the aperture is 10 mW. The iso-intensity surfaces correspond to I(x,y,z)=50, 20, 10, 5 GW/m2.