Fig. 1. (a) Azimuthally polarized beam (APB) electric field (E-field, upper panel) and magnetic field (B-field, lower panel) spatial distributions. The arrows in each panel indicate the polarization direction. The donut-shaped radial B-field polarization distribution is depicted in green, while the blue corresponds to the on-axis longitudinal component (out of plane). (b) The APB (red beam) interacts with a metallic nanoantenna, inducing strong and fast oscillating currents in the inner surface of the antenna (light blue, double headed arrows). As a result of the coherent superposition of the B-fields created by the ultrafast induced currents, the longitudinally polarized magnetic field (blue beam) of the APB is amplified by more than one order of magnitude.

Fig. 2. Vertical section of the considered antenna shapes (all antennas are cylindrically symmetric around the propagation axis). The orifice dimensions are governed by analytical functions (see Table 1, indicated by the red dashed lines). The Gaussian antenna case is parametrized to fit to the parabola shape (green). The shapes are labeled (a)–(e) in the order they are discussed below. The scale bars show the employed wavelength (527.5 nm) relative to the transverse and longitudinal dimensions of the antennas.

Fig. 3. Maximum achievable longitudinal B-field strength (a)–(c) for different antenna thicknesses of the cylindrical antenna, (d)–(f) for the rmin scan, and (g)–(i) for the rmax scan of the conical antenna. (a), (d), (g) In the first column, the maximum longitudinal B-field on the propagation axis is shown along the scanned parameter, together with the B-field of the optimized cylindrical antenna (orange dashed line). (b), (e), (h) The second column reveals the distribution of the B-field strength along the propagation axis, the longitudinal extent of the aperture (gray dashed lines). (c), (f), (i) In the third column, a temporal snapshot of the longitudinally polarized B-field, when the maximum amplitude is reached, is presented for the different scans.

Fig. 4. (a), (b) Total B-field strength for cone orifice radius scans of the rear side [rmin (a)] and the front side [rmax (b)]. The dotted orange line denotes the results retrieved from numerical PIC simulations, whereas the blue dashed line illustrates the predicted B-field strength based on the derived analytical model [see Eq. (3)]. (c), (d) Distribution of the analytically predicted B-field strength along the optical axis for cone orifice radius scans of the rear side [rmin (c)] and the front side [rmax (d)].

Fig. 5. Absolute B-field strength for the parabolic antenna for (a)–(c) scans of rmin, (d)–(f) antenna thickness L, and (g)–(i) cap thickness ΔL. (a), (d), (g) In the first column, the maximum B-field on the propagation axis is shown depending on the scanned parameter. (b), (e), (h) The second column shows the distribution of the B-field strength along the propagation axis, the longitudinal extent of the aperture (gray dashed lines), and the position of the focal point (red, solid line). (c), (f), (i) In the third column, a temporal snapshot of the highest B-field strength is presented for the different scans. The dashed black line in (g) and (h) indicates when the antenna is closed.

Fig. 6. Absolute B-field strength for the Gaussian antenna for scans of rmin (a)–(c), antenna thickness L (d)–(f), and cap thickness ΔL (g)–(i). In the first column (a), (d), (g), the maximum B-field on the propagation axis is shown depending on the scanned parameter. The second column (b), (e), (h) shows the distribution of the B-field strength along the propagation axis, the longitudinal extent of the aperture (gray dashed lines), and the position of the focal point (red, solid line). In the third column (c), (f), (i), a temporal snapshot of the highest B-field strength is presented for the different scans. The dashed black line in (g) and (h) indicates when the antenna is closed.

Fig. 7. Absolute B-field strength for the logarithmic antenna for (a)–(c) scans of κ and (d)–(f) antenna thickness L. (a), (d) In the first column, the maximum B-field on the propagation axis is shown depending on the scanned parameter. (b), (e) The second column shows the distribution of the B-field strength along the propagation axis and the longitudinal extent of the aperture (gray dashed lines). (c), (f) In the third column, a temporal snapshot of the highest B-field strength is presented for the different scans.

Fig. 8. Temporally integrated squared-integrated E-field/B-field (red/blue) and intensity contrast c2B¯2E¯2 for the (a)–(c) cylindrical, (d)–(f) conical, (g)–(i) logarithmic, (j)–(l) parabolic, (m)–(o) closed parabolic, and (p)–(r) Gaussian antenna geometries. For each antenna, the optimal parameters (see above) were used.

Fig. 9. Transverse distribution of the intensity contrast c2B¯2E¯2 for different antenna geometries (see Fig. 8) at the transverse plane in the center of the simulation box. The abscissa shows only positive values for y (equivalent to ρ), as the simulation obeys cylindrical symmetry in the transverse direction.