beams can be used to describe sub-diffracting beams but breaks down when beams are tightly focused. As a consequence we must consider full vectorial solutions of Maxwell’s equations. Here, we have chosen Bessel beams as a basis-set and determined the superposition of Bessel beams which minimized the spot size in a planar finite ROI. Note that the problem of the finite intensity of Bessel beams [@Durnin:1987p7503] is easily circumvented here due to the finite ROI size considered. The monochromatic electric vector field of the vectorial Bessel beam may explicitly be expressed as [@Volke-Sepulveda:2002hk] $$\begin{aligned}
\label{eq:scal-BB}
\nonumber
\mathbf{E}& =&E_0  \exp\left(i\ell\phi+ik_t z)\right) \Bigg((\alpha \mathbf{{e}}_x+\beta \mathbf{{e}}_y) J_\ell(k_t r) \\
\label{eq:s341}
&&+\frac{ik_t}{2k_z}((\alpha+i\beta)\exp(-i\phi)J_{\ell-1}(k_t r)-(\alpha-i\beta)\exp(i\phi)J_{\ell+1}(k_t r))\mathbf{{e}}_z\Bigg)\end{aligned}$$ where $k_t=k_0\sin(\theta) $ and $k_z=k_0\cos(\theta) $ are the transversal and longitudinal wave vectors with $\theta$ the characteristic cone angle of the Bessel beam. $\mathbf{{e}}_{x}$, $\mathbf{{e}}_{y}$ and $\mathbf{{e}}_{z}$ are the unit vectors in the Cartesian coordinate system. The parameter $\ell$ corresponds to the azimuthal topological charge of the beam while $\alpha$ and $\beta$ are associated with the polarization state of the beam. The magnetic field $\mathbf{H}$ was deduced according to Maxwell’s equations. Figure \[fig7\] shows a comparison between the Airy disk, the Bessel beam and the OEi optimized spot considering a 