numerical aperture of NA$=0.1$. As in the case of the LG beams, squeezing the focal spot is accompanied by side bands and a loss in efficiency shown by the Strehl ratio (see Fig. \[fig8\]).

![Intensity cross sections: (a) Airy disk for the maximum numerical aperture considered NA$=\sin(\theta_{max})=0.1$. The yellow dashed circle shows the position of the smallest zero-intensity circle taken as the ROI inside which the spot size is calculated. The spot size is normalized to the spot size of the reference Bessel beam. (b) Reference Bessel beam corresponding to the largest cone angle $\theta_{max}$. The spot size of the reference Bessel beam is denoted as $w_{\textrm{B}}$. (c) OEi spot size optimized beam for a superposition of Bessel beams ($\theta\in[0,\theta_{max}]$) for a large ROI highlighted by the dashed yellow circle. Strehl ratio: $2\%$. (d) OEi spot size optimized beam for a small ROI. Strehl ratio: 0.$2\%$. The gray-scaled region shows the sidebands while the color range the ROI. Notice that the two scales are different.[]{data-label="fig7"}](MaziluFig7.pdf){width="11cm"}

![(a) Relative spot size $\Delta r/w_{\textrm{B}}$ of the Bessel beam superposition as a function of the relative ROI radius $R/R_{\textrm{B}}$. The spot size $w_{\textrm{B}}$ and the ROI radius $R_{\textrm{B}}$ are associated with the reference Bessel 