of the aperture.

The electric amplitude $|\bf E|$ of most efficient transmission OEi is shown in Fig. \[fig:oei\]a illustrating scattering of the aperture. The OEi on its own in Fig. \[fig:oei\]b. The transmission enhancement factor, with respect to the tightest Bessel beam achievable for a numerical aperture of NA=0.8 (Fig. \[fig:oei\]c), is 2.1 and 1.55 with respect to the Airy diffraction limited disk with the same numerical aperture.

In the second example, we use the optical momentum operator associated with the quadratic measure $m^{(\mathbf{F\cdot u_z})}$ as defined in Table \[tab\]. The momentum OEi with the largest positive eigenvalue corresponds to the field profile (Fig. \[fig:oei\] e-f) giving the largest optical force on the microparticle. Figure \[fig:oei\]d shows the field amplitude $|\bf E|$ of this OEi on its own and scattering of the microparticle (Fig. \[fig:oei\]e). The optical force enhancement factor, with respect to the plane wave illumination (Fig. \[fig:oei\]f), is of 49.3 and of 1.33 with respect to the Airy diffraction limited disk with the same numerical aperture.

Discussion and Conclusion
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We have experimentally and theoretically demonstrated an approach based on optical eigenmodes that enables the minimization of the free space spot size of a beam. Using full vectorial 