  Confining light illumination in the three dimensions of space is a challenge
for various applications. Among these, optogenetic methods developed for live
experiments in cell biology would benefit from such a localized illumination as
it would improve the spatial resolution of diffusive photosensitive proteins
leading to spatially constrained biological responses in specific subcellular
organelles. Here, we describe a method to create and move a focused evanescent
spot across the field of view of a high numerical aperture microscope
objective, using a digital micro-mirror device (DMD). We show that, after
correcting the optical aberrations, light is confined within a spot of
sub-micron lateral size and $\sim$100~nm axial depth, resulting in a volume of
illumination drastically smaller than the one generated by a standard
propagative focus. This evanescent focus is sufficient to induce a more intense
and localized recruitment compared to a propagative focus on the optogenetic
system CRY2-CIBN, improving the resolution of its pattern of activation.
