Shaping an evanescent focus of light for high spatial resolution optogenetic activations in live cells

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.