Evanescent field patterning by wavefront shaping.

Evanescent waves are widely used in microscopy for imaging cell structures close to the substrate with a high signal-to-background ratio, since the excitation is confined in a 100 nm-thick layer. We present a method to pattern the intensity of evanescent waves in the transverse plane for applications requiring localized illumination in the 3 dimensions, such as sub-cellular optogenetic activation. These evanescent patterns are generated using a digital micro-mirror device (DMD) in the pupil plane to block propagative waves and perform wavefront shaping in the supercritical annulus. A binary hologram, computed by an iterative Fourier transform algorithm, is displayed in this annular region to produce arbitrary patterns in the sample plane. We studied the uniformity and spatial resolution of these patterns, and confirmed their limited penetration depth. By providing micrometer-scale control of light intensity in a confined layer, this approach could enable the generation of biochemical signals and gradients with high accuracy.