In turbid materials (which scatter light due to spatial variations in refractive index), a converging wavefront that would come to a sharp focus in free space is obviously prevented from doing so. However, if a phase-based spatial-light modulator is placed in the light beam and adjusted via a digital feedback system, a focus can be created deep inside the turbid medium. In fact, because the scattered light reaches the focus with a numerical aperture (NA) greater than that of the input beam, the focal spot can be made smaller than that which would occur in free space—a result potentially important for imaging inside turbid materials such as biological tissue.
Now, researchers at the University of Twente (Enschede, The Netherlands) and the FOM Institute for Atomic and Molecular Physics (Amsterdam, The Netherlands) have experimentally demonstrated this effect. Using zinc oxide (an extreme scatterer with a mean free path of 0.7 µm) ranging from 7.5 to 25 µm thick with a few dyed fluorescent polystyrene nanospheres (with radii of either 80 nm or 150 nm) embedded in it as local intensity probes, they showed that linearly polarized 532 nm light at an incoming NA of 0.95 could be focused to an area (which they examined using an objective with a 1.49 NA) having only 68% of the normally smallest spot area. Contact Elbert van Putten at [email protected].