All-optical dynamic focusing of light via coherent absorption in a plasmonic metasurface

Vision, microscopy, imaging, optical data projection and storage all depend on focusing of light. Dynamic focusing is conventionally achieved with mechanically reconfigurable lenses, spatial light modulators or microfluidics. Here we demonstrate that dynamic control of focusing can be achieved through coherent interaction of optical waves on a thin beam splitter. We use a nanostructured plasmonic metasurface of subwavelength thickness as the beam splitter, allowing operation in the regimes of coherent absorption and coherent transparency. Focusing of light resulting from illumination of the plasmonic metasurface with a Fresnel zone pattern is controlled by another patterned beam projected on the same metasurface. By altering the control pattern, its phase, or its intensity, we switch the lens function on and off, and alter the focal spot’s depth, diameter and intensity. Switching occurs as fast as the control beam is modulated and therefore tens of gigahertz modulation bandwidth is possible with electro-optical modulators, which is orders of magnitude faster than conventional dynamic focusing technologies. Plasmonic metasurfaces: all-optical dynamic focusing realized Ultrafast, all-optical focusing of a light beam has been realized by using ultrathin plasmonic metasurfaces. Dynamic focusing is usually realized using mechanically reconfigurable lenses, microfluidics or spatial light modulators. Now, Maria Papaioannou and co-workers from the University of Southampton, UK, have experimentally demonstrated an alternative approach that involves the coherent interaction of light waves on an ultrathin beam splitter. It can switch the focusing function on and off and control the size, location and intensity of the focal spot—potentially at gigahertz speeds. The scheme operates by illuminating the sub-wavelength thickness plasmonic metasurface from each side with coherent control signals that are images of Fresnel-zone plate patterns. Phase modulation of one of the control signals controls absorption in the metasurface and effectively generates a reconfigurable zone plate with variable focusing properties.