Abstract
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.
Highlights
A light beam is focused by spatially varying its amplitude or phase distribution across the beam
Conventional convex lenses rely on the optical thickness of materials such as glass to introduce suitable phase delays, while Fresnel zone plates block light or introduce phase differences at certain distances from their centre in order to achieve constructive interference at the focus
We show that coherent control of the interaction of light with a metasurface in a standing wave can effectively create a dynamically adjustable Fresnel zone plate
Summary
A light beam is focused by spatially varying its amplitude or phase distribution across the beam. Dynamic focusing is realized by moving several solid lenses relative to each other[1], by elastic deformation[2], by varying curvature and optical thickness of microfluidic lenses[3,4,5,6], by reorientation of liquid crystals[7] or by varying spatial intensity or phase profiles using spatial light modulators[8,9,10] Such techniques are based on moving solid or liquid parts, or they rely on the reorientation of liquid crystal cells, making sub-millisecond response times difficult to achieve. Here we report dynamic control over optical focusing based on the linear interaction of light with light without moving parts, see
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