Wavefront detection using curved nanoscale apertures

Abstract

In this paper, we report the experimental demonstration of wavefront sensing using curved subwavelength apertures. The demonstrated subwavelength structure has an ability to convert the shape of an incident wavefront into a two-dimensional array of focused surface plasmon (SP) waves. The detection principle is based on the phase-dependent spatial displacement of the focal point in circular apertures. The unit-cell structure of the demonstrated device consists of a circular disk surrounded by subwavelength concentric rings to excite highly enhanced SP waves at a designed wavelength. The excited surface waves, carrying the wavefront information of the incident beam, constructively interfere with each other and can be focused inside the disk. For demonstration, a 3-by-3 concentric-ring aperture array was fabricated and experimentally characterized. The focused SP waves within each of the fabricated 3-by-3 apertures under an optical excitation beam with a concave wavefront move toward the center of the array. When the shape of the incident wavefront was changed to convex, the focused spots in the array move away from the center. By measuring the spatial displacement of the focused spot, the shape of the incident wavefront can be directly measured. The demonstrated approach does not require complicated 3-D integration or optical alignment and offers a very high spatial resolution in wavefront detection, and thus, it has great potential for revolutionizing existing wavefront sensors.