Light-Emitting Metasurfaces: Focusing and Beaming of Spontaneous Emission
Dielectric metasurfaces consist of arrays of high-index resonators wherein the electromagnetic phase and amplitude are engineered by changing the shape or size of the resonator. While phased-array metasurfaces have been extensively studied as flat alternatives for wavefront shaping, their capability to tailor spontaneous emission still remains an open question. In this work, we demonstrate focusing and beaming of photoluminescence (PL) using metasurface lenses and beam deflectors. Transmission phase is varied by modifying the radius of approximately-1-um-tall GaN nanopillars at the emission wavelength (510-590 nm) of embedded InGaN quantum wells. Typical metasurface lenses or beam deflectors are designed to focus or direct normally incident light. Light emission from the embedded GaN/InGaN quantum wells, however, is peaked beyond the critical angle. To redirect these oblique rays, we develop off-axis phased-array lens and beam deflector equations. Using real-space imaging accompanied by back-focal-plane imaging, we observe that the 2-D metalenses generate focused emission at the designed focal lengths. Moreover, the beam deflectors show preferred emission along varying output momenta. The presented results are especially promising, since they facilitate development of light sources with metasurface-derived functionalities such as direct focusing and directionality with low beam divergence.