  The improvement of light-emitting diodes (LEDs) is one of the major goals of
optoelectronics and photonics research. While emission rate enhancement is
certainly one of the targets, in this regard, for LED integration to complex
photonic devices, one would require to have, additionally, precise control of
the wavefront of the emitted light. Metasurfaces are spatial arrangements of
engineered scatters that may enable this light manipulation capability with
unprecedented resolution. Most of these devices, however, are only able to
function properly under irradiation of light with a large spatial coherence,
typically normally incident lasers. LEDs, on the other hand, have angularly
broad, Lambertian-like emission patterns characterized by a low spatial
coherence, which makes the integration of metasurface devices on LED
architectures extremely challenging. A novel concept for metasurface
integration on LED is proposed, using a cavity to increase the LED spatial
coherence through an angular collimation. Due to the resonant character of the
cavity, extending the spatial coherence of the emitted light does not come at
the price of any reduction in the total emitted power. The experimental
demonstration of the proposed concept is implemented on a GaP LED architecture
including a hybrid metallic-Bragg cavity. By integrating a silicon metasurface
on top we demonstrate two different functionalities of these compact devices:
directional LED emission at a desired angle and LED emission of a vortex beam
with an orbital angular momentum. The presented concept is general, being
applicable to other incoherent light sources and enabling metasurfaces designed
for plane waves to work with incoherent light emitters.
