Control of LED radiation with functional dielectric metasurfaces

Abstract

Light-emitting diodes (LEDs) are excellent candidates to replace the widespread conventional fluorescent light sources. This stems from their higher compactness and brightness, as well as their better performance in terms of energy efficiency, long lifetime and high color rendering qualities. Their potential use in a broad range of applications has attracted enormous interest to the LED research and development. This has translated to a rapid improvement of the LED characteristics as well as to their commercialization in past years. Further integration and miniaturization of these devices, for applications such as optical communications, requires a higher level of control in terms of LED emission characteristics and a compact solution for any desired functionality on the output. Metasurfaces (structured surfaces with engineered characteristics) grant an unprecedented control over the wavefront of light, while retaining a subwavelength-thickness feature (relative to the excitation light wavelength). Moreover, in some cases, they also offer opportunities for large-scale industrial fabrication. Among different types of the metasurfaces, those based on dielectric and semiconductor materials typically exhibit lower losses comparing to metallic counterparts, which potentially enhances the efficiency of their integrated devices. The main focus of this dissertation is to develop and demonstrate the compact and novel LED platforms, with light emission characteristics on demand, enabled by means of dielectric metasurfaces. For this purpose, highly efficient metasurfaces based on dielectric and semiconductor materials (Si, TiO2, GaN) have been designed and fabricated. The functionalities realized with these include beam deflectors, polarization beam splitters, complex light field generators and lenses. For the latter, a novel class of metasurfaces with asymmetric radiation profile was engineered. With them, light channeling into a single desired diffractive order has been achieved with efficiencies reaching 99%. Ultra-high angle beam deflection metasurfaces operating in the visible regime were demonstrated using TiO2 for the blue and green and Si for the red part of the spectrum. Based on this concept, a near unity numerical aperture (NA) lens was designed and fabricated, far exceeding any previously reported, both commercial and laboratory experimental models. Moving towards direct metasurface integration on conventional LED platforms, GaN metasurfaces were etched directly into optically thick GaN slabs. Despite the low index contrast between the patterned metasurface and its substrate (both GaN), the metasurfaces exhibited high efficiencies across the operational wavelength range of the LED emission. However, when excited directly with the photoluminescence from the LED, the desired functionality was lost due to the low spatial coherence (Lambertian shape of radiation pattern) of the LED. Hence, the approach for direct integration of the metasurface on top of LED was found to be inefficient. To solve this issue, external and internal cavity design solutions for the LED and metasurface integration were proposed and engineered. The efficient transformation of the Lambertian radiation pattern into plane-wave-like radiation, suitable for the metasurface to work, was experimentally demonstrated. Further integration with the designed metasurfaces allowed to obtain advanced functionalities for the LED emitted light, namely beam deflection and vortex beam generation. Those were realized in a GaP LED architecture via optical pumping. The electrically-driven GaN LED device with a beam deflection functionality was demonstrated using the external cavity method. The results presented in this dissertation constitute a step forward towards compact, advanced, efficient and integrated optical devices, by leveraging on the attractive platform of LEDs and the emerging field of metasurfaces, which enables unprecedented control of light. The method proposed can be applied to any other incoherent light source beyond LEDs, and may find broad applications in optical communications, Li-Fi, displays, sensing, smart lighting and many more.