Control of LED Emission with Functional Dielectric Metasurfaces

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.Comment: 29 pages, 6 figure

A Metalens with Near-Unity Numerical Aperture

The numerical aperture (NA) of a lens determines its ability to focus light and its resolving capability. Having a large NA is a very desirable quality for applications requiring small light-matter interaction volumes or large angular collections. Traditionally, a large NA lens based on light refraction requires precision bulk optics that ends up being expensive and is thus also a specialty item. In contrast, metasurfaces allow the lens designer to circumvent those issues producing high NA lenses in an ultra-flat fashion. However, so far, these have been limited to numerical apertures on the same order of traditional optical components, with experimentally reported values of NA <0.9. Here we demonstrate, both numerically and experimentally, a new approach that results in a diffraction limited flat lens with a near-unity numerical aperture (NA>0.99) and sub-wavelength thickness (~{\lambda}/3), operating with unpolarized light at 715 nm. To demonstrate its imaging capability, the designed lens is applied in a confocal configuration to map color centers in sub-diffractive diamond nanocrystals. This work, based on diffractive elements able to efficiently bend light at angles as large as 82{\deg}, represents a step beyond traditional optical elements and existing flat optics, circumventing the efficiency drop associated to the standard, phase mapping approach.Comment: 12 pages, 5 figure

Control of LED radiation with functional dielectric metasurfaces

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.Doctor of Philosoph

High-efficiency and low-loss gallium nitride dielectric metasurfaces for nanophotonics at visible wavelengths

The dielectric nanophotonics research community is currently exploring transparent material platforms (e.g., TiO2, Si3N4, and GaP) to realize compact high efficiency optical devices at visible wavelengths. Efficient visible-light operation is key to integrating atomic quantum systems for future quantum computing. Gallium nitride (GaN), a III-V semiconductor which is highly transparent at visible wavelengths, is a promising material choice for active, nonlinear, and quantum nanophotonic applications. Here, we present the design and experimental realization of high efficiency beam deflecting and polarization beam splitting metasurfaces consisting of GaN nanostructures etched on the GaN epitaxial substrate itself. We demonstrate a polarization insensitive beam deflecting metasurface with 64% and 90% absolute and relative efficiencies. Further, a polarization beam splitter with an extinction ratio of 8.6/1 (6.2/1) and a transmission of 73% (67%) for p-polarization (s-polarization) is implemented to demonstrate the broad functionality that can be realized on this platform. The metasurfaces in our work exhibit a broadband response in the blue wavelength range of 430–470 nm. This nanophotonic platform of GaN shows the way to off- and on-chip nonlinear and quantum photonic devices working efficiently at blue emission wavelengths common to many atomic quantum emitters such as Ca+ and Sr+ ions