Abstract
While total internal reflection (TIR) lays the foundation for many important applications, foremost fibre optics that revolutionised information technologies, it is undesirable in some other applications such as light-emitting diodes (LEDs), which are a backbone for energy-efficient light sources. In the case of LEDs, TIR prevents photons from escaping the constituent high-index materials. Advances in material science have led to good efficiencies in generating photons from electron–hole pairs, making light extraction the bottleneck of the overall efficiency of LEDs. In recent years, the extraction efficiency has been improved, using nanostructures at the semiconductor/air interface that outcouple trapped photons to the outside continuum. However, the design of geometrical features for light extraction with sizes comparable to or smaller than the optical wavelength always requires sophisticated and time-consuming fabrication, which causes a gap between lab demonstration and industrial-level applications. Inspired by lightning bugs, we propose and realise a disordered metasurface for light extraction throughout the visible spectrum, achieved with single-step fabrication. By applying such a cost-effective light extraction layer, we improve the external quantum efficiency by a factor of 1.65 for commercialised GaN LEDs, demonstrating a substantial potential for global energy-saving and sustainability.
Highlights
Lighting accounts for 15% of global electricity consumption and 5% of worldwide greenhouse gas emissions[1]
We start optimising the light extraction layer based on a metasurface composed of periodic metallic stripes with a square cross-section (Meta-I), as depicted in the topleft panel of Fig. 1e
To demonstrate the light extraction effect of Meta-I, we first take a dielectric substrate with refractive index n = 2 as a reference, which exhibits a critical angle θc
Summary
Lighting accounts for 15% of global electricity consumption and 5% of worldwide greenhouse gas emissions[1]. Achieving energy-efficient light sources is of utmost importance for global development and sustainability. By virtue of its compact size, high efficiency and long lifetime, the light-emitting diode (LED) has developed into the prime candidate for replacing conventional light sources such as incandescent bulbs and fluorescent tubes. With substantial efforts in the past decades, prominent progress has been made in LED materials quality, of both III-Nitrides and organics, leading to an internal quantum efficiency close to unity[3,4]. A considerable fraction of generated photons is trapped inside the high-index materials of LEDs by total internal reflection (TIR), rendering outcoupling efficiency the bottleneck of overall efficiency and energy saving
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