Improving optoelectronic devices performance with metasurfaces
Mohamed Abdelkhalik defended his PhD thesis at the Department of Applied Physics and Science 果冻传媒 on June 12th.
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Optoelectronic devices act as optical-to-electronic or electronic-to-optical transducers, and can convert between optical and electrical signals seamlessly. These devices have been developed over decades by integrating electrons with photons for efficient devices. This development has given rise to numerous applications such as light-emitting diodes (LEDs), lasers, photodetectors, optical communication systems, light sensors, and photovoltaic cells. For his PhD research, Mohamed Abdelkhalik focuses on addressing two applications that are everywhere around us: micro-LEDs (碌LEDs ) and organic photovoltaic cells.
The interaction of electrons (electronic particles) and photons (light particles) can be weak due to their different nature. Recent progress in nanophotonics research has demonstrated fundamental phenomena enabled by the resonant interaction of designed nanostructured surfaces, also known as metasurfaces, that can be integrated into optoelectronic devices to enhance this weak interaction.
In close collaboration with Lumileds (a world leader manufacturer of LEDs), PhD researcher worked on resonant metasurfaces and commercial LED architectures to directly translate the effects of resonant nanophotonics into technology improvements. In particular, he focused on two applications that are commonplace in society: micro-LEDs and organic photovoltaic cells.
Micro-LEDs
Micro-LEDs present a unique advantage compared to conventional LEDs due to their small dimensions within the micro-scale range, leading to ultra-high resolution displays. However, micro-LEDs suffer from a very low Light Extraction Efficiency (LEE), which is limiting the device's efficiency. Moreover, they emit in all directions, which usually requires a reshaping of this emission using secondary optics (small lenses or mirrors that need to be incorporated to the LEDs).
The directional light emission from these devices plays an important role in various applications, such as augmented reality (AR) and virtual reality (VR).
In his work, Abdelkhalik demonstrates that metasurfaces integrated with emitters can be a potential alternative to conventional optical elements for a new class of compact, integrated, and high-performance micro-LEDs. Due to recent advances in nanofabrication techniques, it has become feasible to fabricate metasurfaces with dimensions comparable to those of the wavelength of light and over extended areas.
Reshaping
The experiments of Abdelkhalik reveal significant enhancements in LEE and a reshaping of the far-field emission. The electroluminescence (EL) enhancement factor ranged from 2.6 to 21.4 compared to reference devices without metasurfaces.
Furthermore, it has been achieved a beaming light effect with an enhancement factor of 5.8 within the emission cone of 卤30掳, facilitated by the presence of the metasurfaces. The far-field emission can be tailored for specific applications, ranging all the way from omnidirectional emission (emission in all directions) to beamed emission in specific directions.
This capability has a substantial impact on enabling high-brightness displays. Importantly, these design principles are applicable across a range of device dimensions, from micro-LEDs to mini-LEDs.
Metasurfaces are proposed as a solution to enable both directional control of the emission and radiation enhancement in compact devices. To gain a deeper insight into the samples and device performance involving metasurfaces, Abdelkhalik conducted numerical simulations to fine-tune the device structure and assess the enhancement of emission, and in particular its impact on far-field emission.
Photovoltaic cells
Other relevant optoelectronic devices are photovoltaic cells. In his thesis, Abdelkhalik focused on thin-film solar cell technology, including organic molecules for organic solar cells (OSCs).
However, OSCs exhibit limitations, notably a relatively short diffusion length of charges and degradation over time. Therefore, he proposed an approach to enhance charge diffusion in OSCs cells using strong light-matter coupling.
Strong light-matter coupling refers to the interaction in a cavity between photons and charges in the form of excitons, which causes their hybridization and formation of new quasi-particles, called polaritons. Abdelkhalik explores the impact of introducing resonant metasurfaces in OSCs.
He demonstrated using experiments the effects of strong light-matter coupling on different types of OSCs. While in his work, Abdelkhalik showed promising results pointing towards an improvement of the organic material due to strong light-matter coupling, the overall efficiency of the OSCs is not yet improved due to the reduction of light that is absorbed. This work, which has led to 3 patents and 5 publications, has been done in close collaboration with Lumileds, a world-leading multinational working in LEDs.
Title of PhD thesis: . Supervisors: Jaime G贸mez Rivas and Femius Koenderink.