Light-emitting diodes (LEDs) are widely used, and improving their light extraction efficiency is a significant area of research. Conventional semiconductor LEDs suffer from two primary limitations: critical angle loss, where light incident at angles greater than the critical angle undergoes total internal reflection, and Fresnel loss, where a significant portion of light is reflected at the LED chip/encapsulant interface, even at angles below the critical angle. Previous attempts to address these issues include using higher refractive index encapsulants, creating hemispherical LED chips, and using coatings with spatially-gradient refractive indices or microlens arrays. However, these methods often present challenges in manufacturing and cost-effectiveness. The use of nanoparticle-loaded epoxy has also been explored but faces difficulties in controlling nanoparticle density and arrangement without agglomeration or compromising transparency. This paper proposes a new approach: using a monolayer of sub-wavelength metallic nanoparticles arranged as a 'meta-grid' on the LED chip within its encapsulating packaging to reduce Fresnel reflection loss by destructive interference.

Extensive research has focused on enhancing light extraction efficiency in LEDs. Techniques explored include using high-refractive-index encapsulants like chalcogenide glasses, although these are difficult to mass-produce. Hemispherical LED chips reduce total internal reflection but are bulky and complex to manufacture. Other approaches involve spatially-gradient refractive index materials and microlens arrays to improve light transmission. Nanoparticle-based approaches aim to increase the effective refractive index of the encapsulant, but challenges remain in controlling nanoparticle distribution and avoiding agglomeration, maintaining transparency, and adapting to various LED emission wavelengths. The proposed method addresses these challenges by utilizing a meta-grid of plasmonic nanoparticles to reduce Fresnel reflections, offering a potentially simpler and more scalable solution.

The study uses a theoretical model to analyze the optical transmission through a four-layer stack: the LED chip, the encapsulant, the nanoparticle meta-grid, and air. The meta-grid is represented by an effective film using effective medium theory. The analysis focuses on maximizing transmittance within the photon escape cone by controlling the nanoparticle meta-grid parameters (nanoparticle radius (R), interparticle gap (g), and height (h) from the chip surface). Silver nanospheres were chosen due to their strong plasmonic resonance and minimal absorption losses. The effects of varying R, g, and h on the transmission spectra were investigated using both analytical calculations and full-wave simulations. The analytical and simulation results show good agreement, validating the theoretical model. For a specific case of a red LED (peak emission wavelength of 625 nm), the optimal parameters for the NP meta-grid were determined by exploring a wide range of h, R, and g values. The optimal transmittance, R, and g values at each h were then plotted to identify optimal configurations. The Fabry-Perot effect between the chip/encapsulant interface and the NP meta-grid is discussed as the mechanism for enhanced transmission. The analysis also considered the effects of incident angle and polarization on the transmittance.

The study demonstrated that a meta-grid of silver nanospheres significantly enhances light transmission across the LED chip/encapsulant interface. Varying the nanoparticle radius (R), interparticle gap (g), and height (h) influences the transmission spectrum, primarily by shifting and broadening the extinction peak associated with localized plasmon resonances. Reducing the inter-particle gap (g) redshifts and broadens the transmission dip (extinction peak), while increasing height (h) leads to some spectral broadening. Increasing the nanoparticle radius (R) blueshifts and narrows the transmission dip. For a red LED with a peak emission wavelength of 625nm, optimization of the meta-grid parameters (h, R, g) resulted in transmittance values greater than or equal to 98.5%. The optimized parameters were found to be periodic with respect to height (h), indicating the Fabry-Perot effect plays a crucial role in transmission enhancement. The results of full-wave simulations closely matched analytical predictions, validating the theoretical model. The analysis further investigated the transmittance at various incident angles (below the critical angle) for different polarizations (s, p, unpolarized).

The findings demonstrate the potential of using a nanoparticle meta-grid to significantly improve the light extraction efficiency of LEDs. By controlling the meta-grid parameters, Fresnel loss can be substantially minimized, leading to increased light output and energy efficiency. The Fabry-Perot effect is identified as the underlying mechanism, demonstrating the importance of precise control over the distance between the meta-grid and the LED chip surface. The close agreement between theoretical predictions and simulation results validates the effectiveness of the proposed design. This method offers a relatively simple and potentially cost-effective way to enhance LED performance, offering a promising alternative to more complex and challenging existing approaches.

This research successfully demonstrated that a carefully designed meta-grid of plasmonic nanoparticles can significantly improve light extraction from LEDs by reducing Fresnel losses. The optimal design parameters (nanoparticle size, spacing, and height) were identified, and the Fabry-Perot effect was confirmed as the underlying mechanism for the observed enhancement. This approach offers a promising path toward improved LED efficiency and longevity, with the potential for seamless integration into existing manufacturing processes. Future work could explore other materials for the meta-grid, investigate different lattice structures, and optimize the design for LEDs with diverse emission spectra and applications.

The study primarily focused on a specific type of red LED and silver nanoparticles. The optimal design parameters might vary for different LED types and materials. The theoretical model assumes a simplified four-layer structure and may not fully account for complexities in real-world LED structures. Further experimental validation is needed to confirm the theoretical predictions and assess the feasibility of large-scale fabrication.