Jin-Young Na

Microstructured Air Cavities as High-Index Contrast Substrates with Strong Diffraction for Light-Emitting Diodes

Two-dimensional high-index-contrast dielectric gratings exhibit unconventional transmission and reflection due to their morphologies. For light-emitting devices, these characteristics help guided modes defeat total internal reflections, thereby enhancing the outcoupling efficiency into an ambient medium. However, the outcoupling ability is typically impeded by the limited index contrast given by pattern media. Here, we report strong-diffraction, high-index-contrast cavity engineered substrates (CESs) in which hexagonally arranged hemispherical air cavities are covered with a 80 nm thick crystallized alumina shell. Wavelength-resolved diffraction measurements and Fourier analysis on GaN-grown CESs reveal that the high-index-contrast air/alumina core/shell patterns lead to dramatic excitation of the low-order diffraction modes. Large-area (1075 × 750 μm(2)) blue-emitting InGaN/GaN light-emitting diodes (LEDs) fabricated on a 3 μm pitch CES exhibit ∼39% enhancement in the optical power compared to state-of-the-art, patterned-sapphire-substrate LEDs, while preserving all of the electrical metrics that are relevant to LED devices. Full-vectorial simulations quantitatively demonstrate the enhanced optical power of CES LEDs and show a progressive increase in the extraction efficiency as the air cavity volume is expanded. This trend in light extraction is observed for both lateral- and flip-chip-geometry LEDs. Measurements of far-field profiles indicate a substantial beaming effect for CES LEDs, despite their few-micron-pitch pattern. Near-to-far-field transformation simulations and polarization analysis demonstrate that the improved extraction efficiency of CES LEDs is ascribed to the increase in emissions via the top escape route and to the extraction of transverse-magnetic polarized light.

Design principles for morphologies of antireflection patterns for solar absorbing applications.

Two-dimensional surface texturing is a widespread technology for imparting broadband antireflection, yet its design rules are not completely understood. The dependence of the reflectance spectrum of a periodically patterned glass film on various structural parameters (e.g., pitch, height, shape, and fill factor) has been investigated by means of full-vectorial numerical simulations. An average weighted reflectivity accounting for the AM1.5G solar spectrum (λ=300-1000  nm) was sinusoidally modulated by a rod patterns height, and was minimized for pitches of 400-600 nm. When a rationally optimized cone pattern was used, the average weighted reflectivity was less than 0.5%, for incident angles of up to 40° off normal. The broadband antireflection of a cone pattern was reproduced well by a graded refractive index film model corresponding to its geometry, with the addition of a diffraction effect resulting from its periodicity. The broadband antireflection ability of optimized cone patterns is not limited to the glass material, but rather is generically applicable to other semiconductor materials, including Si and GaAs. The design rules developed herein represent a key step in the development of light-absorbing devices, such as solar cells.

Tailoring evanescent wave coupling in GaN/AlGaN core/cladding light-emitting diodes

We investigated GaN/AlGaN core/cladding light-emitting diodes (LEDs) that surpass the limit of light extraction that is typically exhibited by conventional GaN LED structures. Full-vectorial electromagnetic calculations confirmed that with an upper two-dimensional dielectric pattern, the extraction efficiency of the subwavelength-thick GaN/AlGaN waveguide increased by ~50% relative to that of the homogeneous GaN structure. Electric-field intensity profiles demonstrated that this significantly enhanced light extraction was a result of the thin waveguide effect in which the evanescent field of guided modes penetrates further into the upper dielectric pattern. The waveguide effect was investigated at various refractive indices and thicknesses of the cladding.

Reverse layered transfer technology as an innovative manufacturing process for high-efficiency OLED lightings (Conference Presentation)

OLED lightings are getting more attention from industry since characteristic of surface lighting gives design flexibility and energy saving. Transparent conductive electrodes (TCEs) and internal out-coupling layer are essential components to determine OLED performance. Typical out-coupling layer is combination of scattering structure and planarizing layer. ~m planarizing layer smooths rough topology causing from the scattering structure so that sufficiently flat TCE can be coated on it. However undesirable gap between the TCE and the scattering structure occurs inevitably by the insertion of ~m planarizing layer. This leads to diminish out-coupling effect because the scattering structure is located optically too far from TCE to reduce wave guiding loss around TCE. Thus, it is important to bring the scattering structure close to TCE so as to reduce the wave guiding loss around TCE. Here, we present an innovative manufacturing process named “Reverse Layered Transfer process” that makes TCE directly contact with scattering structure, resulting in high out-coupling efficacy. The flexible OLED applied with Reverse Layered Transfer showed 1.8 times higher luminous efficacy than a rigid OLED based on normal process using ref ITO TCE. TCE integrated with out-coupling layer using “Reverse Layered Transfer process” and its device property will be described, together with experimental results and theoretical explanation.

Broadband Omnidirectional Diffuse Mirrors with Hierarchically Designed All-Dielectric Surfaces

An electromagnetic wave with a single wave vector can be converted into multiple partial ones, with discrete or continuum wave vectors, by means of diffraction or scattering elements; this phenomenon is called optical diffusion. Optical diffusion is a crucial light–matter interaction problem, particularly for lighting applications that require uniform illumination. However, omnidirectional diffuse mirrors with minimal absorption loss have not been reported thus far. Here, we demonstrate the high-diffusivity, low-absorption reflecting surfaces, on which hexagonally arranged Al2O3 cones, with a pitch of 3 μm, are conformally covered with HfO2/Al2O3 multilayers. Spectrally resolved far-field measurements reveal that the hierarchically patterned surface diffuses reflected light uniformly over the entire range of azimuthal and polar angles at broadband wavelengths (λ = 400–800 nm), distinct to two-dimensional Al2O3 or Al patterned surfaces. Such omnidirectional optical diffusion is clearly identified by means ...

Design of high-efficiency nanorod emitters Using Optical Cavity Effects

We studied the extraction efficiency of nanorod emitters by performing full-vectorial electromagnetic simulations. The result indicated that the extraction efficiency was dramatically changed by the radial position of a light generation layer.

Reflection properties of nano textured distributed Bragg mirrors

We designed nano-textured distributed Bragg reflectors and investigated their reflectance properties by conducting finite-difference-time-domain, mostly focusing on the effect of surface roughness.

Design principles of ultra high transmittance dielectric/metal/dielectric electrodes

We designed a high-transmittance dielectric/Ag/ITO electrode for high-efficiency GaN-based light-emitting diodes by using the scattering matrix method. The optimized multilayer dielectric/Ag/ITO electrode yielded a transmittance of > 0.90 with an approximately 10-nm-thick Ag layer.