Chia-Hsin Chao

Directional light extraction enhancement from GaN-based film-transferred photonic crystal light-emitting diodes

Experimental investigation of the directionality in the far-field pattern and light extraction enhancement in collected cone were performed in GaN-based film-transferred photonic crystal (PhC) light-emitting diodes (FTLEDs). Angular-resolved measurement revealed directional profile and azimuthal anisotropy in the far-field distribution with guided modes extraction. Good agreement according to Bragg’s diffraction theory and free photon band structure were achieved. The light enhancement in PhC FTLEDs compared to non-PhC FTLEDs within the collection cone angle was obtained according to measured three-dimensional far-field patterns. In a ±20° collection cone, collected light was enhanced by a factor of ∼2.4 for the collimated PhC FTLED.

Polarized light emission from photonic crystal light-emitting diodes

We have experimentally studied polarization characteristics of the two-dimensional photonic crystal (PhC) light-emitting diodes (LEDs) using an annular structure with square lattice and observed a strong polarization dependence of the lattice constant and orientation of the PhC. The extracted light from the GaN PhC LEDs has P∕S ratios of 5.5 (∼85% polarization light) for light propagating in the ΓX direction and 2.1 (∼68% polarization light) for the ΓM direction, respectively. Based on the couple mode theory, the dependence of polarization behaviors on different lattice constant and orientation was found to be in good agreement with theoretical discussion.

Anisotropy of light extraction from GaN two-dimensional photonic crystals

Angular distribution of light diffracted out of the plane of two-dimensional photonic crystals (PhC) has been studied in the azimuthal direction with a specially designed waveguide structure. The optical images of the light extraction patterns from the guided photoluminescence light are obtained with laser excitation in the center of the annual structure made on GaN multilayer. For increasing lattice constant, symmetric patterns with varying number of petals according to the symmetry of the PhC are observed. The appearance and disappearance of the petals can be explained using the Ewald construction in the reciprocal space. The dependence of light extraction on the lattice constant and orientation, and incident angle are investigated.

Highly-directional emission patterns based on near single guided mode extraction from GaN-based ultrathin microcavity light-emitting diodes with photonic crystals

This study investigates the distribution of highly-directional far-field emission on GaN-based ultrathin microcavity light-emitting diodes (uMCLEDs) with photonic crystals (PhCs). The ultrathin 550 nm cavity, PhC lattice constant of 370 nm, and hole depth of 250 nm in the GaN PhC uMCLED provide near single guided mode extraction and a pattern of high directionality radiation. Angular-spectral-resolved electroluminescence measurements reveal photon-band structure agreement with the fundamental mode effective refractive index dispersion curve. In addition, GaN PhC uMCLED increase the output power extraction efficiency by 145.9% (∼2.46×) compared with GaN non-PhC uMCLED, and a directional far-field emission pattern at half intensity of nearly ±15°.

Anisotropy of light extraction from two-dimensional photonic crystal light-emitting diodes

Anisotropic light extraction of photonic crystal (PhC) light-emitting diodes in the azimuthal direction has been investigated with an annular structure of triangular PhC lattice. The optical images of the photoluminescence light extraction are obtained with laser excitation. For increasing lattice constant, sixfold symmetric patterns with varying numbers of petals in multiples of six are observed and analyzed. A map of the anisotropy for various lattice constants and numerical apertures is constructed. Several features of light propagations associated with the PhC are observed including the focusing and collimating behaviors.

Structural Effects on Highly Directional Far-Field Emission Patterns of GaN-Based Micro-Cavity Light-Emitting Diodes With Photonic Crystals

This study theoretically and experimentally investigates the highly directional far-field emission patterns of GaN photonic crystal (PhC) micro-cavity light-emitting diodes (MCLEDs) depending on varying structural parameters. Angular-spectra-resolved electroluminescence measurements reveals the behavior of guided-mode extraction which is significantly affected by the structural parameters of GaN PhC MCLEDs, where the GaN cavity thickness decides the extracted guided mode numbers, PhC lattice constant influences the distribution of far-field emission, and PhC hole depth affects the interaction with guided modes. The proposed GaN ultrathin MCLED (uMCLED) with PhC lattice constant of 420 nm and deep hole depth of 250 nm exhibited a maximum output light extraction efficiency of 248% under one-watt input power compared to GaN non-PhC uMCLED and produced a directional far-field emission pattern at half intensity near 17^ . The present results indicate that highly directional light extraction enhancement could contribute to developments of many applications, especially for etendue-limited applications such as pico- projectors.

Far-field of GaN film-transferred green light-emitting diodes with two-dimensional photonic crystals.

Far-field distributions of GaN-based photonic crystal (PhC) film-transferred light-emitting diodes (FT-LEDs) were investigated. The thickness of the device is about 840 nm. The emission wavelength is around 520 nm. The PhC region is a square lattice with a/lambda around 0.5. Angular-resolved measurements in the Gamma-X and Gamma-M directions were made in the polarized-resolved manner. Guided mode extraction behavior in agreement with the two-dimensional free-photon band calculation was observed. In addition, the pseudo-TM behavior for the non-collinearly coupled modes was observed. The azimuthal-mapping of the angular-resolved spectra revealed the evolution of the collinearly and the non-collinearly coupled modes. Furthermore, the light enhancement of approximately 2.7x and the collimation angle about 102.3 degrees were achieved.

Enhanced Vertical Extraction Efficiency From a Thin-Film InGaN–GaN Light-Emitting Diode Using a 2-D Photonic Crystal and an Omnidirectional Reflector

An InGaN-GaN thin-film vertical-type light-emitting diode with a two-dimensional photonic crystal (PC) on the emitting surface and a TiO-SiO omnidirectional reflector on the bottom was fabricated. The device was investigated by performing a series of experiments and numerical computations. Electroluminescence measurement revealed a strong extraction enhancement in the vertical direction at 433-nm wavelength. The emission spectrum of the light was found to be strongly modified by the PC to have a significantly narrow linewidth of 5 nm. Our experimental results were in accord with those obtained from our numerical findings.

Far-Field and Near-Field Distribution of GaN-Based Photonic Crystal LEDs With Guided Mode Extraction

The near-field and far-field distribution of GaN LEDs with square photonic crystal (PhC) lattice are experimentally investigated. The optical images of the near-field pattern are obtained from the guided electroluminescent light generated at the center of a special annular structure. For increasing lattice constant, symmetric patterns with varying number of petals according to the symmetry of the PhC are observed. The far-field distribution is studied with GaN-based film-transferred PhC LEDs with the thickness about 1500 nm. Angular-resolved measurements under electrical injection revealed guided modes extraction behavior and polarization based on the Braggs diffraction with the PhC lattice. Good agreement with the two-dimension free photon band structure is obtained.

71‐1: Development of Micro‐Pixellated GaN LED Array Micro‐Display System

This paper developed a micro-pixellated GaN LED (μ-LED) array micro-display system. The μ-LED array has the features of high luminance, small optical machine size, high collimation, and lost cost. A time-separate driving method is proposed to drive the micro-display system for releasing the LED over-current and efficiency thermal degradation issues.