Yu-Feng Yin

GaN-based LEDs surrounded with a two-dimensional nanohole photonic crystal structure for effective laterally guided mode coupling

Traditional implementation of photonic crystals (PhCs) on LED light emission surfaces results in weak coupling of light with the PhCs. Here we introduce a GaN-based LED surrounded with a nanohole PhC structure along the mesa edges. The laterally guided modes in the epi-structure can be effectively coupled with the two-dimensional periodic structure. The proposed structure results in the improvement of LED light extraction and provides flexibility of radiation directionality control.

Interactions of Diffraction Modes Contributed From Surface Photonic Crystals and Nanoholes in a GaN-Based Light-Emitting Diode

Photonic crystals (PhCs) were typically fabricated on the mesa surface of an LED to improve light extraction, which is regarded as the weak coupling between the laterally propagated light in the epilayers and the surface nanostructure. Here, we report GaN-based LEDs with the PhC structure on the mesa surface and nanohole reflectors surrounding the light-emitting mesa. The output power of the new LED structure is higher than that of the device with only surface PhCs due to the enhanced diffraction of low-order modes propagated in the lateral direction, in addition to the higher order mode light diffraction from the surface PhCs. From the relative angular spectra, the interaction of in-plane optical wave with the nanoholes (which are etched through multiple quantum wells) is much stronger than that with surface PhCs, suggesting an efficient light diffraction to the surface normal by nanoholes.

On the Radiation Profiles and Light Extraction of Vertical LEDs With Hybrid Nanopattern and Truncated Microdome Surface Textures

The n-side-up vertical light emitting diodes (LEDs) have the advantage of carving the surface of the thick n-GaN layer to improve light extraction and to adjust radiation profiles. In this paper, a two-step surface patterning is employed with the focus on understanding angular light diffractions from both nanopatterns and truncated microdomes. Light output enhancement of the LEDs with hybrid surface texturing is investigated experimentally and theoretically. The results suggest that light is diffracted through the grating effect and curved sidewalls when interacting with truncated microdomes, resulting in a maximum enhancement 64° away from the normal surface. On the other hand, nearly omni-directional enhancement was found from the randomly scattered nanopatterns. As for the hybrid structure, since guided modes in the semiconductor layers are diffracted by either nanopatterns or microdomes, the percentage increase of light extraction from the hybrid structure is approximately the linear superposition of both types of surface textures. The results suggest an interesting guideline to improve LED light output and to adjust angular radiation with the mutlistep surface patterning.

Far-field self-focusing and -defocusing radiation behaviors of the electroluminescent light sources due to negative refraction

In recent years, researchers have demonstrated negative refraction theoretically and experimentally by pumping optical power into photonic crystal (PhC) or waveguide structures. The concept of negative refraction can be used to create a perfect lens that focuses an object smaller than the wavelength. By inserting two-dimensional PhCs into the peripheral of a semiconductor light emitting structure, this study presents an electroluminescent device with negative refraction in the visible wavelength range. This approach produces polarization dependent collimation behavior in far-field radiation patterns. The modal dispersion of negative refraction results in strong group velocity modulation, and self-focusing and -defocusing behaviors are apparent from light extraction. This study further verifies experimental results by using theoretic calculations based on equifrequency contours.

Correlation of angular light profiles of light-emitting diodes to spatial spontaneous emissions from photonic crystals

Typically, photonic crystal light-emitting diodes employ shallow nanostructures which only higher-order optical modes can be interacted with. Here, both the shallow photonic crystals and nanohole arrays (etched through active layers) are fabricated, which serve to diffract, respectively, higher and lower optical modes in the active layer. Our results indicate that the photon lifetime can be controlled by adjusting the geometry of shallow nanostructures and nanohole arrays. The angular emission profiles are thus determined by the dominance of higher- and lower-order mode quality factors in the band structure.

Polarization ratio enhancement of a-plane GaN light emitting diodes by asymmetric two-dimensional photonic crystals

Fabricating photonic crystals (PhCs) on GaN based non-polar light emitting diodes (LEDs) is an effective way to increase light extraction and meanwhile to preserve or improve polarization ratio. In this work, a-plane GaN LEDs with two-dimensional PhCs were demonstrated. With the E // m polarized modes (which mean the optical polarization with the electric field parallel to m-axis) as the target of diffraction, we matched E//m modes to the photonic bands and aligned E//c modes to fall within the photonic band gap. The results show stronger E//m but weaker E//c mode diffractions on both c- and m-axes. At the vertical direction, the polarization ratio is enhanced from 45.8% for the planar device to 52.3% for the LEDs with PhCs.

High-speed modulation from the fast mode extraction of a photonic crystal light-emitting diode

Using light-emitting diodes (LEDs) for visible light communication has become an alternative choice of radio source due to channel crowding of the radio-frequency (RF) signal. The modulation bandwidth of LEDs is usually limited by the spontaneous carrier lifetime in multiple quantum wells. Here, sub-GHz modulation of GaN-based LED employing photonic crystal (PhC) nanostructure is demonstrated. The guided photonic modes of the LEDs are modulated by the RF signal. Both carrier lifetime of lower- and higher-order modes are studied in time-resolved photoluminescence (TRPL) at room temperature. The f-3dB-J curve of the PhC LED exhibits a higher bandwidth than the typical LED structure. At 11.41 kA/cm2, the optical −3-dB bandwidth (f-3dB) up to 234 MHz of the PhC LED (PhCLED) is achieved. Our studies on TRPL at different wavelengths and frequency response at different injection current densities conclude that the higher operation speed is attributed to faster radiative carrier recombination of extracted guided ...

Mechanisms of the Asymmetric Light Output Enhancements in \(a\) -Plane GaN Light-Emitting Diodes With Photonic Crystals

The unique properties of nonpolar GaN light-emitting diodes (LEDs) have the advantages of generating polarized light emission. The employment of asymmetric 2-D photonic crystals (PhCs) can further enhance the light polarization ratio. In addition, it was generally recognized that the Purcell effect can increase the internal quantum efficiency of the LEDs with PhCs. In this paper, we study the properties of optical modes from different crystal planes. The Purcell effect is analyzed based on the PhCs and material crystal orientations. With different transition probability of the polarized photons in valence bands, the corresponding Purcell effect enhancement on the quantum efficiency varies.

High optical bandwidth GaN based photonic-crystal light-emitting diodes

Light emitting diodes (LEDs) for visible light communication (VLC) as radio sources is a solution to channel crowding of radio frequency (RF) signal. However, for the application on high-speed communication, getting higher bandwidth of LEDs is always the problem which is limited by the spontaneous carrier lifetime in the multiple quantum wells. In this paper, we proposed GaN-based LEDs accompanied with photonic crystal (PhC) nanostructure for high speed communication. Using the characteristic of photonic band selection in photonic crystal structure, the guided modes are modulated by RF signal. The PhC can also provide faster mode extraction. From time resolved photoluminescence (TRPL) at room temperature, carrier lifetime of both lower- and higher-order modes is shortened. By observing f-3dB -J curve, it reveals that the bandwidth of PhC LEDs is higher than that of typical LED. The optical - 3-dB bandwidth (f-3dB) can be achieved up to 240 MHz in the PhC LED (PhCLED). We conclude that the higher operation speed can be obtained due to faster radiative carrier recombination of extracted guided modes from the PhC nanostructure.

Spatially adjusted spontaneous emissions from photonic crystals embedded light-emitting diodes

In this work, the angular light output enhancements of LEDs were investigated from the spontaneous emission and light scattering of devices with different photonic crystal (PhC) geometries. The emitted photon coupled into a leaky mode is differentiated by the manipulation of the quality factor in various spatial frequencies. Therefore, light extraction in this light-emitting device is determined by the modal extraction lengths and the quality factor obtained from the measured photonic bands. Furthermore, the higher- and lower-order mode spontaneous emissions are affected by the nonradiative process in the PhC structures with different periods. In our cases, the photonic crystal device with the largest period of 500 nm exhibits the highest lower-order mode extraction and quality factor. As a result, a self-collimation behavior toward the surface-normal is demonstrated in the 3D far-field pattern of such a device. We conclude that, with the coherent light scattering from the PhC region, the spontaneous emission of the material and spatial behavior of the extracted mode can be both managed by the proper design of the device.