Vertical Light Extraction Research Articles

Sub-Micron Monolithic Full-Color Nanorod LEDs on A Single Substrate

The extended reality (XR) display, including augmented reality (AR) and virtual reality (VR), requires ultrahigh-resolution and high-luminance pixel technology. GaN-based micro-LEDs are the most promising for high-density pixel applications. However, it is necessary to simultaneously fabricate full-color R/G/B emitters on a single substrate. Moreover, the efficiency droop at long wavelengths with high-indium InGaN still remains a challenge. The InGaN nanorod has the potential to realize monolithic R/G/B LEDs on a single substrate, and the Al reflector has a light reflectance that can dramatically improve light extraction efficiency. Here, we designed sub-micron monolithic full-color R/G/B nanorod LEDs with an Al core-shell reflector on single chip. The optimal structures for light confinement and vertical light extraction by single InGaN nanorod R/G/B LED structures were identified. The single nanorod with Al core-shell structure dramatically increased the light extraction efficiency, and showed an improvement of more than 20%, especially in the red wavelength. As a result, by introducing a long-length nanorod structure, we have successfully designed a new submicron pixel with full-color R/G/B LEDs operating within 1 μm × 1 μm size. This unique design will be a key factor in enabling the next generation of AR and VR displays that require ultra-small and compact pixels.

3‐4: Highly Efficient Green Top‐Emission Light‐Emitting Diodes Based on Indium‐Phosphide Quantum Dots

One of the major challenges for novel display technologies based on quantum dot light‐emitting (QLED) was replacing toxic emission layers. Indium phosphide (InP) quantum dots were considered as the most promising alternative materials. In this work, the optimal vertical light extraction could be obtained by simulating the different thickness of electron transport layer (ETL) in devices. The Mg‐doped ZnO (ZnMgO) electron transport layer was adopted to further balance the carriers injection and transport. Finally, top‐emission devices with optimized light extraction and balanced carriers shown higher current efficiency and narrower full width at half maximum (FWHM) of electroluminescent peaks (EL).

Single- and Twin-Photons Emitted from Fiber-Coupled Quantum Dots in a Distributed Bragg Reflector Cavity.

In this work, we develop single-mode fiber devices of an InAs/GaAs quantum dot (QD) by bonding a fiber array with large smooth facet, small core, and small numerical aperture to QDs in a distributed Bragg reflector planar cavity with vertical light extraction that prove mode overlap and efficient output for plug-and-play stable use and extensive study. Modulated Si doping as electron reservoir builds electric field and level tunnel coupling to reduce fine-structure splitting (FSS) and populate dominant XX and higher excitons XX+ and XXX. Epoxy package thermal stress induces light hole (lh) with various behaviors related to the donor field: lh h1 confined with more anisotropy shows an additional XZ line (its space to the traditional X lines reflects the field intensity) and larger FSS; lh h2 delocalized to wetting layer shows a fast h2–h1 decay; lh h2 confined shows D3h symmetric higher excitons with slow h2–h1 decay and more confined h1 to raise h1–h1 Coulomb interaction.

Theoretically Enhance the Vertical Light Extraction Efficiency of the AlGaN-Based Nanowire Photonic Crystal Ultraviolet Light-Emitting Diodes by Selecting the Band Gap

Nanowire photonic crystal (PhC) structure is one of promising approaches to enhance vertical light extraction efficiency (LEE) of ultraviolet light emitting diodes (UV LEDs). There are lots of investigations to study the structure of nanowire UV LEDs by using the finite-difference time-domain (FDTD) method. The optimal LEE are obtained by using some optimization algorithms. However, it needs a broad window, not the best values of geometrical parameters of nanowires in practical fabrication. In this paper, the vertical LEE of UV LEDs are theoretically enhanced by selecting the band gaps of AlGaN-based nanowire PhC structure. It illustrates that all the gaps can inhibit the horizontal propagation of light. However, the inhibiting ability of Gap 1 are weaker than them of Gap 2 and Gap 3. The reason is that the radii of nanowires in Gap 1 are too small to support enough guided modes. As a comparison, both Gap 2 and Gap 3 can efficiently inhibit the radiated mode propagation, and finally enhance the vertical LEE. In addition, the variations of height, number, and refractive index of nanowires on vertical LEE and optical band gaps are discussed. This work has benefits for the practical production of nanowire PhC UV LEDs.

Improving the Vertical Light Extraction Efficiency of GaN-Based Thin-Film Flip-Chip LED With Double Embedded Photonic Crystals

The vertical light-extraction efficiency (LEE) of the thin-film flip-chip light-emitting diodes with the embedded photonic crystal (PhC) is investigated using the finite-difference time-domain method. This paper systematically analyzes the dependence of the vertical LEE on the vertical structure and the embedded PhC parameters. It is found that the introduction of the p-side embedded PhC and n-side embedded PhC does not significantly destroy the microcavity resonant effects. In particular, a twofolds enhancement in the vertical LEE is obtained for the optimized structure by scanning the double embedded PhCs parameters and the physical mechanisms for the enhancement of the vertical LEE are discussed.

Improvement of vertical light extraction from GaN‐based LEDs on moth‐eye patterned sapphire substrates

A newly developed moth‐eye patterned sapphire substrate (MPSS) technology increased light extraction efficiency (LEE) in the vertical direction in nitride‐based blue light emitting diodes (LEDs). MPSS is characterized by a submicron‐scale periodical structure that exerts the Bragg diffraction effect at the interface between gallium nitride (GaN) and sapphire. Through simulations and experiments, it was revealed that emphasis of the diffraction effect by optimizing the MPSS structure enhanced vertical emission from LEDs. As a result, resin‐encapsulated, large‐sized LEDs on MPSS with a large diffraction effect achieved a high LEE of 0.76 that was comparable to that of commercially available thin‐film LEDs.Bird's‐eye view of scanning electron microscope (SEM) image of MPSS.

Fabrication of Novel Two-Dimensional Nanopatterned Conductive PEDOT:PSS Films for Organic Optoelectronic Applications

This paper presents a novel strategy to fabricate two-dimensional poly(3,4 ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) photonic crystals (PCs) combining electron beam lithography (EBL) and plasma etching (PE) processes. The surface morphology of PEDOT:PSS PCs after mild oxygen plasma treatment was investigated by scanning electron microscopy. The effects on light extraction are studied experimentally. Vertical extraction of light was found to be strongly dependent on the geometric parameters of the PCs. By changing the lattice type from triangular to square and the geometrical parameters of the photonic structures, the resonance peak could be tuned from a narrow blue emission at 445 nm up to a green emission at 525 nm with a full width at half-maximum of 20 nm, which is in good agreement with Bragg's diffraction theory and free photon band structure. Both finite-difference time-domain and plane wave expansion methods are used to calculate the resonant frequencies and the photonic band structures in the two-dimensional photonic crystals showing a very good agreement with the experiment results. A 2D nanopatterned transparent anode was also fabricated onto a flexible polyethylene terephthalate (PET) substrate and it was integrated into an organic light-emitting diode (OLED). The obtained results fully confirm the feasibility of the developed process of micro/nano patterning PEDOT:PSS. Engineered polymer electrodes prepared by this unique method are useful in a wide variety of high-performance flexible organic optoelectronics.

Bragg extraction of light in 2D photonic Thue–Morse quasicrystals patterned in active CdSe/CdS nanorod–polymer nanocomposites

In this paper two-dimensional (2D) photonic Thue-Morse quasicrystals (ThMo-PQCs) in active CdSe/CdS nanorod (NR) doped polymer nanocomposites are proposed and experimentally demonstrated. Active PQCs and undoped lattices have been prepared in a one-step fabrication process by an electron beam lithography technique (EBL) and the effects on light extraction and emission directionality are studied experimentally. Vertical extraction of light was found to be strongly dependent on both the geometric parameters of the ThMo-PQCs and the presence of NR dopants. By changing the geometrical parameters of the photonic structures, the resonance peak could be tuned from a narrow bluish green emission at 543 nm up to a red-NIR emission at 711 nm with a full width at half-maximum of 22 nm which is in good agreement with Bragg's diffraction theory and free photon band structure. Angular resolved measurements revealed a directional profile in the far-field distribution with guided mode extraction in both doped and undoped PQCs and an enhancement as high as 6.5-fold in light extraction was achieved in the doped photonic structures. These experimental results indicate the critical role of the CdSe/CdS NRs in improving the light extraction efficiency of 2D ThMo-PQCs for solid-state lighting and lasing.

Structural optimization of GaN blue light LED with double layers of photonic crystals

The GaN-based blue light LED with double layer photonic crystals is designed to enhance the vertical light extraction efficiency. The effective index approximation method is proposed and used to solve the distribution of the modes in the LED model. The geometrical parameters including the depth of the top photonic crystal d, the thickness of the embedded photonic crystal T, and the distance between the active layer and the embedded photonic crystal D are optimized. Compared with other numerical optimization, the mode analysis used in this work can dramatically save computation time and reduce complexity. In addition, it can provide more theoretical details about the influence of these geometrical parameters on light extraction efficiency. It can be found that when the surface photonic crystal satisfies the condition of d / nPhCs, high order modes localized in the structure are still not cut off but the low order modes obtain less power since they are pushed away from the active layer. Hence, the light extraction efficiency reaches its maximum. The cap layer modes can be excited by the embedded photonic crystals, when this layer satisfies 100 nm T 300 nm and 100 nm D 200 nm, the cap layer modes gather much power from the active layer and interact with the surface photonic crystals more efficiently. Hence the light extraction efficiency is dramatically improved. With our optimized parameters, the light extraction efficiency can be achieved to be up 4 times that of ordinary LED. These results shows a significant promise of designing a high-efficiency GaN-based blue light LED.

Complex-coupled photonic crystal THz lasers with independent loss and refractive index modulation

Compared to near infra-red photonic crystal (PhC) band-edge lasers, achieving vertical emission with quantum cascade (QC) material operating in the THz range needs dedicated engineering because the TM polarized emission of QCLs favors in-plane emitting schemes and the currently used double plasmon waveguide, prevents vertical light extraction. We present an approach with independent refractive index and extraction losses modulation. The extraction losses are obtained with small extracting holes located at appropriate positions. The modal operation of the PhC is shown to critically depend on the external losses introduced. Very high surface emission power for optimum loss extractor design is achieved.

Novel hybrid organic/inorganic 2D quasiperiodic PC: from diffraction pattern to vertical light extraction

Recently, important efforts have been dedicated to the realization of a fascinating class of new photonic materials or metamaterials, known as photonic quasicrystals (PQCs), in which the lack of the translational symmetry is compensated by rotational symmetries not achievable by the conventional periodic crystals. As ever, more advanced functionality is demanded and one strategy is the introduction of non-linear and/or active functionality in photonic materials. In this view, core/shell nanorods (NRs) are a promising active material for light-emitting applications. In this article a two-dimensional (2D) hybrid a 2D octagonal PQC which consists of air rods in an organic/inorganic nanocomposite is proposed and experimentally demonstrated. The nanocomposite was prepared by incorporating CdSe/CdS core/shell NRs into a polymer matrix. The PQC was realized by electron beam lithography (EBL) technique. Scanning electron microscopy, far field diffraction and spectra measurements are used to characterize the experimental structure. The vertical extraction of the light, by the coupling of the modes guided by the PQC slab to the free radiation via Bragg scattering, consists of a narrow red emissions band at 690 nm with a full width at half-maximum (FWHM) of 21.5 nm. The original characteristics of hybrid materials based on polymers and colloidal NRs, able to combine the unique optical properties of the inorganic moiety with the processability of the host matrix, are extremely appealing in view of their technological impact on the development of new high performing optical devices such as organic light-emitting diodes, ultra-low threshold lasers, and non-linear devices.PACS: 81.07.Pr Organic-inorganic hybrid nanostructures, 81.16.-c Methods of nanofabrication and processing, 42.70.Qs Photonic band-gap materials.

Design of superperiodic photonic-crystal light-emitting plates with highly directive luminance characteristics

A “superperiodic” photonic-crystal (PhC) light-emitting diode (LED) design exhibiting high-luminance patterns is proposed for applications that require highly directive vertical light extraction. The superperiodic design employs a periodic arrangement of defective missing holes on an otherwise perfect periodic pattern of holes for the basic two-dimensional PhC structure. Avoiding the fragile semiconductor membrane structure that is popular in current PhC laser designs, the design utilizes a thick epitaxy structure with electromagnetic density-of-mode singularity. This singularity is created by the defect mode in a three-dimensional frequency versus wave vector relation within the band matched with the gain spectrum of the LED that is placed on top of the base reflector plate material. The superperiodic structure of this design is shown to provide an effective means for sharpening the vertical beam in the far field, based on discrete-Fourier transformation theory. Subsequent simulations based on the finite-difference time-domain analysis verify the effectiveness of the design principle on GaN epitaxy structures with various bottom layers.