Photonic crystal light-emitting diodes fabricated by microsphere lithography

All-dielectric concentration of electromagnetic fields at the nanoscale: the role of photonic nanojets

The photonic nanojet (PNJ) is a narrow high-energy beam that was originally found on the back side of all-dielectric spherical structures. It is a unique type of energy concentration mode. The field of PNJs has experienced rapid growth in the past decade: nonspherical and even pixelized PNJ generators based on new physics and principles along with extended photonic applications from linear optics to nonlinear optics have driven the re-evaluation of the role of PNJs in optics and photonics. In this article, we give a comprehensive review for the emerging sub-topics in the past decade with a focus on two specific areas: (1) PNJ generators based on natural materials, artificial materials and nanostructures, and even programmable systems instead of conventional dielectric geometries such as microspheres, cubes, and trihedral prisms, and (2) the emerging novel applications in both linear and nonlinear optics that are built upon the specific features of PNJs. The extraordinary features of PNJs including subwavelength concentration of electromagnetic energy, high intensity focusing spot, and lower Joule heating as compared to plasmonic resonance systems, have made PNJs attractive to diverse fields spanning from optical imaging, nanofabrication, and integrated photonics to biosensing, optical tweezers, and disease diagnosis.

Large-Area Sub-Wavelength Optical Patterning via Long-Range Ordered Polymer Lens Array

Fabrication of large-area, highly orderly, and high-resolution nanostructures in a cost-effective fashion prompts advances in nanotechnology. Herein, for the first time, we demonstrate a unique strategy to prepare a long-range highly regular polymer lens from photoresist nanotrenches based templates, which are obtained from underexposure. The relationship between exposure dose and the cross-sectional morphology of produced photoresist nanostructures is revealed for the first time. The polymer lens arrays are repeatedly used for rapid generation of sub-100 nm nanopatterns across centimeter-scale areas. The light focusing properties of the nanoscale polymer lens are investigated by both simulation and experiment. It is found that the geometry, size of the lens, and the exposure dose can be deployed to adjust the produced feature size, spacing, and shapes. Because the polymer lenses are derived from top-down photolithography, the nearly perfect long-range periodicity of produced nanopatterns is ensured, and the feature shapes can be flexibly designed. Because this nanolithographic strategy enables subwavelength periodical nanopatterns with controllable feature size, geometry, and composition in a cost-effective manner, it can be optimized as a viable and potent nanofabrication tool for various technological applications.

Step-Controllable Electric-Field-Assisted Nanoimprint Lithography for Uneven Large-Area Substrates

Large-area nanostructures are widely used in various fields, but fabrication on large-area uneven substrates poses a significant challenge. This study demonstrates a step-controllable electric-field-assisted nanoimprint lithography (e-NIL) method that can achieve conformal contact with uneven substrates for high fidelity nanostructuring. Experiments are used to demonstrate the method where a substrate coated with liquid resist is brought into contact with a flexible template driven by the applied electric field. Theoretical analysis based on the elasticity theory and electro-hydrodynamic theory is carried out. Effective voltage range and the saturation voltage are also discussed. A step-controllable release of flexible template is proposed and demonstrated to ensure the continuous contact between the template and an uneven substrate. This prevents formation of air traps and allows large area conformal contact to be achieved. A combination of Vacuum-electric field assisted step-controllable e-NIL is implemented in the developed prototype. Finally, photonic crystal nanostructures are successfully fabricated on a 4 in., 158 μm bow gallium nitride light-emitting diode epitaxial wafer using the proposed method, which enhance the light extraction property.

Enhancement of light output power from LEDs based on monolayer colloidal crystal

One of the major challenges for the application of GaN-based light emitting diodes (LEDs) in solid-state lighting lies in the low light output power (LOP). Embedding nanostructures in LEDs has attracted considerable interest because they may improve the LOP of GaN-based LEDs efficiently. Recent advances in nanostructures derived from monolayer colloidal crystal (MCC) have made remarkable progress in enhancing the performance of GaN-based LEDs. In this review, the current state of the art in this field is highlighted with an emphasis on the fabrication of ordered nanostructures using large-area, high-quality MCCs and their demonstrated applications in enhancement of LOP from GaN-based LEDs. We describe the remarkable achievements that have improved the internal quantum efficiency, the light extraction efficiency, or both from LEDs by taking advantages of diverse functions that the nanostructures provided. Finally, a perspective on the future development of enhancement of LOP by using the nanostructures derived from MCC is presented.

Luminescent elastomeric Janus particles

We report on a low-cost and low-tech method for the preparation of luminescent micro- and millimeter elastomeric particles with asymmetric morphology. The method of fabrication consists in UV-irradiating soft urethane/urea fluorescent spheres, which are then extracted in toluene and dried. Wrinkles appear on the irradiated portions of the particles surfaces, and the spatial periodicity can be controlled with variation in UV irradiation time and the amount of the luminescent compound. The spheres are thus composed of an urethane/urea network in which the tris(8-hydroxyquinolinato)aluminum (Alq3) fluorescent compound was incorporated. The asymmetric morphology and the optical properties of the resultant particles have been confirmed by scanning electron microscopy, atomic force microscopy, optical microscopy, and UV-Vis spectrophotometry. The system shows negligible leaching, and the encapsulation of the Alq3 without recourse to covalent bonding to the polymeric matrix has the advantage of allowing the tuning of the spheres morphology and fluorescence.

Electrically driven nanopillars for THz quantum cascade lasers

In this work we present a rapid and parallel process for the fabrication of large scale arrays of electrically driven nanopillars for THz quantum cascade active media. We demonstrate electrical injection of pillars of 200 nm diameter and 2 µm height, over a surface of 1 mm(2). THz electroluminescence from the nanopillars is reported. This result is a promising step toward the realization of zero-dimensional structure for terahertz quantum cascade lasers.

Emission characteristics of photonic crystal light-emitting diodes

Experimentally measured optical properties of photonic crystal LEDs are reported here. Photonic crystal and photonic quasi-crystal structures were fabricated on GaN epilayer LED wafer material using both direct-write electron beam lithography and nanoimprint lithography. Some of these structures were processed to make finished LEDs. Both electroluminescence and photoluminescence measurements were performed on these structures. Devices were characterized for their current-voltage characteristics, emission spectra, far-field emission pattern, and angular emission pattern. These results are useful for fabricating photonic crystal LEDs and assessing their operational properties.

Colloidal self-assembly meets nanofabrication: from two-dimensional colloidal crystals to nanostructure arrays

Self-assembly of colloidal microspheres or nanospheres is an effective strategy for fabrication of ordered nanostructures. By combination of colloidal self-assembly with nanofabrication techniques, two-dimensional (2D) colloidal crystals have been employed as masks or templates for evaporation, deposition, etching, and imprinting, etc. These methods are defined as "colloidal lithography", which is now recognized as a facile, inexpensive, and repeatable nanofabrication technique. This paper presents an overview of 2D colloidal crystals and nanostructure arrays fabricated by colloidal lithography. First, different methods for fabricating self-assembled 2D colloidal crystals and complex 2D colloidal crystal structures are summarized. After that, according to the nanofabrication strategy employed in colloidal lithography, related works are reviewed as colloidal-crystal-assisted evaporation, deposition, etching, imprinting, and dewetting, respectively.

Enhanced efficiency of light emitting diodes with a nano-patterned gallium nitride surface realized by soft UV nanoimprint lithography

This paper reports a significant enhancement in the extraction efficiency of nano-patterned GaN light emitting diodes (LED) realized by soft UV nanoimprint lithography. The 2 inch soft stamp was fabricated using a replication stamp of anodic alumina oxide (AAO) membrane. The light output power was enhanced by 10.9% compared to that of the LED sample without a nano-patterned surface. Up to 41% enhancement in photoluminescence intensity was obtained from the nano-patterned GaN LED sample. The method is simple, cheap and suitable for mass production.

Fabrication and characterization of subwavelength nanostructures on freestanding GaN slab

We develop a novel way to fabricate subwavelength nanostructures on the freestanding GaN slab using a GaN-on-silicon system by combining self-assemble technique and backside thinning method. Silicon substrate beneath the GaN slab is removed by bulk silicon micromachining, generating the freestanding GaN slab and eliminating silicon absorption of the emitted light. Fast atom beam (FAB) etching is conducted to thin the freestanding GaN slab from the backside, reducing the number of confined modes inside the GaN slab. With self-assembled silica nanospheres acting as an etching mask, subwavelength nanostructures are realized on the GaN surface by FAB etching. The reflection losses at the GaN interfaces are thus suppressed. When the InGaN/GaN multiple quantum wells (MQWs) active layers are excited, the light extraction efficiency is significantly improved for the freestanding nanostructured GaN slab. This work provides a very practical approach to fabricate freestanding nanostructures on the GaN-on-silicon system for further improving the light extraction efficiency.

Evaluation of InGaN/GaN light-emitting diodes of circular geometry

Blue GaN light emitting diodes (LEDs) in the shape of cuboids and circular disks have been fabricated by laser micromachining. The proposed circular geometry serves to enhance overall light extraction on a macro-scale and to improve uniformity of the emission pattern due to the rotational symmetry of the chip. Analysis of the chip shaping effect is carried out by ray-tracing simulations and further supported with mathematical modeling using ideal LED models, and subsequently verified with fabricated devices. In comparison, a 10% improvement in overall emission was observed for circular LEDs over the regular cuboids, consistent with simulations and calculations. The measured emission pattern from the circular LED confirms the axial symmetry of the emission beam.

Design of vertically-stacked polychromatic light-emitting diodes

A new design for a polychromatic light-emitting diode (LED) is proposed and demonstrated. LED chips of the primary colors are physically stacked on top of each other. Light emitted from each layer of the stack passes through each other, and thus is mixed naturally without additional optics. As a color-tunable device, a wide range of colors can be generated, making it suitable for display purposes. As a phosphor-free white light LED, luminous efficacy of 30 lm/watt was achieved.

Polychromatic light-emitting diodes with a fluorescent nanosphere opal coating

A hexagonally close-packed array consisting of fluorescent nanospheres was coated onto short-wavelength GaN light-emitting diodes to demonstrate polychromatic white light emission. The spherical particles self-assemble into ordered three-dimensional opal structures, performing the role of color conversion to generate a polychromatic spectrum with smooth and uniform emission patterns. Different ratios of green and orange-red fluorescent nanospheres were mixed and coated onto high-extraction-efficiency micro-LEDs. Four devices with different shades of white emission were demonstrated. Device A, with a high content of orange-red nanospheres, offers the highest CRI value of 80, whereas device C with a well-balanced ratio of green and orange-red nanospheres exhibits color characteristics closest to ideal white with CIE coordinate at (0.34, 0.34). At 20 mA driving current, the luminous efficacy of the devices A, B, C, and D are 40.5 lm W(-1), 57.7 lm W(-1), 63.1 lm W(-1), and 67.2 lm W(-1) respectively, while the correlated color temperatures (CCTs) of the corresponding devices are 3587, 4778, 5271, and 13 000 K.