Metasurfaces have been strongly investigated to realize a paradigm shift from classical optics. Unlikely the classical optics dealing with light rays in accordance with geometric-optic principles, metasurfaces allow the control of wavefront in subwavelength thickness on flat surfaces [1,2]. As conventional convex lenses, metalenses on flat surface without any macroscopic surface curvature have tremendous capability in future flat optics. They can also be used to replace the expensive compound lens systems in a number of consumer electronic products such as optical storages, digital cameras, microscopes, etc. Providing a flat, form factor, metalenses have a large degree of freedom in designing miniaturized sensors as well.
Typically, metasurfaces are composed of an array of planar nanostructures that locally provide the change of amplitude or phase of light in reflection or transmission. The phase modulation on each nanostructure leads to the change of wavefront for a new class of flat optical elements [3-5]. For example, high-contrast transmit arrays (HCTAs) have been reported to demonstrate subwavelength-thick lenses with high-numerical aperture and large focal efficiency [6,7]. This is a promising approach to make metasurfaces consisting of an array of subwavelength dielectric nanoposts on flat surface; however, a full coverage of phase from 0 to 2 pi is not readily achievable due to phase defects, when the post diameter is chosen to be varied. The phase defects are originated from resonances at the wavelength of interest, hindering a gradual increase of phase with respect to the variation of post diameters. Such defects deteriorate optical performance compared with conventional curved lenses, particularly in focal spot sizes and focusing efficiencies.
Here we propose a novel method to repair phase defects and achieve a full, 2 pi phase coverage with free of defects, which provides complete phase matches with theoretical calculations. We apply this method to demonstrate the convex-lens-like metalens with high numerical aperture (NA), small focal spot size, and high focusing efficiency in near-infrared region (1.55 microns). Together with the theoretical design and simulation, we prepare metalenses using silicon photolithography and nanofabrication and analyze experimental observations. The measured full width at half maximum (FWHM) of the focal spot and focusing efficiency show a high performance for numerical apertures of 0.3 ~0.7. This achievement offers considerable opportunities for various applications using metasurfaces based on controlled wavefront with free of defects.
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Patterning of colloidal quantum dot (QD) of a nanometer resolution is important for potential applications in micro- or nanophotonics. Several patterning techniques such as polymer composites, molecular key-lock methods, inkjet printing, and the microcontact printing of QDs have been successfully developed and applied to various plasmonic applications. However, these methods are not easily adapted to conventional complementary metal-oxide semiconductor (CMOS)-compatible processes because of either limits in fabrication resolutions or difficulties in sub-100-nm alignment. Here, we present an adaptation of a conventional lift-off method for the patterning of colloidal QDs. This simple method can be later applied to CMOS processes by changing electron beam lithography to photolithography for building up photon-generation elements in various planar geometries. Various shapes formed by colloidal QD clusters such as straight lines, rings, and dot patterns with sub-100-nm size could be fabricated. The patterned structures show sub-10-nm positioning with good fluorescence properties and well-defined sidewall profiles. To demonstrate the applicability of our method, we present a surface plasmon generator from a QD cluster.
We report the enhanced electroluminescence (EL) of GaN light-emitting diodes (LEDs) on glass substrates. We found that GaN morphology affected the EL and achieved enhanced EL of GaN-LEDs on glass by identifying the optimal GaN morphology having both high crystallinity and compatibility for device fabrication. At proper growth temperature, GaN crystallinity was improved with increasing GaN crystal size irrespective of the GaN crystallographic orientation, as determined by spatially resolved cathodoluminescent spectroscopy. The optimized GaN LEDs on glass composed of the nearly single-crystalline GaN pyramid arrays exhibited excellent microscopic EL uniformity and luminance values of ~ 9100 cd/m<sup>2</sup> at the peak wavelength of 495 nm. The EL color could be adjusted mainly by varying the quantum well temperature. In addition, new growth methods for achieving high GaN crystallinity at a low growth temperature (e.g. ~700°C) were briefly reviewed and attempted by adopting selective heating. We expect that performance of the GaN LEDs on glass can be much enhanced by enhancing GaN crystallinity and p-GaN coating, and evolvement of low-temperature growth of high-quality GaN might even customize ordinary glass as a substrate, which enables high-performance, low-cost lighting or display.
The patterning of colloidal quantum dots with nanometer resolution is essential for their application in photonics and plasmonics. Several patterning approaches, such as the use of polymer composites, molecular lock-and-key methods, inkjet printing, and microcontact printing of quantum dots, have limits in fabrication resolution, positioning and the variation of structural shapes. Herein, we present an adaptation of a conventional liftoff method for patterning colloidal quantum dots. This simple method is easy and requires no complicated processes. Using this method, we formed straight lines, rings, and dot patterns of colloidal quantum dots on metallic substrates. Notably, patterned lines approximately 10 nm wide were fabricated. The patterned structures display high resolution, accurate positioning, and well-defined sidewall profiles. To demonstrate the applicability of our method, we present a surface plasmon generator elaborated from quantum dots.
Light-emitting displays with colloidal quantum-dot (QD) have recently received considerable attention due to advantages
of QD property such as high quantum yields, extremely narrow emission, spectral tunability, and a higher stability than
other existing luminophores. However, the difficulty of patterning red, green, and blue (RGB) pixels of three individual
QDs with controlled interfaces has prevented from developing a full-color QD display with acceptable quantum
efficiency. In this talk, the issues of QD EL and successful embodiment of full-color QD display by the solvent-free
transfer printing of QD pattern will be presented. Modulated QD assemblies exhibit the excellent morphology, wellordered
QD structure, and clearly defined interfaces, which result in significant enhancements in the charge
transport/balance in the QD layer. A large-area full-color QD displays on a glass substrate, and even on a flexible
substrate can be realized in this manner with the control of nano-interfaces and carrier behaviors.