Nanolens Array Research Articles

Laser‐Assisted Nanotexturing for Flexible Ultrathin Crystalline Si Solar Cells

Ultrathin (UT) crystalline Si wafers, which are more flexible than conventional ones, can apply to curved surfaces, enabling a wide range of applications such as building‐integrated photovoltaics, vehicle‐integrated photovoltaics, and wearable devices. Thinner wafers require more effective light trapping; thus, surface texturing in microscale is a common approach to compensate for the reduced thickness by enhancing the light pathlength. Microscale textures, however, deteriorate the mechanical flexibility due to stress concentration in the valley of the microtextures. In this study, a laser‐assisted nanotexturing process is proposed for enhanced flexibility of the UT Si solar cells with a 50 μm thickness while maintaining light‐trapping performances. A nanolens array is used to focus laser onto the Si wafers, inducing the formation of nanoparticle etch masks for nanopyramid texturing in an alkaline solution. The origin of the enhanced flexibility of the nanotextured Si wafers is discussed by a micromechanics simulation study. Herein, nanotexturing technique is applied to UT Si‐based passivated emitter rear cells and the enhanced flexibility of the cells with a 26 mm critical bending radius is demonstrated. Also, it is shown that the nanotextured Si wafer provides a higher efficiency of 18.68%, whereas the microtextured one exhibits 18.10%.

An aluminum-based reflective nanolens array that enhances the effectiveness of a continuous-flow ultraviolet disinfection system for livestock water

Climate change has worsened droughts and floods, and created conditions more likely to lead to pathogen contamination of surface water and groundwater. Thus, there is a growing need to disinfect livestock water. Ultraviolet (UV) irradiation is widely accepted as an appropriate method for disinfecting livestock water, as it does not produce hazardous chemical compounds and kills pathogens. However, UV-based disinfection inevitably consumes electricity, so it is necessary to improve UV disinfection effectiveness. Aluminum-based reflective nanolens arrays that enhanced the effectiveness of a continuous-flow UV water disinfection system were developed using electrochemical and chemical processes, including electropolishing and two-step anodization. A continuous UV disinfection system was custom designed and the parts were produced using a three-dimensional printer. Electropolished aluminum was anodized at 40 and 80 V in 0.3 M oxalic acid, at 120 and 160 V in 1.0 M phosphoric acid, and at 200 and 240 V in 1.5 M citric acid. The average nanolens diameters (D) of the aluminum-based reflective nanolens arrays prepared using 40, 80, 120, 160, 200, and 240 V anodization were 95.44, 160.98, 226.64, 309.90, 296.32, and 339.68 nm, respectively. Simple UV reflection behind irradiated water disinfected Escherichia coli O157:H7 in water more than did the non-reflective control. UV reflection and focusing behind irradiated water using an aluminum-based reflective nanolens array disinfected E. coli O157:H7 more than did simple UV reflection. Such enhancement of the UV disinfection effectiveness was significantly effective when a nanolens array with D 226.64 nm, close to the wavelength of the irradiated UV (254 nm), was used.

Hierarchical macro to nano press molding of optical glasses by using metallic glasses

The fabrication of ideal optical components is a world-wide grand challenge facing us. Benefiting from the unique thermoplastic nature, precision glass molding (PGM) is considered as a promising method for rapid and precise fabrication of optical components. The success and proliferation of such methods critically rely on the manufacturing of robust and durable master molds. Although silicon-based molds relying on photolithographic forming are facile to manufacture, they are brittle and have limited longevity when used as the molds. To address this challenge, here, we propose a hierarchical macro to nano mold preparation method based on the thermoplastic forming ability of metallic glasses (MGs), which excels in micro and nano scale fabrication. After subsequently crystallizing the as-formed MG, we demonstrate that the crystalline MG is suitable as an excellent mold for molding optical glasses with service temperature up to 600 °C. Using this ‘spawning’ process, several metallic molds including macroscopic lens arrays, microscale gratings, and nano-lens arrays were successfully prepared, respectively. The minimum feature size of the mold is 300 nm and the optimum surface roughness is 7.43 nm. Correspondingly, we prepared hierarchical optical glass components with excellent optical properties using MG as molds.

Improvement of light intensity and efficiency of n-ZnO/NiO/p-GaN heterojunction-based white light emitting diodes using micro-/nanolens array

Our study proposes a technique to enhance light extraction efficiency of light emitting diodes (LEDs) by incorporating various micro-/nanolens arrays (MNLAs) on the substrate layer, which in turn increases the external quantum efficiency (EQE) of the LEDs. To simulate the LEDs, we utilized the finite difference time domain method. To achieve a white LED, we inserted a thin layer of NiO at the interface between the n-type ZnO and the p-type GaN. The basic n-ZnO/NiO/p-GaN heterojunction-based LED exhibited an EQE of 10.99% where the effective refractive index of the LED structure was 1.48. The EQE was further increased by engraving various planoconvex or planoconcave MNLA on the top surface of the substrate layer. A maximum EQE of 12.4% was achieved for convex-1 type (lens height of 0.5 μm and radius of 0.4 μm) elliptical lens engraved LED where the effective refractive index was 1.4. In addition, the peak electroluminescence (EL) light intensity of convex-1 lens-based LED was twice than the light intensity observed in basic LED. Because of excellent EL spectrum and significant amount of light throughout the visible spectrum, the proposed convex-1 structure-based LED can be considered as a prospective candidate for white LED.

Nucleation and growth of optically efficient organic nano-lens arrays fabricated by a vacuum deposition

There are few techniques applicable to top-emitting organic light emitting diode (TOLED) panels, as organic films are easily deteriorated by heat stress, oxygen and moisture exposure when a light extraction structure is fabricated on top of TOLEDs. Recently, we have reported that a N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′diamine nano-lens array (NLA) fabricated by using a simple and highly process-compatible vacuum deposition works as an optically efficient structure when NLA with a lens size of several hundred nanometers is deposited on the oxide layer of the TOLED. To achieve the desirable optical properties of TOLEDs, the vacuum NLA process was controlled to obtain big nano-lenses with a diameter of several hundred nanometers, indicating the importance of understanding of the nucleation and growth behavior of NLA. Here, the systematic NLA experiments using glass and alumina substrates indicate that nano-lenses are formed by nucleation and growth of nuclei. Very interestingly, the nucleation is completed in the early stage of the deposition process, followed by growth without decreasing the total number of nano-lenses per area, and finally followed by growth with decreasing the total number due to Ostwald ripening. Compared to alumina substrates, smaller sizes and greater rates of nucleation of NLAs in glass substrates are explained by the difference in substrate surface energy combined with a classical thermodynamic theory using the difference in Gibbs free energy.

Aluminum based reflective nanolens arrays to improve the effectiveness of ultraviolet inactivation of Escherichia coli O157:H7 and Listeria monocytogenes in water and a sucrose solution.

Aluminum based reflective nanolens arrays were developed via a series of aluminum electropolishing and anodization steps with subsequent selective dissolution of anodic aluminum oxide (AAO). The diameter of nanolenses (d) on arrays can be controlled by altering electrolytes and voltages used for aluminum anodization. The d values of arrays produced by anodization in 0.3M oxalic acid at 40, 60, and 80V, and in 1.0M phosphoric acid at 100, 110, and 120V were 71.94, 121.90, and 161.53nm, and 220.16, 252.06, and 274.78nm, respectively. The effectiveness of UV (254nm) inactivation of Escherichia coli O157:H7 and Listeria monocytogenes at concentrations of 5-6 log CFU/mL in water and in a 10% (w/v) sucrose solution was improved using a nanolens array having a d value of 252.06nm.

A novel fabrication technique for three-dimensional concave nanolens arrays

A novel facile technique is proposed for fabricating three-dimensional (3D) concave nanolens arrays on a silicon substrate. The technique leverages an inherent characteristic of the polymethyl methacrylate (PMMA) resist during inductively coupled plasma (ICP) etching. The tendency for plasma ions to accumulate at the edge of the PMMA resist helps create a local electric field that causes the ions to etch the sidewall of the PMMA resist. This process progressively increases the uncovered area, resulting in a graded etched depth or a concave structure in the substrate. In addition, using a given ICP etching recipe, the time required for a PMMA resist to be removed by sidewall etching is determined by its width. The use of PMMA resist of different widths enables one to achieve structures of varying etched depths and thus a 3D lens array. Optical characteristics of the fabricated nanolens were simulated using the FDTD (Finite-difference time-domain) method, and focal lengths ranging from 150 nm to 420 nm were obtained. This type of nanolens is very useful in ultraviolet optical devices and CMOS image sensors.

High-transmittance and focal controllable plano-convex lenses with embedded nanolens bottoms formed by electrowetting on a colloidal monolayer

UV-curable polymer lenses with a nanolens array decorated on the bottom surface via irreversible electrowetting on a hydrophobic colloidal monolayer.

The formation of an optically functional nano-lens array by substrate surface treatment

Recently, a nano-lens array (NLA) technology has been developed to greatly enhance light extraction efficiencies and color stabilities of red, green and blue organic light-emitting diodes. The fabrications of N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (C44H32N2, NPB) NLAs on oxide layers were accomplished by an organic vapor phase deposition during which surface tensions of NPB were increased by its crystallization. To emphasize the importance of the fabrication technology, the kinds of substrate materials on which NLAs are formed should be extended to high surface energy nitrides. Herein, to understand the role of the surface condition of a silicon nitride substrate in the formation of NLA, three types of nitride substrates are prepared by using surface treatment, while one NPB deposition recipe is used. Very interestingly, even though nano-lens is not observed in an untreated substrate with high surface energy, nano-lens is formed by surface treatment and the resulting decrease in surface energy. This indicates that NLA formation is not limited to an oxide substrate, but is determined by the difference in surface energy values between a substrate and a deposited material on the substrate. Furthermore, the morphological change of NPB by surface treatment is extensively discussed using contact angle experiments.

12‐3: A Highly Mass‐producible Nano‐lens Array Technology for Optically Efficient Full‐color Organic Light Emitting Diode Display Applications

Recently, we have reported that a nano‐lens array (NLA) technology using a very simple and fab‐compatible vacuum deposition process contributes to highly efficient and color stable green and blue organic light emitting diodes (OLEDs). Here, the technique is applied to red OLEDs. With NLA, the increase ratio of current efficiency is about 1.57 which is similar to those of green and blue OLEDs (1.56, 1.59). Furthermore, color coordinate change measurement values of red OLEDs in a wide viewing angle ranging from 0° to 60° are reduced as green and blue OLEDs do, indicating that NLA has a great potential for the development of the optically efficient full‐color OLED displays composed of three color OLED pixels.

Fab-compatible nano-lens array integration for optically efficient flexible top-emitting organic light-emitting diodes

This work extends the kinds of organic materials that can form nano-lens arrays (NLAs). We have previously addressed fabrication of a N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) NLA and explained the formation mechanism as NPB crystallization and the resulting increase in surface tension. Here, we report a Tris(8-hydroxyquinolinato)aluminum (Alq3) NLA fabricated by the same deposition method as a NPB NLA. Grazing-incidence small-angle X-ray scattering experiments indicate that the formation mechanism of the Alq3 NLA is the same as that of the NPB NLA. Furthermore, this study broadens the application of NLAs from rigid to flexible organic light-emitting diodes (OLEDs). Here, flexible stainless steel-based devices using a low-temperature thermal release film, atomic layer deposition and NPB NLA processes are fabricated and characterized. With a NLA, current efficiency is increased 1.4 times, indicating that fab-compatible NLA technology is also available for flexible OLEDs.

Fabrication of Highly Packed Plasmonic Nanolens Array Using Polymer Nanoimprinted Nanodots for an Enhanced Fluorescence Substrate.

A simple and cost-effective fabrication method for plasmonic nanolens arrays (PNA) with a narrow gap has been proposed for fabricating enhanced fluorescence substrates, in which the fluorophores interacting with the enhanced electromagnetic field generated by localized surface plasmons provide a higher fluorescence signal. The PNA was fabricated by the sequential depositions of the SiO2 and Ag layers on a UV-nanoimprinted nanodot array with a pitch of 500 nm, a diameter of 250 nm, and a height of 100 nm. During the deposition processes, the shape of the nanodots changed to that of nanolenses, and the gap between the nanolenses was decreased via sidewall deposition. To examine the feasibility of the fabricated PNA for enhanced fluorescence application, a streptavidin-Cy5 (SA-Cy5) conjugate dissolved in a saline buffer solution was spotted on the PNA, and the fluorescence signals of the SA-Cy5 were measured and compared with those on a bare glass substrate. The enhancement factor was affected by the gap between the nanolenses, and the maximum enhancement factor of ~128 was obtained from the PNA with a SiO2 layer thickness of 150 nm and an Ag layer thickness of 100 nm. Finally, an electromagnetic field analysis was used to examine the fluorescence signal enhancement, and was conducted using rigorous coupled wave analysis.

High Light Extraction Efficiency of Deep Ultraviolet LEDs Enhanced Using Nanolens Arrays

To improve the light extraction efficiency (LEE) of AlGaN-based deep ultraviolet LEDs (DUV-LEDs) by simple and effective method, this has greatly attracted ever-growing attention in DUV-LEDs field. Here, nanolens arrays (NLAs) fabricated by nanophotolithography and wet-etched technique is proposed to improve the LEE of DUV-LEDs, and its theoretical feasibility is verified by the Monte Carlo Ray-Trace method and finite element analysis which show an obvious improvement of light and electric field distribution benefit from NLAs structure. By controlling the time of wet etching, the effect of the uniform patterns of NLAs with adjustable morphology on the LEE of DUV-LEDs is systematically studied. Compared with the sandwiched flat lens, experimental results show that the obviously enhancements of the light output power are achieved by using the proposed NLAs to be 13.0%, 21.1%, 24.7%, 13.0%, and 11.5%, respectively, corresponding to their radius of 325, 340, 350, 360, and 400 nm, under the driving current of 300 mA. Therefore, the highest light output power of DUV-LEDs is used the optimized NLAs with radius of 350 nm, and its emission angle also shows the largest improvement of ~14°, suggesting the best LEE.

Optical Analysis of Power Distribution in Top-Emitting Organic Light Emitting Diodes Integrated with Nanolens Array Using Finite Difference Time Domain.

Recently, we have addressed that a formation mechanism of a nanolens array (NLA) fabricated by using a maskless vacuum deposition is explained as the increase in surface tension of organic molecules induced by their crystallization. Here, as another research using finite difference time domain simulations, not electric field intensities but transmitted energies of electromagnetic waves inside and outside top-emitting blue organic light-emitting diodes (TOLEDs), without and with NLAs, are obtained, to easily grasp the effect of NLA formation on the light extraction of TOLEDs. Interestingly, the calculations show that NLA acts as an efficient light extraction structure. With NLA, larger transmitted energies in the direction from emitting layer to air are observed, indicating that NLAs send more light to air otherwise trapped in the devices by reducing the losses by waveguide and absorption. This is more significant for higher refractive index of NLA. Simulation and measurement results are consistent. A successful increase in both light extraction efficiency and color stability of blue TOLEDs, rarely reported before, is accomplished by introducing the highly process-compatible NLA technology using the one-step dry process. Blue TOLEDs integrated with a N, N'-di(1-naphthyl)- N, N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine NLA with a refractive index of 1.8 show a 1.55-times-higher light extraction efficiency, compared to those without it. In addition, viewing angle characteristics are enhanced and image blurring is reduced, indicating that the manufacturer-adaptable technology satisfies the requirements of highly efficient and color-stable top-emission displays.

Integrated parabolic nanolenses on MicroLED color pixels

A parabolic nanolens array coupled to the emission of a nanopillar micro-light emitting diode (LED) color pixel is shown to reduce the far field divergence. For a blue wavelength LED, the total emission is 95% collimated within a 0.5 numerical aperture zone, a 3.5x improvement over the same LED without a lens structure. This corresponds to a half-width at half-maximum (HWHM) line width reduction of 2.85 times. Using a resist reflow and etchback procedure, the nanolens array dimensions and parabolic shape are formed. Experimental measurement of the far field emission shows a HWHM linewidth reduction by a factor of 2x, reducing the divergence over the original LED.

Fabrication of high-aspect-ratio aluminum oxyhydroxide nanopillars for anti-reflection surfaces

This study presents the anti-reflection characteristics of high-aspect-ratio aluminum oxyhydroxide nanopillars prepared by the hydrothermal synthesis method from a nanoporous alumina substrate. Three different alumina-based nanostructures, including nanolens, short-nanopillar, and long-nanopillar were prepared, and the center-to-center distance of the nanostructures was fixed as 380 ± 50 nm. The diameter of the nanopillars was 140 ± 30 nm, and the aspect ratios of the short and long nanopillars were 1.5 and 4.0, respectively. From visual investigation, the surfaces of fabricated aluminum oxyhydroxide nanopillars appear black, although they did not have any black color. The anti-reflection characteristics of high aspect ratio aluminum oxyhydroxide nanopillars were compared with those of an alumina nanolens array. According to the experimental results, the high-aspect-ratio aluminum oxyhydroxide nanopillar array demonstrated remarkable anti-reflection characteristics, with 0.18% reflectance at an incident angle of 15°.

Crystallization-assisted nano-lens array fabrication for highly efficient and color stable organic light emitting diodes.

To date, all deposition equipment has been developed to produce planar films. Thus lens arrays with a lens diameter of <1 mm have been manufactured by combining deposition with other technologies, such as masks, surface treatment, molding etc. Furthermore, a nano-lens array (NLA) with a sufficiently small lens diameter (<1 μm) is necessary to avoid image quality degradation in high resolution displays. In this study, an organic NLA made using a conventional deposition technique - without combining with other techniques - is reported. Very interestingly, grazing-incidence small-angle X-ray scattering (GI-SAXS) experiments indicate that the NLA is formed by the crystallization of organic molecules and the resulting increase in surface tension. The lens diameter can be tuned for use with any kind of light by controlling the process parameters. As an example of their potential applications, we use NLAs as a light extraction film for organic light emitting diodes (OLEDs). The NLA is integrated by directly depositing it on the top electrode of a collection of OLEDs. This is a dry process, meaning that it is fully compatible with the current OLED production process. Devices with NLAs exhibited a light extraction efficiency 1.5 times higher than devices without, which corresponds well with simulation results. The simulations show that this high efficiency is due to the reduction of the guided modes by scattering at the NLA. The NLAs also reduce image blurring, indicating that they increase color stability.

Highly Ordered Arrays of Femtoliter Surface Droplets

Highly ordered arrays of attoliter to femtoliter droplets are created on substrates pre-patterned with smooth circular microdomains. The morphology and volume of the droplets are governed by the droplet growth dynamics during the solvent exchange. These tunable droplet arrays can be potentially exploited for the fabrication of nanolens arrays for photon manipulation, among other applications.

Nonlinear Chiro-Optical Amplification by Plasmonic Nanolens Arrays Formed via Directed Assembly of Gold Nanoparticles

Metal nanoparticle assemblies are promising materials for nanophotonic applications due to novel linear and nonlinear optical properties arising from their plasmon modes. However, scalable fabrication approaches that provide both precision nano- and macroarchitectures, and performance commensurate with design and model predictions, have been limiting. Herein, we demonstrate controlled and efficient nanofocusing of the fundamental and second harmonic frequencies of incident linearly and circularly polarized light using reduced symmetry gold nanoparticle dimers formed by surface-directed assembly of colloidal nanoparticles. Large ordered arrays (>100) of these C∞v heterodimers (ratio of radii R1/R2 = 150 nm/50 nm = 3; gap distance l = 1 ± 0.5 nm) exhibit second harmonic generation and structure-dependent chiro-optic activity with the circular dichroism ratio of individual heterodimers varying less than 20% across the array, demonstrating precision and uniformity at a large scale. These nonlinear optical properties were mediated by interparticle plasmon coupling. Additionally, the versatility of the fabrication is demonstrated on a variety of substrates including flexible polymers. Numerical simulations guide architecture design as well as validating the experimental results, thus confirming the ability to optimize second harmonic yield and induce chiro-optical responses for compact sensors, optical modulators, and tunable light sources by rational design and fabrication of the nanostructures.

Numerical and experimental studies on sub-wavelength focusing in nano-slit arrays of metallic stripes with variable widths

We simulated the distributions of light energy transmission through metallic nano-slits for the optimal design of nano-lens structures and observed experimentally the light focusing effect from the nano-lens array. Contrary to reported research where the dielectric slit sizes are controlled, the current design is based on the distributions of metallic claddings of different sizes. The nano-focusing is generated by the surface plasmon polariton (SPP) effect that is known to overcome the diffraction limit and to induce sub-wavelength focusing. SPPs are collective electron charge oscillations in metallic nano-slits that can be excited by electromagnetic waves, amplifying near-field optical waves at resonance. Here, intensity amplifications in near-fields are maximized for a given light source, mainly based on various distributions of metallic claddings of different sizes in an array. For experimentation gold nano-slit arrays designed from the numerical optimization were fabricated through the lift-off process and confirmed by observations in SEM and confocal microscopes. The SPP effect in nano-slit lenses was explored by light focusing on light-sensitive papers macroscopically and by light transmission microscopically using a dispersive Raman spectrometer in scattering mode.