Two-dimensional Periodic Nanostructures Research Articles

Electrically Tunable All-PCM Visible Plasmonics.

The realization of electrically tunable plasmonic resonances in the ultraviolet (UV) to visible spectral band is particularly important for active nanophotonic device applications. However, the plasmonic resonances in the UV to visible wavelength range cannot be tuned due to the lack of tunable plasmonic materials. Here, we experimentally demonstrate tunable plasmonic resonances at visible wavelengths using a chalcogenide semiconductor alloy such as antimony telluride (Sb2Te3), by switching the structural phase of Sb2Te3 from amorphous to crystalline. We demonstrate the excitation of a propagating surface plasmon with a high plasmonic figure of merit in both amorphous and crystalline phases of Sb2Te3 thin films. We show polarization-dependent and -independent plasmonic resonances by fabricating one and two-dimensional periodic nanostructures in Sb2Te3 thin films, respectively. Moreover, we demonstrate electrically tunable plasmonic resonances using a microheater integrated with the Sb2Te3/Si device. The developed electrically tunable Sb2Te3-based plasmonic devices could find applications in the development of active color filters.

Zeta Potential Dependent Self-Assembly for Very Large Area Nanosphere Lithography.

Nanosphere lithography offers a rapid, low-cost approach for patterning of large-area two-dimensional periodic nanostructures. However, a complete understanding of the nanosphere self-assembly process is necessary to enable further development and scaling of this technology. The self-assembly of nanospheres into two-dimensional periodic arrays has previously been attributed solely to the Marangoni force; however, we demonstrate that the ζ potential of the nanosphere solution is critically important for successful self-assembly to occur. We discuss and demonstrate how this insight can be used to greatly increase self-assembled 2D periodic array areas while decreasing patterning time and cost. As a representative application, we fabricate antireflection nanostructures on a transparent flexible polymer substrate suitable for use as a large-area (270 cm2), broadband, omnidirectional antireflection film.

Modeling of second-harmonic generation in periodic nanostructures by the Fourier modal method with matched coordinates.

We present an advanced formulation of the Fourier modal method for analyzing the second-harmonic generation in multilayers of periodic arrays of nanostructures. In our method, we solve Maxwell's equations in a curvilinear coordinate system, in which the interfaces are defined by surfaces of constant coordinates. Thus, we can apply the correct Fourier factorization rules as well as adaptive spatial resolution to nanostructures with complex cross sections. We extend here the factorization rules to the second-harmonic susceptibility tensor expressed in the curvilinear coordinates. The combination of adaptive curvilinear coordinates and factorization rules allows for efficient calculation of the second-harmonic intensity, as demonstrated for one- and two-dimensional periodic nanostructures.

Plasmon-inducing enhancement for multi-band light emission from ZnO film via periodic ordered Ag nanoparticles

In this article, we report the fabrication of two-dimensional periodic nanostructures of Ag nanoparticles and investigate the effects of the capped Ag nanostructure on the light-emitting properties of ZnO in detail. The experimental data clearly reveal that the light-emitting properties of the periodic Ag/ZnO nanostructures were very different from those of the ZnO films capped with random Ag islands. In particular, not only the near-band-edge emission but also the visible emissions of ZnO was enhanced significantly in the photoluminescence spectra for the periodic Ag/ZnO nanostructures, which may result from the multi surface plasmon modes excited by the periodic nanostructures of Ag/ZnO.

Development of an energy-saving glass using two-dimensional periodic nano-structures

A new type of energy-saving glass, Ag inlaid with SiO2 nano-structures (Inlay SiO2), is numerically developed here to offer modern buildings a passive but promising way of saving power under sunlight. The profile of glass is identified after optimizing optical responses using the genetic algorithm and a fitness function. Wavelength-selective reflectance (R) and transmittance (T) are demonstrated with validated programs based on the rigorous coupled-wave analysis algorithm. The high T and low R in the visible range not only saves energy for lighting but diminishes reflected and stray light. The low T and high R in the near-infrared reduce the cost for air-conditioning. Besides the cost can be further reduced, the superiority of this glass over current commercially available products includes the material commonness, light weight, and freedom from adhesion as well as oxidation. The fabrication tolerance of each dimension is further investigated to determine the key one. Other structure configurations are also approached to explore possibilities of alternates.

Fabrication of high-resolution large-area patterns using EUV interference lithography in a scan-exposure mode

Limited beam spot size is a major limitation of interference lithography. This limits the area of patterning and reduces the pattern homogeneity. We describe a scanning exposure technique to circumvent this problem. We show the generation of uniform and seamless gratings with half-pitches down to 35 nm over an area of several mm2 using EUV interference lithography. The presented technique offers a fast and cost-effective method of fabricating one- and two-dimensional periodic nanostructures with improved uniformity and increased patterning area.

Enhancing the Out-Coupling Efficiency of Organic Light-Emitting Diodes Using Two-Dimensional Periodic Nanostructures

The out-coupling efficiency of planar organic light emitting diodes (OLEDs) is only about 20% due to factors, such as, the total internal reflection, surface plasmon coupling, and metal absorption. Two-dimensional periodic nanostructures, such as, photonic crystals (PhCs) and microlenses arrays offer a potential method to improve the out-coupling efficiency of OLEDs. In this work, we employed the finite-difference time-domain (FDTD) method to explore different mechanisms that embedded PhCs and surface PhCs to improve the out-coupling efficiency. The effects of several parameters, including the filling factor, the depth, and the lattice constant were investigated. The result showed that embedded PhCs play a key role in improving the out-coupling efficiency, and an enhancement factor of 240% was obtained in OLEDs with embedded PhCs, while the enhancement factor of OLEDs with surface PhCs was only 120%. Furthermore, the phenomena was analyzed using the mode theory and it demonstrated that the overlap between the mode and PhCs was related to the distribution of vertical mode profiles. The enhancement of the extraction efficiency in excess of 290% was observed for the optimized OLEDs structure with double PhCs. This proposed structure could be a very promising candidate for high extraction efficiency OLEDs.

Ultraviolet luminescence enhancement of ZnO two-dimensional periodic nanostructures fabricated by the interference of three femtosecond laser beams

We have developed a simple and rapid method for fabricating ZnO periodic nanostructures. By changing the laser polarization combinations, we fabricated different types of two-dimensional (2D) nanostructures on ZnO crystal surfaces by the interference of three femtosecond laser beams. The 2D nanostructures became more regular and uniform when increasing the cross angles between any two laser beams. Compared with the case of the plane surfaces of ZnO crystals, the 2D nanostructures revealed an ultraviolet (UV) luminescence enhancement excited by an 800 nm femtosecond laser beam. We studied the photoluminescence properties of the 2D nanostructures and the mechanisms of the UV luminescence enhancement. Our results indicated that the enhancement was caused by an increase in optical absorption with respect to that of the unaltered ZnO plane surface and by the formation of surface defect states.

The profile optimization of periodic nano-structures for wavelength-selective thermophotovoltaic emitters

This work demonstrates a systematic and efficient way of searching two-dimensional periodic nano-structures with wavelength-selective radiative properties appealing to thermophotovoltaic emitters. The orthogonal array from the Taguchi method benefits the optimization of structure depth, periods, and filling ratios among 1024 combinations. The spectral emittance from optimized structures is above 0.9 at wavelengths between 1.0 and 2.0 μm, but the emittance is below 0.2 at wavelengths between 2.0 and 4.0 μm. Furthermore, such spectral emittance characteristics can be kept for wide ranges of emission direction and either polarization. Representative emittance spectra from an example of convex rectangular structures are plotted and compared to those of concave structures in previous works. The comparison shows that radiative properties of convex structures are dominated by cavity resonances while those of concave structures are the interplay of multiple mechanisms. All numerical results are obtained from validated programs based on the rigorous coupled-wave analysis.

Hybrid-polymer nanostructures forming an anti-reflection film using two-beam interference and ultraviolet nanoimprint lithography

This study presents a new process to fabricate the two-dimensional periodic nanostructures with the specific material of inorganic–organic hybrid polymers (ORMOCER). In this study, the photoresist (PR) originated nanostructures can be fabricated with a two-beam interference method by use of 351 nm-wavelength Argon ion laser light. The replicated mold is performed by Ni electroless plating that Ni mold can be used by ultraviolet nanoimprint lithography (UV-NIL) process imprinting into the hybrid-polymer surface. The hybrid-polymer material is coating on the flexible polyethylene terephthalate (PET). Here, a self-assembled monolayer of fluoroalkyl silane (FAS-SAM) can be effective by means of atmospheric-pressure plasma (AP) chemical vapor modification of Ni mold surface for the duplicated ORMOCER/PET nanostructure film in UV-NIL process.

Fabrication of two-dimensional periodic nanostructures by two-beam interference of femtosecond pulses

Two-dimensional periodic nanostructures on ZnO crystal surface were fabricated by two-beam interference of 790 nm femtosecond laser. The long period is, as usually reported, determined by the interference pattern of two laser beams. Surprisingly, there is another short periodic nanostructures with periods of 220-270 nm embedding in the long periodic structures. We studied the periods, orientation, and the evolution of the short periodic nanostructures, and found them analogous to the self-organized nanostructures induced by single fs laser beam.

Formation of two-dimensional periodic nanostructures on fused quartz, polyimide, and polycrystalline diamond by pulsed four-wave interference laser modification

Atomic force microscopy is used to examine the topography of submicron periodic structures formed on the surfaces of synthetic polycrystalline diamond and polyimide films. The films are deposited on fused quartz substrates by four-wave interference modification using a pulsed 308-nm UV XeCl excimer laser. It is demonstrated that a two-dimensional periodic relief with a submicron period can be formed on the diamond surface directly by laser evaporation in the absence of a photoresist. Depending on the exposure, two mechanisms of polyimide film modification are observed. At exposures less than 100 mJ/cm2, the relief is formed due to swelling at the positions of interference maxima. At exposures greater than 100 mJ/cm2, holes are formed in the films. A periodic relief on the fused quartz surface is formed by using a UV photoresist exposed to pulsed interference laser radiation and subsequent Ar ion etching.