Optical metamaterials at near and mid-IR range fabricated by nanoimprint lithography

Two types of optical metamaterials operating at near-IR and mid-IR frequencies, respectively, have been designed, fabricated by nanoimprint lithography (NIL), and characterized by laser spectroscopic ellipsometry. The structure for the near-IR range was a metal/dielectric/metal stack “fishnet” structure that demonstrated negative permittivity and permeability in the same frequency region and hence exhibited a negative refractive index at a wavelength near 1.7 μm. In the mid-IR range, the metamaterial was an ordered array of fourfold symmetric L-shaped resonators (LSRs) that showed both a dipole plasmon resonance resulting in negative permittivity and a magnetic resonance with negative permeability near wavelengths of 3.7 μm and 5.25 μm, respectively. The optical properties of both metamaterials are in agreement with theoretical predictions. This work demonstrates the feasibility of designing various optical negative-index metamaterials and fabricating them using the nanoimprint lithography as a low-cost, high-throughput fabrication approach. PACS 42.25.Bs; 81.16.Nd; 42.70.-a; 81.07.-b     

Low losses left-handed materials with optimized electric and?magnetic resonance

We propose that the losses in left-handed materials (LHMs) can be significantly affected by changing the coupling relationship between electric and magnetic resonance. A double bowknot shaped structure (DBS) is used to construct the LHMs. And the magnetic resonance of the DBS, which resonated in the case of lower and higher frequencies than the electric resonant dip, is studied in simulation and experiment by tailoring the structural parameters. The case of magnetic resonance located at low electric resonance frequencies band is confirmed to have relatively low losses. Using full wave simulation of prism shaped structure composed of DBS unit cells, we prove the negative refraction behavior in such a frame. This study can serve as a guide for designing other similar metal–dielectric–metal (MDM) in low losses at terahertz or higher frequencies.     

On amorphization and nanocomposite formation in Al-Ni-Ti system by mechanical alloying

Amorphous structure generated by mechanical alloying (MA) is often used as a precursor for generating nanocomposites through controlled devitrification. The amorphous forming composition range of ternary Al-Ni-Ti system was calculated using the extended Miedema’s semi-empirical model. Eleven compositions of this system showing a wide range of negative enthalpy of mixing (−ΔH ^mix) and amorphization (−ΔH ^amor) of the constituent elements were selected for synthesis by MA. The Al₈₈Ni₆Ti₆ alloy with relatively small negative ΔH ^mix (−0.4 kJ/mol) and ΔH^amor (−14.8 kJ/mol) became completely amorphous after 120 h of milling, which is possibly the first report of complete amorphization of an Al-based rare earth element free Al-TM-TM system (TM = transition metal) by MA. The alloys of other compositions selected had much more negative ΔH^mix and H^amor; but they yielded either nanocomposites of partial amorphous and crystalline structure or no amorphous phase at all in the as-milled condition, evidencing a high degree of stability of the intermetallic phases under the MA environment. Hence, the negative ΔH ^mix and ΔH ^amor are not so reliable for predicting the amorphization in the present system by MA     

Influence of parameters of the potential barrier at the metal/polymer interface on the electronic switching in the metal/polymer/metal structure

The influence of the first-order phase transition on the parameters of the potential barrier at the indium/polymer interface has been investigated. It has been established that the phase transition occurring in the metal initiates switching of the polymer insulator into a high-conductivity state. Performed investigations have shown that the main charge transfer mechanism in the metal-polymer-metal structure at high temperatures is the current caused by the electron thermionic emission. The analysis of current-voltage characteristics has demonstrated that the first-order phase transition in indium leads to the variation in the potential barrier height at the metal/polymer interface by Δφ ≈ 0.18 eV. It is this phenomenon that is responsible for the electronic switching.     

A theoretical study of hydrogen adsorption on Li, Be, Na, and Mg atoms attached to aromatic hydrocarbons

The binding energy of a hydrogen molecule on metal atoms (Li, Be, Na, and Mg) attached to aromatic hydrocarbon molecules (benzene and anthracene) was calculated using an ab initio molecular orbital method at the MP2(FC)/cc-pVTZ level with basis set superposition error (BSSE) correction. The energy tended to become more negative as the metal atom had a more positive charge and a smaller radius. The energies of Li₂C₆H₆-H₂, Li₂C₁₄H₁₀-H₂, Na₂C₁₄H₁₀-H₂, and MgC₁₄H₁₀-H₂ were −2.7 to −2.2, −4.0 to −3.1, −2.8 to −0.3, and −1.3 kcal/mol, respectively. Most of these energies were more negative than those on the hydrocarbons without metal atoms (ca. −1 kcal/mol). Analyzing the Lennard–Jones type potential with the parameters determined by the MP2 calculations, it was found that these energies mainly consisted of the induction force caused by the positive charge of the metal atom and the dispersion force from the nearest C₆-ring. The energy of BeC₁₄H₁₀-H₂ was more negative (−8.6 kcal/mol) than of the other complexes. The hydrogen molecule in this complex had a comparatively longer H–H distance and a more positive H₂ charge than the others. These data suggest that the hydrogen adsorption on this complex involves a charge transfer process in addition to physisorption interactions. The hydrogen binding energies in some Li₂C₁₄H₁₀-H₂ systems (∼−4.0 kcal/mol) and BeC₁₄H₁₀-H₂ are promising to operate hydrogen storage/release at ambient temperature with moderate pressure.     

基于正六边形多开口的新型双频带左手材料

本文提出了基于正六边形多开口的新型双频带磁谐振体.在微波衬底材料的一面放置交错多开口的两个正六边形金属环,多开口结构破坏了环间耦合电容,从而使两环形成相对独立的两个谐振网络实现双频带效应.最后将该谐振结构的另一面放置金属导线形成一个双频带的新型左手材料.文中利用HFSS软件仿真和等效参数提取的方法,分析和验证该结构的正确性.     

Metal Sulfide Cluster Complexes and their Biogeochemical Importance in the Environment

Aqueous clusters of FeS, ZnS and CuS constitute a major fraction of the dissolved metal load in anoxic oceanic, sedimentary, freshwater and deep ocean vent environments. Their ubiquity explains how metals are transported in anoxic environmental systems. Thermodynamic and kinetic considerations show that they have high stability in oxic aqueous environments, and are also a significant fraction of the total metal load in oxic river waters. Molecular modeling indicates that the clusters are very similar to the basic structural elements of the first condensed phase forming from aqueous solutions in the Fe–S, Zn–S and Cu–S systems. The structure of the first condensed phase is determined by the structure of the cluster in solution. This provides an alternative explanation of Ostwald’s Rule, where the most soluble, metastable phases form before the stable phases. For example, in the case of FeS, we showed that the first condensed phase is nanoparticulate, metastable mackinawite with a particle size of 2 nm consisting of about 150 FeS subunits, representing the end of a continuum between aqueous FeS clusters and condensed material. These metal sulfide clusters and nanoparticles are significant in biogeochemistry. Metal sulfide clusters reduce sulfide and metal toxicity and help drive ecology. FeS cluster formation drives vent ecology and AgS cluster formation detoxifies Ag in Daphnia magna neonates. We also note a new reaction between FeS and DNA and discuss the potential role of FeS clusters in denaturing DNA. An erratum to this article is available at .     

Si–LiNbO3–air–metal structure for concentrated terahertz emission from ultrashort laser pulses

A planar Si–LiNbO₃–air–metal structure is proposed as a further development of the highly efficient optical-to-terahertz conversion scheme in sandwich structures that was recently demonstrated. The new structure allows one to collect the terahertz emission into one spatial direction and to control its spectrum by varying an air gap between the metal substrate and the LiNbO₃ layer. While the overall increase in the terahertz generation can reach a factor of 2, the spectral density in the interesting for practical application interval 0.5–1.5 THz can be increased by a factor of 3.5–4.     

Solid state lithium ion conductors: Design considerations by thermodynamic approach

The most common previously employed methods of designing useful solid state lithium ion conductors (SSLICs) are reviewed and a new approach for the rational design of advanced SSLICs is described, which makes use of thermodynamic considerations. The described method is based on the Gibbs energy of formation of binary compounds of substitutional or additional cations (including dopants) and is demonstrated by the improvement of the lithium ion conductivity of SSLICs having perovskite-, NASICON- and Li₄SiO₄-type structures. Dopant metal oxides with higher negative Gibbs energies of formation than that of the parent metal oxide increase commonly the lithium ion conduction. The stronger binding forces of the oxide ion with the dopant cation result in an electrostatic shielding of the attractive forces between the lithium ions and the anions which facilitates the ionic motion. Irrespective of the crystal structure, it is expected that this thermodynamic rule holds also for other mobile ionic species. Paper presented at the 8th Euroconference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Investigation of plasmonics resonance infrared bowtie metal antenna

An approximate resonance wavelength equation that varies with metal antenna structure size is developed to design a bowtie gold metal antenna working at near-infrared (IR) wavelength. Bowtie antenna structures with resonance wavelength of 1.06 μm, 1.55 μm and 10.6 μm are designed based on this equation. A finite-difference time domain (FDTD) algorithm with total field scattered field (TFSF) source simulation shows the resonance wavelength of the designed structures being precisely in agreement with the expected wavelengths from the equation. Planar integration of the metal bowtie antennas is discussed as well. Gold nanohole bowtie antenna arrays are fabricated and the near-field optical transmission properties of the nanohole array are investigated with a near-field scanning optical microscope (NSOM). Our experimental results verify the near-field optical transmission performance and further demonstrate that they are in agreement with the theoretical calculation results. The high enhancement efficiency and integration of the metal bowtie antennas open the possibility of a wide application in IR optoelectronics detection and imaging.     

The x-ray LIII absorption discontinuity of ytterbium in the metal and in some of its compounds

The shape and fine structure of the x-ray L_III absorption discontinuity of ytterbium (Z=70) has been studied in the pure metal and in several of its compounds. The shape and the near edge structure of the discontinuity in the metal reflects the band structure. A molecular orbital interpretation for the fine structure near the absorption edge has been proposed for the spectrum of the sesqui-oxide. The average bond lengths for the systems studied have been determined from the measurements on the fine structure employing Lytle’s and Levy’s methods and are compared with the available crystallographic data in the case of the metal, oxide and fluoride.     

Transverse magnetic peak type metal-clad optical waveguide sensor

Transverse magnetic (TM) waves in a four-layer slab waveguide structure are studied for optical sensing applications. The structure consists of a semi-infinite substrate, a thin metal layer, a medium with negative permittivity and permeability as a guiding layer, and a semi-infinite layer as a cover. The proposed sensor is operated in reflection mode in which the angular position of the reflectance peak is used to detect small changes in the refractive index of the cover medium. The optimal structure parameters that correspond to the sharpest and highest peak are presented. The results reveal that for aluminum metal layer, a thickness of about 9 nm represents the optimum metal thickness. Moreover, the thickness, negative permittivity, and negative permeability of the guiding layer are found to have great impacts on the performance of the proposed optical waveguide sensor.     

Electronic structure and magnetic properties of SmCo7?xTix

Electronic structure of SmCo_7−x Ti_x alloy has been studied by means of the spin-polarized MS-Xα method. It is shown that a few of electrons are transferred to Sm(5d) orbital due to orbital hybridization between Sm and Co atoms. The exchange interaction between 5d-3d electrons is stronger, which is the main reason resulting in the long-range ferromagnetic order between Co and Sm atoms. The Curie temperature of SmCo_7−x Ti_x is generally lower than that of pure Co metal, which may be explained by the weaker average of coupling strength between Co sites due to some negative exchange couplings occurring mainly at 2e site. The calculated results for the Sm₅Co₂₈Ti₆ cluster may lead to a better understanding of why SmCo_7−x Ti_x is stable phase. Since the negative interaction of 2e sites weakens and the bonding at E _F strengthens with increasing Ti concentration, which result in the decrease in the free energy of the alloy, the stable ferromagnetic order forms inside SmCo_7−x Ti_x. Considering the localization of 4f states and 5d moment arising from the orbital hybridization, the calculated moment is 9.47 μ _B per formula unit that is in agreement with experiments.     

Lithium insertion/deinsertion mechanisms in the transition metal pnictides LixMPn4

This paper presents a combined experimental and computational study of the electrochemical reactivity of the ternary transition metal pnictides Li_xMPn₄ (M=Ti, V; Pn=P, As) towards Li. These materials show high specific capacities (500<C<1000 mAhg⁻¹) in their first charge/discharge cycles which are holded on to 80% after 20 cycles and are very promising as negative electrode material in Li-ion battery. The high reversible charge/discharge process is associated with a destruction/recovering of the long-range cubic order. On the basis of electronic structure calculations we rationalize this mechanism by means of the contraction/elongation of the M-Pn bonds of the strongly bonded Mpn₄ tetrahedra. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Inverse layer at the surface of n-AlxGa1−xSb solid solution

The surface properties of frequency oscillations of n-Al_xGa_1−xSb solid solution are discussed. The dependence of the barrier height for electrons at the metal/n-Al_xGa_1−xSb boundary on the composition of the solid solution is considered. In studying the characteristics of the surface-barrier structure, it is shown that the position of the Fermi level at the boundary with the metal and at the n-Al_xGa_1−xSb free surface is similar. Pinning of the Fermi level close to the top of the valence band in the solid solution with 0≤x≤0.3 leads to the appearance of a p-type inverse layer at the surface. Experimental proof of the existence of this layer is provided. The hole concentration at the n-Al_xGa_1−xSb surface is estimated, along with the thickness and surface conductivity of the inverse layer. Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 108–113, July, 1998.     

Investigation of the relative stability of solid phases in the La-graphite system based on the method of multicenter atom-atom potentials

The relative stability of crystalline phases of the lanthanum-carbon system with metal contents up to 33.3 at. % is investigated using the method of semiempirical atom-atom potentials. It is shown that the stability of these phases depends on the stoichiometry, the structure and degree of order of the lattice, and also on the crystal size. Among the bulk phases, the most stable is a phase with the calcium carbide structure, followed by phases with the metal graphitide and fluorite structures. In layers with thicknesses of a few nanometers, including 1–4 monolayer films, the energies of formation of lanthanum graphitide and carbide approach one another. This makes the phase transition from carbide to graphitide easier, especially when the film of lanthanum-carbon compound is grown on the surface of single-crystal graphite. Fiz. Tverd. Tela (St. Petersburg) 39, 1879–1882 (October 1997)     

Ionic and temperature aspects of the photoresponse of metallic clusters

A calculation of the electronic response of alkali metal clusters is carried out in the Adiabatic Time-Dependent Local Density Approximation. The role played by the ionic structure is investigated in the framework of second-order pseudopotential perturbation theory. The calculations are carried out at different temperatures, and the effect of temperature in the decay of the collective excitations is analyzed. It is found that the coupling of the electrons to the thermal vibrations of the ions accounts for the width of the plasmon resonances. Presented at the International Conference on “Atomic Nuclei and Metallic Clusters”, Prague, September 1–5, 1997.     

Formation of microtower structures on nanosecond laser ablation of liquid metals

We report the formation of microtower structures, observed on multishot nanosecond laser irradiation of liquid metals (Ga, In, Sn–Pb alloy, Wood’s metal). Ablation in a reactive ambient gas (air, nitrogen, sulfur hexafluoride, nitrogen trifluoride) is shown to lead to a tower-like structure growing on the irradiated surface at a rate of 3–20 μm per pulse depending on laser fluence and the types of metal and ambient gas. The interplay between different processes in the heat-affected zone of the irradiated samples is analyzed, including ablation, thermal expansion, temperature variations of viscosity, surface tension, thermal stresses, capillary effects, and surface chemistry. A clear picture of microtower origin has been established, and qualitative modeling can explain the formation mechanism.     

Sheath characteristics in multi-component plasma with negative ions

Properties of steady state ion sheath formed in front of a negatively biased metal plate under the influence of negative ions have been investigated in collisionless argon/SF₆ plasma. This experiment is carried out at a fixed discharge voltage and fixed filament heating power. In this experiment, the decrement in plasma pre-sheath potential drop as well as positive ion drift velocity toward the plate is experimentally recorded in the presence of negative ions. It is also found that the plasma positive ion density and plasma electron temperature decrease in the presence of negative ions. These factors attribute to the decrease of ion current toward the plate. Hence the usual ion sheath expands. Article presented at the International Conference on the Frontiers of Plasma Physics and Technology, 9–14 December 2002, Bangalore, India.     

Photocatalytic synthesis of silver nanoparticles using polysilane initiator

This study shows that the exposure to visible light of the poly[diphenylsilane-co-methyl(H)silane] solution together with a silver salt, initiates a photocatalytic process which leads to the formation of metal nanoparticles. This phenomenon is a consequence of close-range interactions between the methylhydrosilyls’ σ-conjugated segments and the metal ions at the salt surface. Due to the weak charge transfer complexes thin films casted from solution show a specific morphology with microdomains of various dimensions and shapes in relation with the stage of the process. The polymethylhydrosilane copolymer stabilizes the synthesized nanoparticles in a similar manner as the conventional surfactants do. The polymer chemical structure is not affected during the photocatalytic process and the optical and electronic properties of polysilanes are well preserved.