Photonic absorption bands in the spectra of nanoporous metallic films

The light absorption spectra of multilayer nanoporous metallic films containing spherical dielectric inclusions are calculated in the framework of the self-consistent electrodynamic approach (the Korringa-Kohn-Rostoker method). It is demonstrated that the coupling between Mie plasmons in different layers of nanopores leads to the formation of strong light absorption bands over a wide range of frequencies.     

Crystallographically-oriented single-crystalline copper nanowire arrays electrochemically grown into nanoporous anodic alumina templates

Highly crystallographically-oriented single-crystalline copper nanowire arrays were electrochemically deposited into nanoporous commercial alumina templates. A gold/copper backward contact was needed in the template, while the nanowires were grown from a 0.5 M CuSO₄·5H₂O solution adjusted to pH=1. The kinetics of the growing process is studied by means of current vs. time curves. The pore filling is between 80 to 90%. The structure and morphology of the wires are studied by XRD, SEM and TEM. The wires have an average diameter of 150 nm corresponding to the pore diameter of the template, with the cubic face-centered copper structure. This structure is highly oriented along the [100] direction parallel to the wire axis. The preferential growing along this direction was not previously found in the literature, which may have interesting applications when such direction is required. PACS 81.05.Bx; 82.45.Qr; 81.07.-b     

Bloch-Floquet waves and controlled stop bands in periodic thermo-elastic structures

An algorithm for controlling the stop bands for elastic Bloch-Floquet waves within a periodic structure is proposed. Explicit asymptotic estimates of frequencies of translational and rotational standing waves, together with the numerical estimates of the stop band frequencies, are given. Thermal pre-stress is introduced and used to control the position of the stop bands on the dispersion diagram.     

Resonant transmission in stop bands of acoustic waves in periodic structures

Acoustic resonant transmission (RT) phenomenon found in Bragg couplers made of alternatively high (H) and low (L) impedance materials is theoretically investigated. The existence of the RT within the Bragg stop bands is analytically demonstrated, along with the exact peak values expressed in closed form in terms of the coupler parameters. RT takes place either when the coupler is terminated with an L-layer and has its bottom surface free, or when it is terminated with an H-layer and the bottom clamped. Numerical results are given for the transmission and reflection rates in Bragg couplers made of tungsten and SiO(2) layers.     

Spectroscopy of stop bands in magnetic photonic crystals

The shape of the spectra of continuous visible-range radiation reflection from the surface (111) of a globular photonic crystal filled with magnetic materials was determined. The bands observed in the reflectance spectrum correspond to the appearance of stop bands of the photonic crystal. It was shown that position of the intensity maximum in the observed reflectance spectra depends on the type of a magnetic material introduced into photonic crystal pores.     

Photoluminescence of f-centers in films of anodic alumina

Results are reported from a study of the optical properties of porous alumina films obtained by anodizing in a water solution of sulfuric acid and modified by thermal annealing in air at T ≥ 850°C. A comparative analysis of the data shows that the near-UV and visible photoluminescence of alumina anodized in a sulfuric acid solution is caused primarily by oxygen divacancies (F₂, F₂⁺ _2^{+} , and F₂^{2 +} _2^{2 + } centers), while sulfate ions have little effect on the luminescence properties of anodic alumina in this spectral range.     

Optically tunable photonic stop bands in homogeneous absorbing media

Resonantly absorbing media supporting electromagnetically induced transparency may give rise to specific periodic patterns where a light probe is found to experience a fully developed photonic band gap yet with negligible absorption everywhere. In ultracold atomic samples the gap is found to arise from spatial regions where Autler-Townes splitting and electromagnetically induced transparency alternate with one another and detailed calculations show that accurate and efficient coherent optical control of the gap can be accomplished. The remarkable experimental simplicity of the control scheme would ease quantum nonlinear optics applications.     

Laser micromachining of porous anodic alumina film

A laser direct-write process on porous anodic alumina film is carried out for the fabrication of microstructure on the film, using nanosecond, second harmonic Nd:YAG laser. Laser micromachining can be preformed in two different ways on colored and pore-sealed anodic alumina film, to generate microstructures on the film. Removal of the aluminum substrate before laser irradiation greatly improves the shape characteristics of the microstructure. The depth of microstructures can be controlled by the power and the scanning speed of the laser beam. Several structures with depths from 1 to 35 μm were fabricated on anodic alumina film with good precision and reproducibility. PACS 79.20.Ds; 82.45.Cc; 42.62.Cf     

Structure of alumina nanowires synthesized by chemical etching of anodic alumina membrane

We report the structure of alumina nanowires (ANWs) synthesized by the chemical etching of an anodic alumina membrane (AAM). The AAM was prepared by the two-step anodization method followed by the lift-off process. The field emission transmission electron microscopy analyses showed that the AAM consists of a gel-like structure, an outer layer, and an inner layer, which are distinguished by their anion impurity concentration. The fabricated ANWs appeared to have a two-oxide-layer structure, similar to the core-shell structure in a coaxial cable. The inner oxide layer may be composed of relatively pure alumina and the outer oxide layer of the byproducts of the reaction between alumina and the anodization or etching solution. The fabricated ANWs have a flexible nature, with some of them being sufficiently malleable to form L-shaped ANWs. One possible formation process of the two-oxide-layer structured ANW is discussed.     

Abnormal relaxation kinetics in D-mannitol glass confined by nanoporous alumina

The relaxation kinetics and phase transformations of the confined D-mannitol(DM) in nanoporous alumina are studied in-situ using a high-precision nano-calorimeter. We find that the crystallization behavior can be suppressed when it is confined in nanopores smaller than 50 nm. The confined DM glass has a much smaller fragility(～76) than free DM glass(～125), confirming the enhanced glass-forming ability. It is intriguing that during isothermal annealing both the confined and free DM glasses relaxation kinetics experience two relaxation stages that have distinct activation energies. The relaxation activation energy of the confined glass is about 25%-29% smaller than the free glass, which is attributed to the reduced dimensionality. The abnormal kinetics observed in the confined DM glass open a new avenue for preparing stable glasses.     

Birefringence of nanoporous alumina: dependence on structure parameters

We report on experimental and theoretical investigations of the birefringence of free-standing nanoporous anodic alumina membranes in the optical range. The value of birefringence is analyzed for the samples with different porosities by measuring polarization dependent transmission spectra at different angles of incidence. The experimental data are compared to the results of birefringence simulations in accordance with the modified Bruggeman effective-medium approximation. It is both experimentally and theoretically shown that the birefringence value increases with porosity increases in the low porosity region. The porous alumina samples under investigation possess the greatest value of birefringence (0.062) up to the present. PACS 78.20.-e; 78.67-m; 78.20.Fm; 78.20.Ci     

Roughness evolution of highly ordered nanoporous anodic aluminum oxide films

In this paper, we report on the surface roughness evolution of highly ordered anodic aluminum oxide (AAO) films based on an atomic force microscopy (AFM) study. Root mean square of the surface roughness was measured on AFM images taken from highly ordered AAO films produced by two-step anodization under different conditions including electrolyte type, anodization voltage, and anodization time. Surface roughness of highly ordered AAO films increases step by step through the two-step anodizing process including electropolishing, first-step anodization, dissolution, and second-step anodization. However, increase of the surface roughness is proportional to the anodization voltage and time. The surface roughness of AAO films changes as a function of length scale until it finally approaches a maximum termed the saturation roughness. The variation of roughness of the growth of AAO could be scaled with an anomalous dynamic behavior as it saturates over a critical length scale while the saturation roughness is dependent on the anodizing time and voltage.     

Energy transfer in porous anodic alumina/rhodamine 110 nanocomposites

We have used porous anodic alumina (PAA) films as templates for embedding rhodamine 110 (Rh110) molecules and examined their photoluminescence (PL) properties in detail. The analysis of the polarization memory (PM) of PL strongly suggests that there is a significant energy transfer from PAA to Rh110 molecules. The effect of annealing the PAA layer on the PL properties of the nanocomposite has been studied. The results show that the energy transfer becomes more efficient in annealed PAA.     

Excitation and emission processes of Tb in porous anodic alumina

The strong photoluminescence (PL) of porous anodic alumina (PAA) with terbium deposition is reported. PAA, which has a regular pore morphology, is considered an effective template for fabricating luminescent composites. Tb was deposited onto PAA films by immersion in alcoholic solution with terbium chloride followed by heat treatment. The PL spectra demonstrate typical bands of Tb³⁺ corresponding to ⁵D₄ → ⁷F_j (j = 3, 4, 5, 6,) electron transition, with the maximum at 18,360 cm⁻¹ (⁵D₄ → ⁷F₅). The PL mechanism of Tb³⁺ was systematically studied with annealing temperature. The non-radiative relaxation channel is provided by OH hydroxyls at the surface of porous anodic alumina and, after annealing at 900 °C, the PL yield is highly improved. The PL intensity of Tb³⁺ increases with laser power and a saturation phenomenon, associated with the ratio of Tb³⁺ to Tb⁴⁺ ions, is observed at approximately 90 W/cm². Based on a theoretical model, the optical cross-section σ of terbium in PAA is estimated, with a value close to that of other porous materials doped with the rare-earth elements.     

Nanoporous alumina membrane prepared by nanoindentation and anodic oxidation

The fabrication of nanopatterned surfaces at large scale attracts the interest of research groups from a wide range of areas as biotechnology, nanoelectronics and nanomagnetism. An extended method to pattern the surface in the nanoscale is the fabrication of ordered arrays of nanoelements based on porous templates as Nanoporous Anodic Aluminium Oxide (NAAO). One of the challenges of the NAAO fabrication, based on self-organized methods, is the control of the symmetry and lattice parameter of the ordered nanoporous films. In this work, we present a combined method based on Atomic Force Microscopy (AFM) nanoimprint and anodic oxidation of Al surface. AFM nanoindentations substitute the first anodization process and even more important, allow us to control the symmetry and the lattice parameter of the ordered arrays. In addition, by using AFM nanoimprint method it is possible to select the region were the ordered alumina grows. We demonstrate that square nanoporous arrays of alumina with lattice parameter of 105 nm can be obtained by this method.     

Photoluminescence of anodic alumina membranes: pore size dependence

The photoluminescence (PL) of anodic alumina membranes with different pore sizes is investigated in this paper. We observe that a small pore size leads to a decrease of photoluminescence intensity, an increase of the recombination lifetime, and a broadening of the emission band. The origin of the PL characteristics are discussed together with the decay measurement of the emission band. PACS 78.47.+p; 78.55.Mb     

Nanostructural and photoluminescence features of nanoporous silicon prepared by anodic etching

The nanostructural and photoluminescence (PL) features of nanoporous Si (NPS) were investigated in terms of various process parameters such as current density, etching time and oxidation conditions. The NPS was prepared by electrochemical anodic etching of p-type (0 0 1) Si wafers of 4 Ω cm resistivity in HF solution. The pores are of polygon-type columns with 5, 6 and 7 side walls. The average diameter of the column-shaped pores is critically determined by the current density, while the etching time plays an important role on the pore depth; in particular, when the current densities of 30 and 100 mA/cm² were applied, the pore diameters were 9 nm and 3.3 μm, respectively. The variation in the PL characteristics of the NPS with oxidation condition and etching current density was measured and then related with their structural changes. The aging and thermal treatments produce oxidation and lattice distortion in the NPS. The degree of deviation from the as-prepared NPS during aging or thermal treatment seems to depend on the nanostructure as well as morphology of the NPS. It is found in this study that etching current density plays an important role on such structural features of the NPS.