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This paper reviews recent progress in perfect absorbers of electromagnetic waves that utilize metamaterials (MMs). Various numerical techniques that are inevitable for the research on MMs, along with their pros and cons, are commented on and discussed. Abandoning the conventional periodic-boundary conditions is crucial for understanding the discrepancy between simulation and experiments for certain MMs. Broadband perfect absorption can be realized by using multiple resonances and/or multiple units with different sizes in a unit cell, or by utilizing bad Q-factors by incorporating the electrically resistive components, in addition to the ordinary metallic elements. The classical analogue of electromagnetically-induced transparency can be utilized to realize perfect absorption, by employing either structural asymmetry, or near-field interactions. Perspectives and suggestions for further study are discussed. [ABSTRACT FROM AUTHOR]

Micropatterned ordered arrays of cobalt (Co) anti-dots on a uniform thin nickel (Ni) underlayer, known as bilayered magnetic anti-dot lattices (BMALs), were fabricated using photolithography and a controlled wet-etching process. The magnetization reversals in a rhomboid BMAL under the application of a field along 0° (i.e., easy axis) and 90° (i.e., hard axis) were investigated using field-dependent magnetic-force microscopy, and the angular dependence of squareness (Mr/Ms) and coercivity (Hc) were studied using magneto-optical Kerr effects. Although the magnetic reversals were dominated by domain-wall motions or domain rotations, reflecting the easy and hard axis characteristics, various other complex reversal processes were also found that depends on the history of the field application. The angular dependence of Mr/Ms and Hc in a BMAL system was completely different from that in a single-layer, Co magnetic anti-dot lattice. Interestingly, a unidirectional and uniaxial component representing the overall magnetic anisotropy was revealed in the BMAL system. The details of these complicated magnetization behaviors were investigated and elaborated. [ABSTRACT FROM AUTHOR]

We experimentally and numerically studied the transmission spectra of a combined metamaterial structure whose negative refractive index was supposed to be achievable only with simultaneously negative permittivity and permeability. However, it was found that the negativity of the refractive index of such a structure could be obtained not only when both parameters were negative but also when only one parameter, more specifically the permittivity was negative. These characteristics of combined structure were analyzed in detail by using the standard retrieval effective-medium method. According to the analyses, it can be concluded that the negativity originates from the complex permittivity and permeability. The interplay among the real and the imaginary parts of those parameters is the key to the negative behavior of refractive index. [ABSTRACT FROM AUTHOR]

We studied both theoretically and experimentally the influence of the lattice constant on the resonance frequencies of cut-wire-pair structures, which are essential components in assembling left-handed materials. These structures were designed and fabricated, and the transmission spectra were measured in the microwave-frequency regime. Numerical simulation was performed using the transfer-matrix method. All the numerical results were in good agreement with the experimental data. It was found that the lattice parameters of the cut-wire-pair structure provide sensitive effects on the electric resonance, especially, in the propagation k and the electric field E directions, while the magnetic resonance frequency is nearly unchanged, except for its bandwidth and depth. In addition, we also studied the effect of dielectric layer thickness on the resonance frequencies of the cut-wire-pair structure. These sensitive changes in the electric resonance might have remarkable effects on the effective plasma frequency and therefore on the left-handed behavior of combined structures consisting of cut-wire pairs and continuous wires. [ABSTRACT FROM AUTHOR]

The dielectric functions of Co2Si, CoSi, and CoSi2 compounds have been measured and compared with the calculated ones. All three compounds have very common features in their density of states, especially near the Fermi level: Co 3d-dominated characteristics with very small amounts of Co and Si sp characteristics. Consequently, the features observed in the optical conductivity (OC) spectra of 0.5–5.0 eV for all the compounds also have very similar characteristics. Since in this energy range both the occupied and empty states involved in the interband-absorption peak formation have a 3d character in an energy range, the major factor contributing to the features observed in the OC spectra is the joint-density-of-states (JDOS) effects, and the optical-transition-matrix elements do not play an important role. Co2Si, whose crystal structure is orthorhombic, has a strong optical anisotropy, as expected. The structural dependence of the electronic band structure for Co silicides has also been evaluated by examining the optical properties of the disordered and the crystalline Co-Si alloy films of the CoSi and CoSi2 stoichiometry. The change in the OC spectra can be explained by “simple” JDOS effects in the DOS curves. [ABSTRACT FROM AUTHOR]

An approach named the defect-induced extension of photonic band gaps (PBGs) is proposed, in which a stratified periodic structure is combined with another stratified structure containing defect layers. For comparison, three structures composed of TiO2 and SiO2 alternating layers, perfect, five-defect and combined-photonic crystals (PCs), were fabricated by using an electron-beam evaporation system. The measured PBG width of combined-PCs extended to 370 nm at the normal incidence, nearly three times more than that of perfect ones, 130 nm, but a few ripples are observed within the PBG. In order to elucidate the origin of the ripples, the influence of the refractive-index contrast on the reflectance was analyzed theoretically. When the refractive-index contrast nH/nL≥1.7, the ripples are eliminated. More importantly, an overdoubled omnidirectional PBG is realized at nH/nL=2.56, covering the visible region completely. This approach is considered to be based on the light localization and a heterostructure resulting from introducing the impurity band and compensating for it using PCs with a strictly periodic structure, in which the thickness of every layer remains unchanged. These characteristics distinguish it from other methods proposed before, with the fabrication much easier. [ABSTRACT FROM AUTHOR]

The structural dependence of the magnetic, transport, optical, and magneto-optical (MO) properties of Cu2MnAl Heusler alloy (HA) thin-film samples was investigated. Disordered thin films of Cu2MnAl HA fabricated by vapor-quenching deposition onto substrates cooled by liquid nitrogen behave as highly resistive Pauli paramagnets down to 5 K, while an annealing of the disordered films at 580 K for 1 h restores the L21-type ordered structure, as well as the ferromagnetic order and other physical properties typical for the ordered Cu2MnAl HA film. The film deposition onto substrates held at T>=690 K leads to the formation of a multiphase crystalline structure. A significant influence of the structural disorder on the optical and MO properties of Cu2MnAl was experimentally observed and discussed in terms of the electronic structure of the alloy. [ABSTRACT FROM AUTHOR]

The structural dependence of magnetic and transport properties of nearly stoichiometric Ni[sub 2]MnGa alloy films prepared by flash evaporation is investigated. The films deposited at 720 K have the L2[sub 1] Heusler structure and behave nearly the same as the bulk Ni[sub 2]MnGa alloy, while the films deposited at 150 K have an almost amorphous structure. Disordered Ni[sub 2]MnGa alloy films do not exhibit magnetic ordering down to 4 K, but rather have the characteristics of highly resistive Pauli paramagnets. Annealing of disordered films restores the L2[sub 1] structure. The magnetic and transport properties of the annealed films are similar to those of the films deposited at 720 K. A possible explanation of the loss of magnetic moment in the disordered film is discussed with the aid of first-principles electronic-structure calculations. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The transport and optical properties of Ni[sub 2]MnGa alloy in the martensitic and austenitic states were investigated. The dependence of the temperature coefficient of resistivity on temperature shows kinks at the structural and ferroparamagnetic transitions. Electron-magnon and electron-phonon scattering are analyzed to be the dominant scattering mechanisms of the Ni[sub 2]MnGa alloy in the martensitic and austenitic states, respectively. It was observed that the martensite reveals a higher Debye temperature compared to the austenite, which was explained by a difference in the restoring coefficient between atoms. A Lorentz model that consists of two oscillators was employed to fit the interband contribution to optical conductivity (OC). It is found that the structural transition plays a more important role for the changes in OC spectrum than the thermal-expansion effect. [ABSTRACT FROM AUTHOR]

The influence of structural disorder on the magnetic, transport, and optical properties of Ni[sub 0.50]Al[sub 0.50] alloy films has been investigated. A significantly disordered state was obtained by using vapor-quenching deposition onto substrates cooled by liquid nitrogen. The loss of translational invariance in the disordered state leads to a prominent increase of the magnetic moment of alloy below 50 K. This growth can be explained by the appearance of antistructure Ni atoms (or their clusters) in the disordered state of alloy and their ferromagnetic coupling below 50 K. The temperature dependences of resistivity for both ordered and disordered states of the Ni[sub 0.50]Al[sub 0.50] alloy films exhibit the resistivity minimum at 17-18 K, which has a nonmagnetic nature and can be related mainly to the quantum corrections to the electron-electron interactions in the presence of weak localization. It was shown that the resistivity of the ordered Ni[sub 0.50]Al[sub 0.50] alloy films in 50-300 K temperature range originates mainly from the electron-phonon scattering, while in the disordered state the contribution from the electron-phonon-vibrating impurity scattering becomes dominant. The structural disordering also leads to a noticeable change in the optical properties of alloy, especially in the infrared region. The observed temperature and structural dependences of the resistivity as well as the optical properties in the intraband region confirm the thesis on partial localization of the electronic states near the Fermi level. [ABSTRACT FROM AUTHOR]

The influence of the structural disorder on the transport properties of Co[sub 0.50]Ti[sub 0.50] alloy films in a temperature range of 4.2-300 K has been investigated without and with a magnetic field of 0.5 T. The absence of translational invariance in a disordered state leads to an increase in the resistivity and causes a change from the positive to negative temperature coefficient of resistance. This fact is explained by the partial localization of electronic states near the Fermi level. It was established that a partial structural disordering enhances the role of the electron-phonon-vibrating impurity scattering in the transport properties and also makes the spin-diffusive scattering rather noticeable. The appearance of a low-temperature resistivity minimum for the disordered Co[sub 0.50]Ti[sub 0.50] alloy film arises from the quantum corrections to the electron-electron interactions in the presence of weak localization. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The structural dependence of magnetic and transport properties of nearly stoichiometric Ni(sub 2)MnGa alloy films prepared by flash evaporation is investigated. The film deposited at 720K has almost the same properties of bulk stoichiometric Ni(sub 2)MnGa alloy, while the film deposited at 150 K is substantially disordered and behaves like a highly resistive pauli paramagnet.

The magnetic properties of near-equiatomic Co-Ti alloys were studied by using the first-principles electronic structure calculations. A formation of the antistructure Co atom and its cluster plays an important role to determine the ferromagneti ferromagnetic behavior of the disordered equiatomic alloy or off-stoichiometric Co-rich alloys. A small but considerable amount of charge transfer from the Ti to Co atom is crucial to the structural stability and the magnetic properties of the alloy system. A sharp peak in the calculated density-of-states curve near the Fermi level grows as the Ti concentration decreases, indicating a strong localization of the states, which gives a better understanding of the change in physical properties upon an order [right arrow] disorder transition. Index Terms--Antistructure atom, electronic structures, magnetic properties.

We report the results of a joint experimental and theoretical investigation focused on the magneto-optical (MO) properties of one-dimensional magnetic grating structure made of Ni. It was found that the longitudinal Kerr rotation of the second-order diffracted beam is nearly three times larger than that of the zeroth-order beam. The calculational results further confirmed the experimental ones, and almost perfectly reproduced the measured hysteresis loops of the longitudinal MO Kerr rotation, elucidating the origin of the enhanced MO rotation. [ABSTRACT FROM AUTHOR]

The magneto-optical properties of one-dimensional magnetic grating structure of Co2MnSi film were investigated. By using the interference pattern of two femtosecond laser beams, a selective-area annealing of as-deposited Co2MnSi film was achieved and one-dimensional magnetic grating structures were fabricated. The longitudinal Kerr rotations of the zeroth- and the first-order diffracted beams were measured. The longitudinal Kerr rotation of the first-order diffracted beam is nearly 18 times larger than that of the zeroth-order beam. [ABSTRACT FROM AUTHOR]

A dimerlike-positional-correlation heterostructure is used in one-dimensional photonic crystals to introduce the defect mode with perfect transmittance. The switching behavior of the transmittance, at the defect mode, is demonstrated theoretically. When the normal incident beam is tilted at a negligibly small angle, the perfect transmittance peak vanishes. It is found that this condition causing this phenomenon can be easily met and controlled through elucidation on the mechanism. This finding is significant for potential applications in high-precision filters and optical switches. [ABSTRACT FROM AUTHOR]

We investigated the effect of ion-beam mixing (IBM) on the physical properties of Fe-Si multilayerd films (MLF). By the magnetic, magneto-optical, optical and structural studies, it was found that the IBM results in a formation of metastable B2-phase Fe[sub 2]Si with a perfect crystalline structure, a low coercivity and a Curie temperature of about 550 K. An annealing at 720 K of the ion-beam mixed Fe-Si MLF further destroys the layered structure at the bottom and also decomposes Fe[sub 2]Si to form a metastable magnetically-hard Fe[sub 5]Si[sub 3] phase and, presumably, Fe[sub 3]Si. [ABSTRACT FROM AUTHOR]

The magnetic properties and the optical conductivity spectra of the ordered and disordered Co[sub 1-x]Al[sub x] (x=0.5, 0.46, and 0.38) alloy films were measured. The ordered off-stoichiometric alloy films exhibit a ferromagnetic behavior and the magnetic property is enhanced when the disorderedness is introduced. The electronic band structures, magnetic moments, and optical conductivity spectra of the ordered Co[sub 1-x]Al[sub x] (x=0.5, 0.4375, and 0.375) alloys were calculated using the linearized-muffin-tin-orbital method. The calculated magnetic moments were in a reasonable agreement with the experiments. Both the antistructure Co atoms and their neighboring eight Co atoms contribute to the magnetic moments. The charge transfer between Co and Al atoms is closely related to the magnetic behavior. The minority-spin bands of the antistructure Co atoms form a very intense and sharp density of states peak at the Fermi level, resulting in the magnetic moments. The evolution of the low-energy structures below 2 eV in the measured optical conductivity spectra upon decreasing the Al concentration is explained by an increased number of the antistructure Co atoms. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]