Optical transmission through metal/dielectric multilayer films perforated with periodic subwavelength slits

The transmission of p-polarized plane wave through Ag/SiO₂ multilayer films perforated with periodic subwavelength air slits is investigated by using the finite-difference time-domain (FDTD) method. The results show that the optical transmission property is mediated by the interference among the propagating coupled-SPP modes along the lateral direction inside the SiO₂ layers and the conditions of Fabry-Pérot-like resonance along the longitudinal direction together. When some geometric parameters are suitably initialized, the high transmission peaks can split into more peaks as the functional layer (metal/dielectric/metal sandwich stack) number increases, and the wavelength of the same-order transmission peak exhibits a red shift as the grating period increases.     

Metallic nonlinear magnetooptical nonreciprocal isolator

In advanced optical fiber communication systems where reflection light might cause degradation of the signal quality, optical waveguide isolators play an important role. The effect of a metal layer with negative permittivity on the behavior of nonlinear-magnetooptic isolator is studied. The isolator consists of metal film, nonlinear cladding, and magnetooptic substrate. It is found that difference between forward and backward propagation for TM₀ mode increases with increasing the absolute value of the tuning parameter which is the permittivity of the metal film, ?_f. It is also found that the maximum cut-off thickness of the isolator occurs in self-defocusing case around η = 0.65 and at the highest assumed value of ?_f = −8. These results are helpful in fabricating an isolator with high performance.     

Negative permittivity of ZnO thin films prepared from aluminum and gallium doped ceramics via pulsed-laser deposition

Aluminum and gallium doped zinc oxide thin films with negative dielectric permittivity in the near infrared spectral range are grown by pulsed laser deposition. Composite ceramics comprising ZnO and secondary phase Al₂O₃ or Ga₂O₃ are employed as targets for laser ablation. Films deposited on glass from dense and small-grained ceramic targets show optical transmission larger than 70 % in the visible and reveal an onset of metallic reflectivity in the near infrared at 1100 nm and a crossover to a negative real part of the permittivity at approximately 1500 nm. In comparison to noble metals, doped ZnO shows substantially smaller losses in the near infrared.     

Optical properties of one-dimensional photonic crystals containing graded materials

The optical properties of an one-dimensional photonic crystal containing graded materials are studied theoretically. The graded layers have space dispersive permittivity and magnetic permeability which vary along the direction perpendicular to the surface of the layer. The gradation profiles of permittivity are studied in detail. We show that the structure possesses forbidden band gaps in its transmission spectra and the gradation profiles of permittivity affect the band gaps significantly. For the exponential gradation profile ε₁(x) = α e^βx, the number of the band gaps increases and the total frequency region corresponding to the gaps becomes large with increasing parameter β. On the other hand, the position of band gaps can be changed by the adjustment of the gradation profiles even if possessing same volume-average permittivity in the graded layers. Therefore, we can achieve suitable photonic band gaps by choosing gradation profiles of permittivity.     

Specific features of the behavior of optical properties upon the metal-insulator transition in EuBaCo<Subscript>2</Subscript>O<Subscript>5+δ</Subscript>

The temperature dependences of the optical properties and the electrical resistivity for EuBaCo₂O_5+δ single crystals are investigated. At temperatures below the metal-insulator transition (T _MI = 340 K), the electrical resistivity is well approximated by the relationship ρ = ρ₀exp(T/T ₀)^1/4. The optical band gap E _g = 0.05 eV for the insulating phase is underestimated as compared to the theoretical value. The specific features in the dispersion of the optical conductivity and the real part of the complex permittivity upon the metal-insulator transition are determined. It is demonstrated that the optical response from charge carriers on the metal side of the metal-insulator transition is caused by the redistribution of the spectral weight of the optical conductivity from the high-energy range to the low-energy range and exhibits a strongly incoherent character. The revealed features are associated with the manifestation of the strongly correlated metallic state.     

Metal-diamond semiconductor interface and photodiode application

Carrier transport mechanism at p-diamond/metal interfaces are studied by analyzing dependencies of specific contact resistance (ρ_c) on measurement temperature and acceptor concentration (N_A). A variety of metals, such as Ti, Mo, Cr (carbide-forming metals), Pd, and Co (carbon-soluble metals), are deposited on boron-doped polycrystalline diamond layers, and the ρ_c values are measured by a transmission line method. Thermal annealing which produces metallurgical reactions between diamond and metal reduces Schottky barrier heights of the contact metals to a constant value. It is found that use of a metal compound which does not react with diamond at elevated temperatures is the key to develop the thermally stable Schottky contact material for p-diamond. Along this guideline, we test the suitability of tungsten carbide (WC) and hafnium nitride (HfN) as thermally stable Schottky contacts to develop a thermally stable, deep-ultraviolet (DUV) photodiode using a boron-doped homoepitaxial p-diamond epilayer. Thermal annealing at 500 °C improves the rectifying current-voltage characteristics of the photodiode, resulting in the excellent thermal stability. The discrimination ratio between DUV and visible light is measured to be as large as 10⁶ at a reverse bias voltage as small as 2 V, and it remains almost constant after annealing at 500 °C for 5 h. Metal carbide and nitride contacts for diamond are thus useful for developing a thermally stable diamond DUV photodetector.     

Excitation by light ofω + andω − surface plasma waves in thin metal layers

Calculations with the extended Fresnel formulae show, thatω ₊ andω _? surface plasma waves can be excited by light, using the following optical arrangement: The metal foil is embedded between two dielectric layers of low index of refraction and equal thickness. Both sides of this 3-layer packet are in optical contact with a high index prism. Total reflection is broken byω ₊ andω _? resonances in reflectivity, transmission and absorption. Theω ₊ resonance is mainly transmissive, (transmission through a 800 Å thick silver foil greater than 50% at a wavelength of 5,461 Å) theω _? resonance is mainly absorptive. The polarization of the transmitted light exceeds 99%-theω ₊ resonance is proposed as a principle for a new optical polarizer.     

Optimization of transverse electric peak-type metal-clad waveguide sensor using double-negative materials

In a very recent work, a transverse electric peak-type metal-clad waveguide optical sensor was proposed in which a double-negative material (DNM) was used as a guiding layer. The sensor was found to exhibit a considerable angular shift of the reflectance peak for small changes in the refractive index of the analyte, due to the DNM layer. In this work, the optimization of the structure parameters is investigated to find out the most appropriate metal and its optimal thickness. Moreover, the optimal DNM layer parameters corresponding to the highest sensitivity are explored. Our calculations reveal that metals with high absolute value of the real part of the permittivity correspond to sharper peaks. Moreover, as the absolute value of the real part of both ε and μ of the DNM increases, the reflectance peak becomes sharper and the dip following the peak becomes deeper.     

Making structured metals transparent for ultrabroadband electromagnetic waves and acoustic waves

In this review, we present our recent work on making structured metals transparent for broadband electromagnetic waves and acoustic waves via excitation of surface waves. First, we theoretically show that one-dimensional metallic gratings can become transparent and completely antireflective for extremely broadband electromagnetic waves by relying on surface plasmons or spoof surface plasmons. Second, we experimentally demonstrate that metallic gratings with narrow slits are highly transparent for broadband terahertz waves at oblique incidence and high transmission efficiency is insensitive to the metal thickness. Further, we significantly develop oblique metal gratings transparent for broadband electromagnetic waves (including optical waves and terahertz ones) under normal incidence. In the third, we find the principles of broadband transparency for structured metals can be extended from one-dimensional metallic gratings to two-dimensional cases. Moreover, similar phenomena are found in sonic artificially metallic structures, which present the transparency for broadband acoustic waves. These investigations provide guidelines to develop many novel materials and devices, such as transparent conducting panels, antireflective solar cells, and other broadband metamaterials and stealth technologies.     

Density functional studies of selected metal dioxides

This study investigates the optical properties of selected metal oxides due to their high dielectric constants. The local-spin-density approximation plus Hubbard U (commonly called LDA+U) is used in a study of the structural, mechanical and optical properties of UO₂. The inclusion of a Hubbard U correction to 5f electrons of uranium changes UO₂ from a metal to an insulator and, therefore, has a dramatic effect on the localisation of the electron spin and charge density of uranium. However although the band gap can be reproduced using the effective U parameter, which is equal to 3.5 eV and optical properties were calculated in our previous work, it is difficult to calculate ionic contribution to the static dielectric constant within LDA+U formalism for this compound. It is shown in the present work that the electronic structures of both ceria and thoria exhibit similarities to urania within LDA or PBE functional implementations. Within this functional and linear response theory one can easily calculate static dielectric permittivity and it is shown that in agreement with experiment the predicted values are an order of magnitude larger than the dielectric constant of SiO₂. In this work, high accuracy, first-principles calculations are also used to compare properties of urania versus ceria and thoria and how these similarities can help in understanding these compounds. It is also shown that the B3LYP functional predicts slightly overestimated band gaps for ceria and thoria as well as smaller than experimentally observed electronic contribution to the static dielectric constant, while the index of refraction is well reproduced for thoria.     

Mechanism of the polarization response in the relaxor state of lead scandotantalate single crystals with different levels of ion ordering

The dielectric properties and optical transmission of stoichiometric lead scandotantalate (PST) single crystals in strong electric fields was studied above the temperature of the spontaneous ferroelectric phase transition (T_sp). It is shown that the mechanism of polarization response directly above T_sp is related to induced polarization effects and macrohysteretic behavior only in ac fields above 5 kV/cm. Ananalysis of reciprocal dielectric permittivity carried out over a broad temperature range far above the temperature at which the dielectric permittivity passes through a maximum revealed that specific features of the relaxor behavior manifest themselves up to 400°C even in highly ordered PST crystals.     

Damping of optical phonons in metals and strongly doped semiconductors

The electron-phonon-induced damping of optical phonons arising in metals and strongly doped semiconductors under laser irradiation is investigated. The damping of both short-wave KV_F > ω₀ and long-wave KV_F < ω₀ optical phonons is calculated; K is the wavevector, ω₀ is the frequency of the optical phonon; V_F is the Fermi velocity. The electron- phonon-induced damping is important if the frequency of the optical phonon is larger than two frequencies of acoustic phonons of all branches in the range of the whole Brillouin zone. The damping of a soft transverse optical phonon in narrow-gap ferroelectric-semiconductors is also defined by the electron-phonon interaction. In other cases the main relaxation process for optical phonons in metals is the decay into two acoustic phonons due to lattice anharmonicity.     

Metal grating on a substrate nanostructure for sensor applications

Extraordinary optical transmission via periodic array of sub-wavelength slits in a metal layer on a dielectric substrate is revisited for sensing applications. Numerical case studies using an in-house software tool showed two peaks of enhanced transmission, one being very sensitive to the ambient index and another to the substrate index. Based on this, we designed and realized an optical sensor with sensitivity of the order of 400 nm/RIU.     

Effects of interplay between metal subwavelength slits on extraordinary optical transmission

In this paper we study the extraordinary optical transmission of one-dimensional multi-slits in an ideal metal film.The transmissivity is calculated as a function of various structural parameters.The transmissivity oscillates,with the period being just the light wavelength,as a function of the spacing between slits.As the number of slits increases,the transmissivity varies in one of three ways.It can increase,attenuate,or remain basically unchanged,depending on the spacing between slits.Each way is in an oscillatory manner.The slit interaction responsible for the oscillating transmission strength that depends on slit spacing is the subject of more detailed investigation.The interaction most intuitively manifests as a current distribution in the metal surface between slits.We find that this current is attenuated in an oscillating fashion from the slit corners to the center of the region between two adjacent slits,and we present a mathematical expression for its waveform.     

Optical properties of subwavelength metallic-dielectric multilayers

We present studies on the optical properties of periodic metallic-dielectric (MD) multilayers and numerical results show that there exists, insensitive to the lattice scaling, a transparent band as long as the layer thickness is in the subwavelength ranging. It illustrates the transparent band is controlled by mechanisms beyond the Bragg scattering: the shorter-wavelength band edge comes from the intensive resonant absorption behavior of the metals, while the longer-wavelength band edge is determined by zero (volume) averaged permittivity εeff=0. Moreover, a Lorentz-Drude model for the permittivity of a ε-negative (ENG) metamaterial is used to show that a transparent band may be obtained in a subwavelength structure consisting of ENG multilayers with total length less than both the center wavelength and the half width of the band.     

Eine Methode zur Bestimmung des Verhältnisses σ T /σ S =α der totalen Streuquerschnitte für Streuung von Alkaliatomen thermischer Energie am Triplett- und am Singulettpotential

It has been proved experimentally that the flux of atoms emerging through a hole from a cell containing Rubidium vapour depends on the vapour's spin polarisation. The dimensions of the cell have been smaller than the mean free path of the atoms in order to avoid multiple collisions. The atoms emerged from the cell through a system of aligned slits; the atomic flux was measured by means of a Langmuir-Taylor-Detector. A sufficiently large slit in the rear of the cell opposite to the beam-forming system of slits assured that no atoms in the beam could come from collisions with the cell surface. Hence all atomic particles had to come from interatomic collisions which happened inside the volume of vapour. As these atoms were polarized by using optical pumping techniques the flux of atoms in the beam turned out to be dependent on the polarisation achieved. This dependance could be explained in the following way: First we notice that all the collisions among entirely polarized spin 1/2-particles must occur in the triplet state of the compound molecule whereas unpolarized spin-1/2-particles collide in three of four cases in the triplet state and in one case in the singlet state. Secondly, according to the difference of the triplet- and the singlet-potential, the respective cross sectionsσ _T andσ _S are unequal. Consequently, the flux of atoms scattered from the cell into any direction must depend on the polarisation since the polarisation governs the share of singlet- and triplet-scattering in the total scattering process.     

Percolation phenomenon in barium samarium titanate-silver composite

Ba₄Sm_9.33Ti₁₈O₅₄-Ag (BST-Ag) composites were prepared by a solid-state ceramic route and its dielectric properties were investigated in the vicinity of percolation threshold. The structure and microstructure of the composites were analyzed by X-ray diffraction along with optical and scanning electron microscopy observations. The effects of silver content and frequency on the dielectric properties of BST-Ag composites were studied using a LCR meter. The relative permittivity (ε_r) of the composite increases with silver content below the percolation limit and is in agreement with power law. A 0.14 volume fraction of silver loading increases the relative permittivity of the composite from 50 to 450 at 10 kHz. Addition of 0.15 volume fraction of silver increases the relative permittivity of the composite in the order of 10⁵. It is found that the giant relative permittivity is almost constant for frequencies from 1 kHz to 1 MHz. This high ε_r composite offers the perspectives for application in electromechanical devices.     

Optical properties of electron Fermi-gas of metals at arbitrary temperature and frequency

Optical properties of Fermi-gas of metals are considered at arbitrary temperature (T ≥ ? _F ). The expressions for temperature and frequency dependences of permittivity are obtained from the solution of the quantum kinetic equation. Frequency and temperature dependences of reflectivity of irradiated surface and volume factor of absorption are determined using Frennel’s formulae.