Evanescent Plane Waves in Multilayer Structures

We have considered evanescent plane waves in structures with a layer of a substance with ε, μ < 0 and with a layer of a well-reflecting metal, ε < 0, μ ≥ 1. Waves with increased amplitude as compared with the initial wave have been found to occur, due to which evanescent waves with wave number as in the initial wave but with increased amplitude arise behind these layers. A composite material with ε, μ < 0 at optical frequencies are proposed. Surface waves on a metal layer are considered in detail. It is shown that surface waves with a sufficiently arbitrary wave number can be excited. It is also shown that, on very thin layers, surface waves with wave number exceeding ten times that of a homogeneous plane wave in vacuum can be excited. Propagation losses are calculated. For a silver layer, the wave path can be from 30 up to 100 wavelengths. Practical use in developing techniques for optical transformations of short-wave surface waves in 2D space, similar to those in 3D space, are pointed out.     

Calculation of effective material parameters by field averaging over lattices with non-negligible unit cell size

The Pendry et?al. field averaging method for calculation of effective material parameters is reviewed and its limits explored. The method is then extended so that it can accurately calculate the effective material parameters of lattices where the unit cell size is appreciable but still quasistatic (d??0.1?? ₀). The new algorithm is verified by calculating the effective material properties of periodically placed particles suspended in free space, as the unit cell size becomes appreciable. Results of our proposed formulation are then compared with the Pendry et?al. and conventional volumetric averaging algorithms.     

Near-field photoluminescence of Si-doped GaAs

We have measured surface photoluminescence properties of Si-doped bulk GaAs using a near-field scanning optical microscope. An apertured fiber probe tip is used as an emitter of excitation laser as well as a collector of luminescence from GaAs. Due to the Fabry-Perot etalon effect, the excitation laser is reflected or transmitted with an oscillation period of λ._He-Ne/2 as the gap between the tip and the GaAs surface varies. The luminescence from GaAs also varies with an oscillation period of λ_GaAs/2 due to the same etalon effect. Therefore, the intensity of luminescence light collected by the probe tip shows a beating between two oscillations of different periods. When the probe approaches the GaAs surface, the collected luminescence intensity increases due to tunneling of evanescent wave. On the other hand, when we collect the luminescence using a lens, the intensity also increases due to similar coupling of evanescent wave into propagating wave in spite of a shadowing effect of the wide metal coating.     

Slab waveguide with air core layer and anisotropic left-handed material claddings as a sensor

A three-layer slab waveguide with air core layer and anisotropic left-handed material claddings is investigated for sensing applications. Different from the waveguide mode sensors and surface plasmon resonance sensors in which the analyte is placed in the evanescent field region, the proposed sensor contains the sample in the core region that supports the oscillating field. Due to the strong concentration of the electromagnetic field in the analyte medium, the proposed device exhibits unusual sensitivity enhancement. The simulations revealed that the sensitivity improvement of TE₃ mode compared to conventional evanescent wave sensor is approximately a factor of 20.     

Parametrische Anregung von elektrostatischen Oberflächenwellen in einer inhomogenen Plasmaschicht durch ein amplituden-moduliertes UHF-Pumpfeld

Parametric resonance phenomena are investigated in a plasma layer with thickness d and thin inhomogeneous boundary regions. The modulated UHF electric field is parallel to the plasma layer. We consider both strong and low modulation of the field amplitude and suppose, that the carrier frequency ω₀ of the pump wave is much larger than the Langmuir frequency ω_Le. We find the region for the modulation frequency ω, in which the parametric growing of the asymmetric and symmetric surface waves occurs. The maximum growth rates of these waves, the direction of their propagation and the threshold value of the modulation depth α of the UHF pump field are calculation.     

Effects of a lossy thin plasma-film on metal-diffused LiNbO3 optical waveguides

I intend to investigate the effects of different kinds of partially reflecting metal film (plasma layer) on the wave characteristics of a stratified titanium-diffused LiNbO₃ optical waveguide. The mode number, the equivalent indices, the extinction coefficients, the field distributions and the powers carried in each layer are given as a function of the free-space wavelength, of the diffusion parameters, and of the opticc-axis orientation. Of course, the power of TM-like hybrid waves flows in opposite directions in the plasma and the other regions.For ac-axis varying in the transverse plane, the ith-order hybrid mode is rather complicated in the guide with an aluminum layer. There, in fact, the field mapping can be seen as an overlap of a TE _i with a TM _i+1, mode because of a sudden transformation of the TM₀ mode into a superficial plasma wave (SPW). On the contrary, with a silver layer, the hybrid field is a simple combination of a TE _i with a TM _i mode with no SPW growing.The guide losses assume the lowest values (1 dB/cm) for an Ag layer which is the prime candidate for making electro-optical or acoustic-optical devices. On the other hand, the nickel film causes the highest losses (66 dB/cm).     

The evanescent-wave cavity ring-down spectroscopy technique applied to the investigation of thermally grown oxides on Si(100)

We report about the contribution of thermally grown SiO_x overlayer on the SiO_x/Si interface (with oxidation states Siⁿ⁺, where n = 1, 2, 3, 4) to the optical losses of a resonant spectroscopic cavity. The experiments on Si oxide thin films were performed in evanescent wave for Si samples in contact with a total internal reflection surface of a BK7 prism. The evanescent field can be exploited to investigate properties and processes such as the absorption of thin film or solid/air interface reactions. The results show that the oxide overlayer thickness grows with the thermal exposure time and is limited after more than 7 h of treatment. Transmission electron microscopy has been used for the native oxide thickness measurement and angle-resolved X-ray photoelectron spectroscopy used to determine the thermal oxide thickness. A change of absorption coefficient Δα in the range 100–200 cm^?1 is obtained by evanescent-wave cavity ring-down spectroscopy (EW-CRDS) for thermal silicon oxide overlayer, in agreement with the general trend from literature. The evaluation from the EW-CRDS experiments presents the used setup as a competitive method for measuring the absorption properties of thin overlayer.     

Field enhancement effect of metal probe in evanescent field

Field enhancement effect of metal probe in evanescent field, induced by using a multi-layers structure for exciting surface plasmon resonance （SPR）, is analyzed numerically by utilizing two-dimensional （2D） TM- wave finite difference time-domain （FDTD） method. In this letter, we used a flmdamental mode Gaussian beam to induce evanescent field, and calculated the electric intensity. The results show that compared with the nonmetal probe, the metal probe has a larger field enhancement effect, and its scattering wave induced by field enhancement has a bigger decay coefficient. The field enhancement effect should conclude that the metal probe has an important application in nanolithography.     

Systematic experimental study of instabilities in the sealed CO2 laser

A systematic pressure-current experimental study of the wave instability regions in glow-discharge of different CO₂ laser mixtures has been accomplished. The forward ionization wave F^– has been found to be the main instability. At low currents (< 30 mA) and higher pressures (> 550 Pa) the laser discharge becomes stable. The highest output power at a given mixture ratio was achieved in these quiet regions. The normalized electric field dependences of wavenumber, phase and group velocities of the F^– wave are presented.     

The transmission enhancement of a THz pulse through an Ag/Ag2O layer detected by terahertz time-domain spectroscopy

This paper reports a new way to detect the enhanced transmission of a THz electromagnetic wave through an Ag/Ag2O layer by THz-TDS （time-domain spectroscopy）. As the THz beam illuminates the sub-wavelength Ag particles gained by Ag2O thermal decomposition, the evanescent wave is generated. The evanescent wave is coupled by a 500μm-GaAs substrate, which attaches behind the Ag/Ag2O layer, and then it transmits to the far field to be detected. The experimental results indicate that the transmitting amplitude is enhanced, as well as the frequent shifting and spectra broadening.     

The fabrication of polymer-based evanescent optical waveguide for biosensing

A polymer waveguide was fabricated to amplify the evanescent optical field for biosensing. The structure of waveguide was designed to propagate a normal single mode at the input and output regions for low loss beam coupling and propagation. A sensing region was formed in the middle of the waveguide to activate the evanescent mode and to induce high birefringence by depositing a thin dielectric film with a high refractive index on a single mode waveguide. A polymer waveguide with the dimensions of 7 μm-width and 2.5 μm-thickness was fabricated by photolithography and dry-etching. The active region of the TiO₂ thin film was fabricated with the dimensions of 20 mm-length, 20 nm-thickness and 2 mm-tapered tail. A polarimetric interference technique was used to evaluate the evanescent waveguide biosensor, and biomaterial such as glycerol was tested. The sensitivity of the sensor increased with increasing TiO₂ film thickness. For the fabricated waveguide with a 20 nm-thick TiO₂ film, the measured index change to the lead phase variation of 2π was 1.8 × 10⁻⁴.     

Near-field optical imaging using force detection with new tip-electrode geometry

We propose a new tip-electrode geometry to detect an (optical) evanescent field using noncontact atomic force microscopy. Using a semi-transparent metal electrode on the prism surface, the force sensitivity due to evanescent field in new tip-electrode geometry was enhanced by a factor of about 1000, comparing with that in old tip-electrode geometry where electrode was located behind the prism. Furthermore, this tip-electrode geometry avoids the electrostatic field caused by the residual charges and contact-electrified charges near the prism surface, which affects the force sensitivity due to evanescent field. We demonstrated the high resolution imaging of the evanescent field on the Au film with 15-nm (λ/33) lateral resolution.     

Ta/Ni/Ta multilayered ohmic contacts on n-type SiC

The interface formation, electrical properties and the surface morphology of multilayered Ta/Ni/Ta/SiC contacts were reported in this study. It was found that the conducting behavior of the contacts so fabricated is much dependent on the metal layer thickness and the subsequent annealing temperature. Auger electron spectroscopy (AES) and X-ray diffraction analyses revealed that Ni₂Si and TaC formed as a result of the annealing. The Ni atoms diffused downward to metal/SiC interface and converted into Ni₂Si layer in adjacent to the SiC substrate. The released carbon atoms reacted with Ta atoms to form TaC layer. Ohmic contacts with specific contact resistivity as low as 3 × 10⁻⁴ Ω cm² have been achieved after thermal annealing. The formation of carbon vacancies at the Ni₂Si/SiC interface, probably created by dissociation of SiC and formation of TaC during thermal annealing, should be responsible for the ohmic formation of the annealed Ta/Ni/Ta contacts. The addition of Ta into the Ni metallization scheme to n-SiC restricted the accumulation of carbon atoms left behind during Ni₂Si formation, improving the electrical and microstructure properties.     

Low-frequency degenerate surface modes of an electron-hole plasma

The computations of Flahive and Quinn¹ of the dispersion curves of low frequency degenerate surface (DS) modes propagating along the magnetic field in an electron-hole plasma are extended to higher values of the wavenumber. We find that beyond a certain value of the wavenumber the DS mode re-enters the allowed region of surface wave propagation and tends to an asymptotic frequency ω_R (<ω_LH). These low frequency resonances of an electron-hole plasma are discussed with reference to the experimental observations.     

Crossing resonance of wave fields in a medium with an inhomogeneous coupling parameter

The dynamic susceptibilities (Green’s functions) of the system of two coupled wave fields of different physical natures in a medium with an arbitrary relation between the mean value ? and rms fluctuation Δ? of the coupling parameter have been examined. The self-consistent approximation involving all diagrams with noncrossing correlation lines has been developed for the case where the initial Green’s function of the homogeneous medium describes the system of coupled wave fields. The analysis has been performed for spin and elastic waves. Expressions have been obtained for the diagonal elements G _mm and G _uu of the matrix Green’s function, which describe spin and elastic waves in the case of magnetic and elastic excitations, and for the off-diagonal elements G _mu and G _um , which describe these waves in the case of cross excitation. Change in the forms of these elements has been numerically studied for the case of one-dimensional inhomogeneities with an increase in Δ? and with a decrease in ? under the condition that the sum of the squares of these quantities is conserved: two peaks in the frequency dependences of imaginary parts of G _mm and G _uu are broadened and then joined into one broad peak; a fine structure appears in the form of narrow resonance at the vertex of the Green’s function of one wave field and narrow antiresonance at the vertex of the Green’s function of the other field; peaks of the fine structure are broadened and then disappear with an increase in the correlation wavenumber of the inhomogeneities of the coupling parameter; and the amplitudes of the off-diagonal elements vanish in the limit ? → 0.     

A TiO2 Film/K+ Ion-Exchanged Glass Composite Optical Waveguide and Its Application to a Refractive Index Sensor

A composite optical waveguide for biological and chemical sensors was successfully developed by sputtering a thin TiO₂ film onto the surface of a low loss potassium ion-exchanged optical waveguide. The electric field of the evanescent wave at the film surface was made strong through adiabatic transition of the guided light. The attenuation of the guided light was sufficiently small in the air, and the guided light intensity was changed sensitively with the refractive index of the cladding layer (n_c). Thus, n_c can be sensitively monitored with a detecting sensitivity of about 10^-4.     

Optical transmission through double-layer metallic subwavelength slit arrays

We present measurements of transmission of infrared radiation through double-layer metallic grating structures. Each metal layer contains an array of subwavelength slits and supports transmission resonance in the absence of the other layer. The two metal layers are fabricated in close proximity to allow coupling of the evanescent field on individual layers. The transmission of the double layer is found to be surprisingly large at particular wavelengths, even when no direct line of sight exists through the structure as a result of the lateral shifts between the two layers. We perform numerical simulations using rigorous coupled wave analysis to explain the strong dependence of the peak transmission on the lateral shift between the metal layers.     

The Field-Dependent Magnetization of d5–d7 Metal β−Diketonate Complexes and Their Macromolecular Polymers

A vibrating sample magnetometer (VSM) has been used to study the field-dependent magnetization, M(H), of the d⁵?d⁷ metal acetates [M(OAc)₂.nH₂O], metal β?diketonate complexes [M(tba)₂(H₂O)₂] and the macromolecular polymers [M(tba)₂(4,4-bipy)]_n (where, M = Mn(II), Fe(II), and Co(II), OAc = O₂CCH₃, tba = deprotonated 3-benzoyl-1.1.1-trifluoroacetone, and 4,4-bipy = 4,4′-bipyridine). The magnetic field strength (H) was applied in the range of 0?10⁴ O_e at ambient temperature (ca. 23°C). The experimental results showed that the d⁵?d⁷ metal acetate, complexes and polymers exhibit low paramagnetic properties excepting [Fe(tba)₂(4,4-bipy)]_n polymer, which had negative magnetization M(emu/g) showing diamagnetic properties in the range 0?10⁴ O_e. The magnetization was almost equal to zero without an applied magnetic field (H(O_e)) for each d⁵?d⁷ metal acetate, complex, and polymer. The linear M(H) curve had a magnetic saturation for iron and manganese acetate species at the magnetic field strengths of 3.1 × 10³ and 4.7 × 10³ Oe, respectively. The external magnetic field reached 9.0 × 10³ O_e without any saturation magnetization for the cobalt compounds. The coordination effect of 3-benzoyl-1.1.1-trifluoroacetone (H-tba) and 4,4′-bipyridine (4,4′-bipy) ligands on the field-dependent magnetization M(H) and paramagnetic behavior of d⁵?d⁷ metal atoms is discussed. The field-dependent magnetization M(H) curves of metal β?diketonate complexes and the polymers including d⁵?d⁷ metal acetates showed a weak field octahedral geometry.     

Pseudo volume plasmon in arrays of doped and un-doped semiconductors

We present a theoretical work which shows that for a metamaterial consisting of a periodic array of doped and un-doped semiconductors it is possible to define a frequency ?? _t corresponding to a pseudo volume plasmon. ?? _t?depends on the thicknesses and on the dielectric constants of the components of the metamaterial and on the plasma frequency of the doped semiconductor. As its homologue in noble metal, the pseudo volume plasmon is the collective oscillation of charges present in the metallic part of the metamaterial leading to a pure longitudinal electric wave. We show that ?? _t is the degeneracy frequency between the anti-symmetric mode in a transverse magnetic field and the mode in a transverse electric field. We demonstrate that this degeneracy is due to the periodicity of the structure, which transforms the imaginary solution of a metal?Cdielectric interface into a real solution in the case of the periodic metamaterial.     

Domain growth and aging scaling in coarsening disordered systems

We explore the electronic and transport properties out of a biased multilayer hexagonal boron nitride (h-BN) by first-principles calculations. The band gaps of multilayer h-BN decrease almost linearly with increasing perpendicular electric field, irrespective of the layer number N and stacking manner. The critical electric filed (E ₀) required to close the band gap decreases with the increasing N and can be approximated by E ₀ = 3.2 / (N ? 1) (eV). We provide a quantum transport simulation of a dual-gated 4-layer h-BN with graphene electrodes. The transmission gap in this device can be effectively reduced by double gates, and a high on-off ratio of 3000 is obtained with relatively low voltage. This renders biased MLh-BN a promising channel in field effect transistor fabrication.