Dielectric recovery of short A-C arcs between low-boiling-point electrodes

The re-ignition characteristics (variation of re-ignition voltage with time after current zero) of short alternating current arcs between plane brass electrodes in air were studied by observing the average re-ignition voltages on the screen of a cathode-ray oscilloscope and controlling the rates of rise of voltage by varying the shunting capacitance and hence the natural period of oscillation of the reactors used to limit the current. The shape of these characteristics and the effects on them of varying the electrode separation, air pressure, and current strength were determined.

The results show that short arc spaces recover dielectric strength in two distinct stages. The first stage agrees in shape and magnitude with a previously developed theory that all voltage is concentrated across a partially deionized space charge layer which increases its breakdown voltage with diminishing density of ionization in the field-tree space. The second stage appears to follow complete deionization by the electric field due to displacement of the field-free region by the space charge layer, its magnitude and shape appearing to be due simply to increase in gas density due to cooling. Temperatures calculated from this second stage and ion densities determined from the first stage by means of the space charge equation and an extrapolation of the temperature curve are consistent with recent measurements of arc value by other methods. Analysis or the decrease with time of the apparent ion density shows that diffusion alone is adequate to explain the results and that volume recombination is not. The effects on the characteristics of variations in the parameters investigated are found to be in accord with previous results and with the theory if deionization mainly by diffusion be assumed.

Scattering by two degenerate anisotropic modes in square-lattice dielectric photonic crystals

We systematically investigate the square-lattice dielectric photonic crystals that have been used to demonstrate flat slab imaging experimentally. A right-handed Bloch mode is found in the left-handed frequency region by using the plane wave expansion method to analyze the photonic band structure and equifrequency contours. Using the multiple scattering theory, numerical simulations demonstrate that the left-handed mode and the right-handed mode are excited simultaneously by a point source and result in two kinds of transmitted waves. Impacted by the evanescent waves, superposition of these transmitted waves brings on complicated near field distributions such as the so-called imaging and its disappearance.

Subwavelength imaging by a dielectric-tube photonic crystal

In contrast to previous two-dimensional coated photonic crystals, in this paper we propose a left-handed one that is made of dielectric tubes arranged in a close-packed hexagonal lattice. Without metallic cores, this structure is low-loss and convenient to fabricate. Negative refraction and its resulting focusing are investigated by dispersion characteristic analysis and numerical simulation of the field pattern. With proper modification at the interface, the image is improved. With better isotropy than that with noncircular rods, planoconcave lenses made by dielectric tubes focus a Gaussian beam exactly at R//n - 1/.

Subwavelength imaging with two symmetrical interfaces by dielectric-tube photonic crystals

As distinct from coated photonic crystals, in this paper we propose a novel one that is made of dielectric tubes arranged in a close-packet square lattice. Without metallic cores, this structure is low-loss and convenient to fabricate. A left-handed frequency region is found in the second band by dispersion characteristic analysis. Without inactive modes for the transverse electric mode, negative refraction and subwavelength imaging are demonstrated by the finite-difference time-domain simulations with two symmetrical interfaces, i.e. Gamma X and Gamma M.

Effect of La3+ on microwave dielectric properties of (Pb0.45Ca0.55) (Fe0.5Nb0.5)O-3 ceramics

In this work, microwave dielectric properties of A-site substitution by La3+ in (Pb0.45Ca0.55) (Fe0.5Nb0.5) 03 system were investigated. Microwave dielectric properties of A-site charge unbalance substitution of [(Pb0.45Ca0.55)(1-x) La-x] (Fe0.5Nb0.5)O-3(+) (P45CLFN) were improved because the solid solution of small amount of surplus La3+ with (Pb, Ca)(2+) could eliminate oxygen vacancies, and the formation of secondary phase (pyrochlore) was also caused by surplus La3+. The decreasing of dielectric constant with the increase of La3+ content is due to the formation of pyrochlore. The grain size is changed slightly and Q(f) values (7000 similar to 7300 GHz) are almost unchanged at x = 0.02 similar to 0.10, but the temperature coefficient of resonant frequency (TCF) are increased and changed from negative to positive. TCF is zero at x 0.075 with Q(f) = 7267 GHz and K = 89. TCF of all specimens are within +/- 5 x 10(-6)degrees C-1.

Design and properties analysis of multi-layer dielectric used in pulse compressor gratings

Used in chirped-pulse amplification system and based on multi-layer thin film stack, pulse compressor gratings (PCG) are etched by ion-beam and holographic techniques. Diffraction efficiency and laser-induced damage threshold rely on the structural parameters of gratings. On the other hand, they depend greatly on the design of multi-layer. A theoretic design is given for dielectric multi-layer, which is exposed at 413.1 nm and used at 1053 nm. The influences of coating design on optical characters are described in detail. The analysis shows that a coating stack of H3L (H2L) (boolean AND) 9H0.5L2.01H meets the specifications of PCG well. And there is good agreement of transmission between experimental and the theoretic design. (c) 2005 Elsevier GmbH. All rights reserved.

Investigation of laser-induced damage on multi-layer dielectric gratings

We investigate mechanisms of laser induced damage thresholds (LIDTs) of multi-layer dielectric gratings (AIDG,). It is found that the laser damage thresholds of MDGs and unstructured dielectric multi-layer coatings (the substrate of MDG) are 3.15J/cm(2) and 9.32 J/cm(2), respectively, at 1064nm (12ns) with the Littrow angle 51.2 degrees and the TEM00 mode. The laser-induced damage mechanism of multi-layer dielectric is presented with the analysis of the following factors: The dominant factor is the pollution on the corrugated surface, which is induced by the complex manufacture process of multi-layer dielectric gratings; another is the electric field distribution along the corrugated surface. The third reason is due to the reduction in stoichiometry of oxide films, resulting from the manufacture process of etching.

Laser induced damage of multi-layer dielectric used in pulse compressor gratings

Laser induced damage threshold (LIDT) of multi-layer dielectric used in pulse compressor gratings (PCG) was investigated. The sample was prepared by e-beam evaporation (EBE). LIDT was detected following ISO standard 11254-1.2. It was found that LIDTs of normal and 51.2 deg. incidence (transverse electric (TE) mode) were 14.14 and 9.31 J/cm2, respectively. A Nomarski microscope was employed to map the damage morphology, and it was found that the damage behavior was pit-concave-plat structure for normal incidence, while it was pit structure for 51.2 deg. incidence with TE mode. The electric field distribution was calculated to illuminate the difference of LIDT between the two incident cases.

Optimization of near-field optical field of multi-layer dielectric gratings for pulse compressor

Multi-layer dielectric (MLD) gratings for pulse compressors in high-energy laser systems should provide high diffraction efficiency as well as high laser induced damage thresholds (LIDT). Nonuniform optical near-field distribution is one of the important factors to limit their damage resistant capabilities. Electric field distributions in the gratings and multi-layer film region are analyzed by using Fourier modal method. Optimization of peak electric field in the gratings ridge is performed with a merit function, including both diffraction efficiency and electric field enhancement when the top layer material is HfO2 and SiO2, respectively. A set of optimized gratings parameters is obtained for each structure, which reduce the peak electric field within the gratings ridge to being respective 1.39 and 1.84 times the value of incident light respectively. Finally, we also discuss the effects of gratings refractive index, gratings sidewall angle and incident angle on peak electric field in the gratings ridge. (c) 2006 Elsevier B.V. All rights reserved.

Theoretical analysis of optical properties of dielectric coatings dependence on substrate subsurface defects

A model for refractive index of stratified dielectric substrate was put forward according to theories of inhomogeneous coatings. The substrate was divided into surface layer, subsurface layer and bulk layer along the normal direction of its surface. Both the surface layer (separated into N-1 sublayers of uniform thickness) and subsurface layer (separated into N-2 sublayers of uniform thickness), whose refractive indices have different statistical distributions, are equivalent to inhomogeneous coatings, respectively. And theoretical deduction was carried Out by employing characteristic matrix method of optical coatings. An example of mathematical calculation for optical properties of dielectric coatings had been presented. The computing results indicate that substrate subsurface defects can bring about additional bulk scattering and change propagation characteristic in thin film and Substrate. Therefore, reflectance, reflective phase shift and phase difference of an assembly of coatings and substrate deviate from ideal conditions. The model will provide some beneficial theory directions for improving optical properties of dielectric coatings via substrate surface modification. (c) 2005 Elsevier B.V. All rights reserved.

Theoretical analysis of influence of substrate microdefects on polarized light properties of dielectric coatings

A model for refractive index of stratified dielectric substrate was presented according to inhomogeneous coatings theories. The substrate was divided into surface layer, subsurface layer and bulk layer along the normal direction of its surface. The former two layers were equivalent to inhomogeneous coatings. Theoretical deduction was executed by employing the characteristic matrix method of optical coatings, and one mathematical calculation example was presented. The results indicate that reflectance, reflective phase shift and phase difference of polarized light deviate from ideal conditions. It shows that substrate microdefects can induce volume scattering and change propagation characteristic of light both in coatings and substrate. (c) 2005 Elsevier GmbH. All rights reserved.

Development in laser induced extrinsic absorption damage mechanism of dielectric films

Absorption of host and the temperature-dependence of absorption coefficient have been considered in evaluating temperatures distribution in films, when laser pulse irradiates on films. Absorption of dielectric materials experience three stages with the increase of temperature: multi-photon absorption; single photon absorption; metallic absorption. These different absorption mechanisms correspond to different band gap energies of materials, which will decrease when the temperature of materials increases. evaluating results indicate that absorption of host increases rapidly when the laser pulse will be over. If absorption of host and the temperature-dependence of absorption are considered, the material temperatures in films will be increased by a factor of four.

Characteristics of high reflection mirror with an SiO2 top layer for multilayer dielectric grating

The high reflection (HR) mirror composed of dielectric stacks with excellent spectrum characteristics and high damage resistant ability is critical for fabricating multilayer dielectric (MLD) grating for pulse compressor. The selection of the SiO2 material as the top layer of the HR mirror for grating fabrication is beneficial for improving the laser-induced damage threshold of MLD grating as well as minimizing the standing-wave effect in the photoresist during the exposure process. Based on an (HLL) H-9 design comprising quarter-waves of HfO2 ( H) and half-waves of SiO2 ( L), we obtain an optimal design of the HR mirror for MLD grating, the SiO2 top layer of which is optimized with a merit function including both the diffraction efficiency of the MLD grating and the electric field enhancement in the grating. Dependence of the performance of the MLD grating on the fabrication error of the dielectric mirror is analysed in detail. The HR mirror is also fabricated by E-beam evaporation, which shows good spectral characteristics at the exposure wavelength of 413 nm and at the operation wavelength of 1053 nm and an average damage threshold of 10 J cm(-2) for a 12 ns pulse.