Over the last few years arrays of evanescently coupled waveguides have been brought into focus as a particular representation of functionalized optical materials, in which the dispersion and diffraction of propagating light can be specifically tuned Moreover, it turns out that the light evolution in these systems shares fundamental similarities to the quantum evolution of particle wavefunctions, so that waveguide arrays can act as a model system for emulating quantum mechanics Recently, a novel technique was developed with which waveguides can be directly 'written' into various optical bulk materials using femtosecond laser pulses, which allows for the realization of a variety of innovative concepts which are not feasible using other fabrication methods The aim of this tutorial is to give an introduction to this topic

https://iopscience.iop.org/article/10.1088/0953-4075/43/16/163001/pdf

Discrete optics in femtosecond-laser-written photonic structures

Water is not restricted to moderate temperatures and low pressures, but can exist up to very high temperatures, far above its critical point at 647 K. In this supercritical regime, water can be gradually compressed from gas-like to liquid-like densities. The resulting dense supercritical states have extraordinary properties which can be tuned by temperature and pressure, and form the basis for innovative technologies. This Review covers the current knowledge of the major properties of supercritical water and its solutions with nonpolar, polar, and ionic compounds, and of the underlying molecular processes.

Supercritical water as a solvent.

Negative hydrogen/deuterium ions can be formed by processes occurring in the plasma volume and on surfaces facing the plasma. The principal mechanisms leading to the formation of these negative ions are dissociative electron attachment to ro-vibrationally excited hydrogen/deuterium molecules when the reaction takes place in the plasma volume, and the direct electron transfer from the low work function metal surface to the hydrogen/deuterium atoms when formation occurs on the surface. The existing theoretical models and reported experimental results on these two mechanisms are summarized. Performance of the negative hydrogen/deuterium ion sources that emerged from studies of these mechanisms is reviewed. Contemporary negative ion sources do not have negative ion production electrodes of original surface type sources but are operated with caesium with their structures nearly identical to volume production type sources. Reasons for enhanced negative ion current due to caesium addition to these sources are discussed.

Negative hydrogen ion production mechanisms

The present invention provides microwave plasma nozzle array systems (10, 70, 230, and 310) and methods for configuring microwave plasma nozzle arrays (37, 99, and 337). The microwaves are transmitted to a microwave cavity (323) in a specific manner and form an interference pattern (66) that includes high-energy regions (69) within the microwave cavity (32). The high-energy regions (69) are controlled by the phases and the wavelengths of the microwaves. A plurality of nozzle elements (36) is provided in the array (37). Each of the nozzle elements (36) has a portion (116) partially disposed in the microwave cavity (32) and receives a gas for passing therethrough. The nozzle elements (36) receive microwave energy from one of the high-energy regions (69). Each of the nozzle elements (36) includes a rod-shaped conductor (114) having a tip (117) that focuses on the microwaves and a plasma (38) is then generated using the received gas.

Plasma nozzle array for providing uniform scalable microwave plasma generation

We report on the experimental observation of two-dimensional surface waves localized at the edge or in the corner of femtosecond laser-written waveguide arrays in fused silica. Increasing the power of the input beam allows one to observe a clear transition from a linear diffraction pattern to localized nonlinear surface states, which can exist at the interface only above a certain power threshold. This constitutes the first ever experimental observation of two-dimensional nonlinear surface solitons in optics.

https://www.icfo.eu/images/publications/J07-028.pdf

Observation of Two-Dimensional Surface Solitons in Asymmetric Waveguide Arrays

A review on recent development in self-consistent description of high-frequency discharges is presented. The discussions are based on fluid models of travelling-wave-sustained plasmas and results for the axial structure of discharges in different gases. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Guided-wave-produced plasmas

A pulsed inductively-coupled radiofrequency plasma in oxygen is investigated by means of time-resolved microwave interferometry in a wide pressure range from 0.5 to 200 Pa. In the afterglow a peak of the electron density is observed. The effect is maximum for pressures around 50 Pa. The time-resolved measurements of the electron density are interpreted in the framework of a fluid model. This model points out the significance of negative ions. The overall electron density is comparatively small. Attachment and detachment processes nearly balance during the power-on phase. But when the power is switched off, the electron temperature drops very quickly. This means that the production of new negative ions is inhibited so that the negative ions are destroyed by collisions. These reactions quickly set free electrons in the afterglow and are the reason for the observed peak in the electron density after switching off the power.

https://iopscience.iop.org/article/10.1088/0963-0252/15/2/001/pdf

Electron release in the afterglow of a pulsed inductively-coupled radiofrequency oxygen plasma

Free-fall regime maintenance of hydrogen discharges is analyzed based on numerical solutions of a set of equations involving the balance equations of the charged particles [electrons, the three types of the positive ions (H+, H2+, and H3+), and negative H− ions] and of the neutral species (hydrogen atoms H and vibrationally excited molecules), the momentum equations of the positive ions, the electron energy balance equation, and the Poisson equation, all together 25 differential equations. The obtained results for varying discharge radius show strong accumulation of the negative ions in the on-axis region of the discharge when the discharge radius is small, which leads to a concept for a design of a volume-production based source as a matrix of small radius discharges. The variation in the negative ion density with changing gas pressure and electron density at the discharge axis is also analyzed.

A small radius hydrogen discharge: An effective source of volume produced negative ions

This article presents a fluid-plasma model of the free-fall regime of maintenance of high-frequency discharges in hydrogen. The obtained results are for the radial profiles of the concentrations and the velocities of electrons, positive H+, H2+, and H3+ ions, negative H− ions, potential of the radial dc electric field, and electron temperature. The importance of the directed motion of the charged particles in the radial dc electric field, the negative ion behavior in the discharge, and the description of the discharge characteristics by continuous radial profiles, which smoothly cover the total cross section of discharge, are stresses. A strong impact of the negative ions on the formation of the self-consistent discharge structure is shown. The discussions are in terms of changing gas pressure and electron concentration at the discharge axis.

Low pressure hydrogen discharges

The results from a two-dimensional model of hydrogen discharges sustained in a single-chamber small radius plasma source presented in this study show that when the plasma maintenance is nonlocal, the conditions ensuring high concentration of the negative ions are formed by the behavior of the entire discharge structure and, in particular, of the fluxes in the discharge. The traditionally accepted requirements for low-electron temperature and high-electron density formulated based on the locality of the discharge behavior can no longer be employed. The obtained results show strong accumulation of negative ions in the discharge center, which results from their flux in the dc electric field, not from local balance of the ions there.

Kh. Tarnev

Papers

Guided-wave-produced plasmas

Electron release in the afterglow of a pulsed inductively-coupled radiofrequency oxygen plasma

A small radius hydrogen discharge: An effective source of volume produced negative ions

Low pressure hydrogen discharges

Negative hydrogen ion maintenance in small radius discharges: Two-dimensional modeling