Crystals2014, 4(4), 427-438; doi:10.3390/cryst4040427 - published 14 October 2014 Show/Hide Abstract
Abstract: We survey our recent results on the observation and studies of the effects accompanying the dynamical Bragg diffraction in one-dimensional photonic crystals (PhC). Contrary to the kinematic Bragg diffraction, the dynamical one considers a continuous interaction between the waves travelling within a spatially-periodic structure and is the most pronounced in the so called Laue geometry, leading to a number of exciting phenomena. In the described experiments, we study the PhC based on porous silicon or porous quartz, made by the electrochemical etching of crystalline silicon with the consequent thermal annealing. Importantly, these PhC are approximately hundreds of microns thick and contain a few hundreds of periods, so that the experiments in the Laue diffraction scheme are available. We discuss the effect of the temporal splitting of femtosecond laser pulses and show that the effect is quite sensitive to the polarization and the phase of a femtosecond laser pulse. We also show the experimental realization of the Pendular effect in porous quartz PhC and demonstrate the experimental conditions for the total spatial switching of the output radiation between the transmitted and diffracted directions. All described effects are of high interest for the control over the light propagation based on PhC structures.
Crystals2014, 4(3), 417-426; doi:10.3390/cryst4030417 - published 17 September 2014 Show/Hide Abstract
Abstract: We revisit the standard Nicolson–Ross–Weir method of effective permittivity and permeability restoration of photonic structures for the case of subwavelength metal-dielectric multilayers. We show that the direct application of the standard method yields a false zero-epsilon point and an associated spurious permeability resonance. We show how this artifact can be worked around by the use of the cycle shift operator to the periodic multilayer in question.
Crystals2014, 4(3), 404-416; doi:10.3390/cryst4030404 - published 5 September 2014 Show/Hide Abstract
Abstract: Crystal structures of five hexaethylguanidinium salts (PF6−1, FeCl4−2, CuCl42−·H2O 3, CoBr42−/Br−·H2O 4, bistriflimide 5) were determined. Short interionic contacts were identified. Cyclic voltammetry of 2 revealed an electrochemical window from +1.0 to −1.5 V with a single Fe(III)/Fe(II) redox event at −0.27 V on a gold electrode versus Ag/AgCl.
Crystals2014, 4(3), 390-403; doi:10.3390/cryst4030390 - published 3 September 2014 Show/Hide Abstract
Abstract: The geometrical shape of ions in crystals and the concept of ionic radii are re-considered. The re-investigation is motivated by the fact that a spherical modelling is justified for p valence shell ions on cubic lattice sites only. For the majority of point groups, however, the ionic radius must be assumed to be an anisotropic quantity. An appropriate modelling of p valence ions then has to be performed by ellipsoids. The approach is tested for pyrite-structured dichalcogenides MX2, with chalcogen ions X = O, S, Se and Te. The latter are found to exhibit the shape of ellipsoids being compressed along the <111> symmetry axes, with two radii r|| and r⊥ describing their spatial extension. Based on this ansatz, accurate interatomic M–X distances can be derived and a consistent geometrical model emerges for pyrite-structured compounds. Remarkably, the volumes of chalcogen ions are found to vary only little in different MX2 compounds, suggesting the ionic volume rather than the ionic radius to behave as a crystal-chemical constant.
Abstract: The effect of composition and temperature on the large field behavior of C cut and poled (d32-mode) rhombohedral relaxor ferroelectric 0.24PIN-(1−x)PMN-xPT single crystals was characterized under electromechanical loading and the relative phase energy determined. The electric field and stress induced polarization and strain response and field dependent material properties are reported for two concentrations of lead titanate (PT), with one PT concentration closer to the morphotropic phase boundary, at low and high temperature. A thermodynamic analysis to determine the relative energy levels of the rhombohedral and orthorhombic phases is based on the path integration of the measured data to determine external work done to drive the phase transition. The effect of heat generated by irreversible strain and electric displacement increments (hysteresis in the phase transformation) was removed from the work done during the loading cycle and the relative Helmholtz free energy density levels of the phases was determined.
Abstract: Relaxor-PbTiO3 piezoelectric single crystals have been widely used in a broad range of electromechanical devices, including piezoelectric sensors, actuators, and transducers. This paper reviews the unique properties of these single crystals for piezoelectric sensors. Design, fabrication and characterization of various relaxor-PT single crystal piezoelectric sensors and their applications are presented and compared with their piezoelectric ceramic counterparts. Newly applicable fields and future trends of relaxor-PT sensors are also suggested in this review paper.