Pump Requirements for Parametric Amplification and Generation of Surface Waves on YZ LiNbO3
Absfract-Expressions are developed for estimating the threshold electric fields required for parametric amplification or generation of surface waves in piezoelectric materials utilizing the nonlinear characteristics of the material. When applied to Y Z LiNbOa, they yield values in excess of 20 kV/cm under the most optimistic, yet still realistic, conditions. These values are too Iarge for amplification or generation to be considered for practical applications. Several pump electrode geometries and circuit matching techniques were investigated to determine the best obtainable pump circuit performance. An interdigital electrode separated from the crystal by a small gap was found to be the most efficient pump electrode geometry and was capable of producing approximately ZkV/cm of field per watt’‘2 of input pump power.
Nonlinear Materials for Acoustic‐Surface‐Wave Convolver
Acoustic‐surface‐wave convolution by nonlinear mixing has been observed in cubic, hexagonal, and trigonal crystals. The magnitudes of the phenomenological nonlinear surface‐wave coupling constants are tabulated and compared with the linear electromechanical coupling factor 2(ΔV/V). It has been found that the basal plane of PZT‐8 has a larger nonlinear coupling constant than that of the commonly used Y‐cut Z‐propagating LiNbO3.
Collagen dynamics in articular cartilage under osmotic pressure
Cartilage is a complex biological tissue consisting of collagen, proteoglycans and water. The structure and molecular mobility of the collagen component of cartilage were studied by 13C solid‐state NMR spectroscopy as a function of hydration. The hydration level of cartilage was adjusted between fully hydrated (∼80 wt% H2O) and highly dehydrated (∼30 wt% H2O) using the osmotic stress technique. Thus, the conditions of mechanical load could be simulated and the response of the tissue macromolecules to mechanical stress is reported. From the NMR measurements, the following results were obtained. (i) Measurements of motionally averaged dipolar 1H–13C couplings were carried out to study the segmental mobility in cartilage collagen at full hydration. Backbone segments undergo fast motions with amplitudes of ∼35° whereas the collagen side‐chains are somewhat more mobile with amplitudes between 40 and 50°. In spite of the high water content of cartilage, collagen remains essentially rigid. (ii) No chemical shift changes were observed in 13C cross‐polarization magic angle spinning spectra of cartilage tissue at varying hydration indicating that the collagen structure was not altered by application of high osmotic stress. (iii) The 1H–13C dipolar coupling values detected for collagen signals respond to dehydration. The dipolar coupling values gradually increase upon cartilage dehydration, reaching rigid limit values at ∼30 wt% H2O. This indicates that collagen is essentially dehydrated in cartilage tissue under very high mechanical load, which provides insights into the elastic properties of cartilage collagen, although the mechanical pressures applied here exceed the physiological limit. Copyright © 2006 John Wiley & Sons, Ltd.