Increasing demand for raw materials means that alternatives to indium-tin oxide are desired for optically transparent electrode applications. Carbon nanotube, metal nanowire networks and regular metal grids have been investigated as possible options. In this review, these materials and recently rediscovered graphene are compared with the usual transparent conductive oxides.

Past achievements and future challenges in the development of optically transparent electrodes

Ferroelectric lithium niobate (LiNbO3) is widely used in integrated and guided-wave optics because of its favorable optical, piezoelectric, electro-optic, elastic, photoelastic, and photorefractive properties. However, detailed summaries of its pertinent physical properties and crystal structure are not readily available. In this tutorial paper, the important tensor physical properties and their mathematical descriptions are compiled and presented. The essential features of the structure of lithium niobate, including its hexagonal and rhombohedral unit cells, are illustrated and the principal (Cartesian) axes used in the description of the anisotropic properties are specified relative to the crystal structure. Errors in property coefficient values and structure information that have been propagated in the literature are corrected.

Lithium niobate: Summary of physical properties and crystal structure

Acoustic metamaterials can manipulate and control sound waves in ways that are not possible in conventional materials. Metamaterials with zero, or even negative, refractive index for sound offer new possibilities for acoustic imaging and for the control of sound at subwavelength scales. The combination of transformation acoustics theory and highly anisotropic acoustic metamaterials enables precise control over the deformation of sound fields, which can be used, for example, to hide or cloak objects from incident acoustic energy. Active acoustic metamaterials use external control to create effective material properties that are not possible with passive structures and have led to the development of dynamically reconfigurable, loss-compensating and parity–time-symmetric materials for sound manipulation. Challenges remain, including the development of efficient techniques for fabricating large-scale metamaterial structures and converting laboratory experiments into useful devices. In this Review, we outline the designs and properties of materials with unusual acoustic parameters (for example, negative refractive index), discuss examples of extreme manipulation of sound and, finally, provide an overview of future directions in the field. Acoustic metamaterials can be used manipulate sound waves with a high degree of control. Their applications include acoustic imaging and cloaking. This Review outlines the designs and properties of these materials, discussing transformation acoustics theory, anisotropic materials and active acoustic metamaterials.

Controlling sound with acoustic metamaterials

Piezoelectric and piezomagnetic materials and their function in transducers

3 – Piezoelectric and Piezomagnetic Materials and Their Function in Transducers

A plane elastic wave propagating in a piezoelectric crystal may be accompanied by longitudinal electric fields which provide an additional elastic stiffness When the crystal is also semiconducting, these fields produce currents and space charge resulting in acoustic dispersion and loss A linear theory of this effect is developed, taking into account drift, diffusion, and trapping of carriers for both extrinsic and intrinsic semiconductors Conductivity modulation sets an upper limit on strain amplitude for a linear theory The directional characteristics and the magnitude of the effects are illustrated for CdS and GaAs The Appendix treats the interaction of an arbitrary acoustic plane wave with the electromagnetic fields in a piezoelectric crystal (based on a treatment by Kyame [J J Kyame, J Acoust Soc Am 21, 159 (1949); 26, 990 (1954)]) and further shows explicitly that only the effects of longitudinal electric fields need be considered

Elastic Wave Propagation in Piezoelectric Semiconductors

Radiofrequency waves at 15 and 45 Mc are transduced into pressure waves at the same frequencies, and are applied to a CdS crystal. The waves are then converted by a second transducer into a r-f output wave. The signal amplification is found as a function of the illumination intensity on the crystal, and the electric field parallel to the pressure waves. Positive gain may be attained for suitable values of the electric field. (T.F.H.)

Ultrasonic Amplification in CdS

Piezoelectricity and Conductivity in ZnO and CdS

Acousto-Electric Explanation of Non-Ohmic Behavior in Piezoelectric Semiconductors and Bismuth

A large anisotropy in the electrical conductivity of CdS appearing below 30\ifmmode^\circ\else\textdegree\fi{}K is reported. This anisotropy casts serious doubt on the customary view that impurity hopping supplies the low-temperature conductivity. We present a new model for this conductivity based upon the electronic screening of the piezoelectric polarization around dislocations. Numerical solutions of the highly nonlinear Poisson equation quantitatively justify this model.

A. R. Hutson

Papers

Elastic Wave Propagation in Piezoelectric Semiconductors

Ultrasonic Amplification in CdS

Piezoelectricity and Conductivity in ZnO and CdS

Acousto-Electric Explanation of Non-Ohmic Behavior in Piezoelectric Semiconductors and Bismuth

Role of Dislocations in the Electrical Conductivity of CdS