Spatial-transformation-based wave manipulations in electromagnetics and acoustics: From theory to applications

Transformation optics (TO) or transformation acoustics (TA) theory provides an elegant framework for designing intricate wave-material interaction devices. Combining TO or TA with emerging metamaterial technology, the propagating waves inside metamaterials can be guided at will by mapping a preset spatial pattern onto a specific constituent parameter distribution, thus achieving desired functionalities. In this review, we mainly focus on recent progress in TO and TA, in terms of basic theory and representative applications. Firstly, we introduce the fundamental principles of TO and TA. Subsequently, we present some related spatial-transformation-based applications in electromagnetics, such as cloaks, beam manipulation devices and applications in nonlinear responses, and some typical applications in acoustics, namely underwater ultrasound invisibility, sound imaging lenses and parity-time symmetry. Furthermore, we discuss transformation-based devices for acoustic-electromagnetic-joint control. Finally, we summarize current spatial-transformation-based wave modulations and outline some future opportunities in this research field.