  In the past two decades, artificial structures known as metamaterials have
been found to exhibit extraordinary material properties that enable the
unprecedented manipulation of electromagnetic waves, elastic waves, molecules,
and particles. Phenomena such as negative refraction, bandgaps, near perfect
wave absorption, wave focusing, negative Poissons ratio, negative thermal
conductivity, etc., all are possible with these materials. Metamaterials were
originally theorized and fabricated in electrodynamics, but research into their
applications has expanded into acoustics, thermodynamics, seismology, classical
mechanics, and mass transport. In this Research Update we summarize the
history, current state of progress, and emerging directions of metamaterials by
field, focusing the unifying principles at the foundation of each discipline.
We discuss the different designs and mechanisms behind metamaterials as well as
the governing equations and effective material parameters for each field. Also,
current and potential applications for metamaterials are discussed. Finally, we
provide an outlook on future progress in the emerging field of metamaterials.
