Devices and machines that convert energy from one form to another are a common thread woven through modern society, ranging from the generation of electrical energy from solar radiation to power our homes, to the creation of mechanical motion from the chemical energy of gasoline to power our cars. Increasing the efficiency of these devices and machines, or inventing alternatives, is arguably one of the major themes permeating engineering. In most instances, the design of a machine determines the optimal bandwidth, frequency, and magnitude of the input and output energy or power. The energy conversion process occurs at a surface or within a functional material at the heart of the device.