There are a number of ways to excite a population of vibrations, i.e., create a hot optical phonon population. For example, if the electron population is excited, using a laser or electric field, they will typically relax by emitting optical phonons. Additionally, a hot molecular gas can impart its vibrations to a crystal when chemisorbed. Regardless of method, the conversion efficiency is limited by the optical phonon temperature achieved as compared to the electron temperature within the device due to Carnot's theorem.