Sound amplification by stimulated emission of radiation phonon laser action had been proposed in a wide range of physical systems such as nanomechanics, semiconductors, nanomagnets and paramagnetic ions in a lattice. Finding materials that stimulate emission was needed for the development of the SASER. The generation of coherent phonons in a double-barrier semiconductor heterostructure was first proposed around 1990. The transformation of the electric potential energy in a vibrational mode of the lattice is remarkably facilitated by the electronic confinement in a double-barrier structure. On this basis, physicists were searching for materials in which stimulated emission rather than spontaneous emission, is the dominant decay process. A device
Stimulated emission in such an excited state may thus result in radiation which has a narrow spectrum (centered around one wavelength of light), but the individual photons would have no common phase relationship and would also emanate in random directions. This is the mechanism of fluorescence and thermal emission. An external electromagnetic field at a frequency associated with a transition can affect the quantum mechanical state of the atom without being absorbed. As the electron in the atom makes a transition between two stationary states (neither of which shows a dipole field), it enters a transition state which does have a dipole