Sound amplification by stimulated emission of radiation vector in the second proposed scheme, is supposed to be determined by magnitude of in-plane magnetic field. Therefore, such kind of SASER is tunable (i.e. its wavelength of operation can be altered in a controlled manner). Common semiconductor lasers can be realised only in direct gap semiconductors. The reasoning behind that is that a pair of electron and hole near minima of their bands in an indirect gap semiconductor can recombine only with production of a phonon and a photon, due to energy and momentum conservation laws. This kind of process is weak in comparison with electron-hole recombination in a
Stimulated emission to the number of atoms in the lower state, "N". Einstein showed that the coefficient for this transition must be identical to that for stimulated emission: Thus absorption and stimulated emission are reverse processes proceeding at somewhat different rates. Another way of viewing this is to look at the "net" stimulated emission or absorption viewing it as a single process. The net rate of transitions from "E" to "E" due to this combined process can be found by adding their respective rates, given above: Thus a net power is released into the electric field equal to the photon energy "hν"