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The laser is based on a fundamental principle in physics, the (L)ight (A)mplification by (S)timulated (E)mission of (R)adiation. This concept theoretically predicted by Albert Einstein in 1916 and experimentally demonstrated for the first time in 1961 can be adopted for the phonon, a vibrational quantum in a crystal which consists of a regular arrangement or lattice of atoms in space. Phonons can be absorbed or emitted by electrons in the crystal. A net amplification of phonons requires that their number emitted per second via stimulated emission is larger than that absorbed per second. In other words, there must be more electrons emitting than absorbing a phonon. This condition is illustrated schematically in Fig. 1 where the electron energy is plotted as a function of the electron momentum k, following roughly a parabolic dependence. For a thermal equilibrium distribution of electrons at room temperature, electron states at higher energies have a smaller population than those at lower energies, resulting in a net phonon absorption. Stimulated emission of a phonon can only prevail if a so-called population inversion exists between two electronic states separated by both the energy and the momentum of the corresponding phonon in the crystal. For optical phonons, the latter condition is very difficult to fulfill because of their comparably large energy.
Researchers from the Max-Born-Institute in Berlin, Germany, the Sandia National Laboratories, Albuquerque, New Mexico and the State University of New York at Buffalo, New York, USA, have now demonstrated the amplification of optical phonons in a specially designed metal-semiconductor nanostructure [Fig. 1(c)]. The system consists of a metallic dog-bone antenna on top of...
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