Acoustic phonons exhibit a linear relationship between frequency and phonon wavevector for long wavelengths. Many phonon dispersion curves have been measured by inelastic neutron scattering. Infrared spectroscopic ellipsometry (IRSE) can probe lattice vibrational (phonon) and free charge carrier (plasmon) properties. The strength of electron phonon interaction is determined by the dimensionless coupling constant formula 1. This velocity saturation phenomenon results from a process called "optical phonon scattering". Excitation can also be achieved at energies below that of zero phonon emission. With increasing temperature, phonon concentration increases and causes increased scattering. Despite its 2-D nature, graphene has 3 acoustic phonon modes. Tests on phonon dispersion curves yield sufficient accuracy compared to the experimental data. The thermodynamic properties of a solid are directly related to its phonon structure. The single phonon is called an "elementary excitation". Consider an illustration of a transverse phonon below. Two schemes for steady stimulated phonon generation were mentioned. "weakly coupled" because the anharmonic energy is small compared with the phonon bandwidth. This is the phonon driven RESET operation.