Studies of Nonequilibrium Dynamics in The Time Domain

Abstract

With but one exception, the material to be covered in this lecture concerns the excitation, and subsequent dynamics in the time domain, of nonequilibrium phonons in solids. This being the case, one needs to be aware of the natural time-scale associated with phonon dynamics; namely, the “lifetime” over which the excitation exists. It is now known that the lifetime of optical phonons in a simple solid is of the order of picoseconds, whereas those of acoustic phonons varies strongly with phonon frequency (υ) and phonon branch. Typically, the lifetimes (τ) of 1 THz longitudinal acoustic (LA) phonons is of the order of microseconds and varies with the inverse fifth power of the frequency (see e.g., Baumgartner et al., 1981), such that τ(υ) 10⁵⁴ υ^-5. On the other hand, in the absence of any impurity scattering it has been postulated that transverse acoustic (TA) phonons in the dispersive regime may have lifetimes of the order of seconds (Orbach and Vredevoe, 1964). I consider here only those cases in which even TA phonons possess lifetimes ≤ 10^-6 s; hence to a regime 10^-12 < τ < 10^-6 which may be called the short to ultrashort time domain. I will further restrict myself to the transport of so-called high frequency phonons, i.e., υ ≥ 200 GHz.