Acoustic phonon lifetime is larger than optical phonon?

[leftbank007]

for example, in GaAs, the LA phonon lifetime can be ~850 ns, while LO phonon lifetime is only about 5 ps. Why such huge difference occurs? Thanks!

 

[BigJDubs]

The large difference in lifetimes for LA and LO phonons in GaAs is due to the differences in the way they interact with the lattice. LO phonons interact mainly through anharmonic processes, which are much faster than the harmonic processes responsible for the longer lifetime of LA phonons. Anharmonic interactions involve more complex energy exchange mechanisms, which generally lead to a shorter lifetime.