  Gold nanoparticles (GNPs) may serve as "devices" to emit electromagnetic
radiation in the terahertz (THz) range, whereby the energy is delivered by
radio frequency or microwave photons which won't by themselves induce
transitions between sparse confinement-shaped electron levels of a GNP, but may
borrow the energy from longitudinal acoustic phonons to overcome the
confinement gap. Upon excitation, the Fermi electron cannot relax otherwise
than via emitting a THz photon, the other relaxation channels being blocked by
force of shape and size considerations. Within this general scope that has been
already outlined earlier, the present work specifically discusses two-phonon
processes, namely (i) a combined absorption-emission of two phonons from the
top of the longitudinal acoustic branch, and (ii) an absorption of two such
phonons with nearly identical wavevectors. The case (i) may serve as a source
of "soft" THz radiation (at ~0.54 THz), the case (ii) the "hard" THz radiation
at 8.7 THz. Numerical estimates are done for crystalline particles in the shape
of rhombicuboctahedra, of 5 - 7 nm "diameter". A technical realisation of this
idea is briefly discussed, assuming the deposition of GNPs onto / within the
substrate of Teflon, the material sustaining high temperatures and transparent
in the THz range.
