  We demonstrate theoretically that ultrafast heating of metal nanoparticles by
the laser pulse should lead to the generation of coherent terahertz (THz)
radiation during the heat redistribution process. It is shown that after the
femtosecond laser pulse action the time-dependent gradient of the electronic
temperature induces low-frequency particle polarization with the characteristic
timescale of about fractions of picosecond. In the case of the directed
metallic pattern, the THz pulse waveform can be controlled by changing geometry
of the individual particle. The generation mechanism proposed in this Letter
can be used for interpretation of the recent experiments on the THz generation
from metallic nanoparticles and nanostructures.
