  We propose a microscopic theory of terahertz (THz) radiation generation on
metal gratings under the action of femtosecond laser pulses. In contrast to
previous models, only low-frequency currents inside the metal are considered
without involving electron emission. The presented model is based on
plasmon-enhanced thermal effects and explains the resonant character of
optical-to-THz conversion giving an adequate estimation for the full signal
energy. Numerical modeling reproduces specific experimental features like
delayed character of THz response and low conversion efficiency when the
grating depth is too large.
