  We present a novel scheme to obtain robust, narrowband, and tunable THz
emission by using a nano-dimensional overdense plasma target that is irradiated
by two counter-propagating detuned laser pulses. So far, no narrowband THz
sources with a field strength of GV/m-level have been reported from laser-solid
interaction. We report intense THz pulses at beat-frequency ($\simeq$30THz)
generated due to strong plasma current produced by the beat ponderomotive force
in the colliding region, with an unprecedentedly high peak field strength of
11.9GV/m and spectral width $(\Delta f/f$$\simeq$$5.3\%)$ from 2D PIC
simulations. Such an extremely bright narrowband THz source is suitable for
various ambitious applications.
