  Terahertz radiation generation from liquid water has long been considered to
be impossible due to strong absorption. A few very recent works reported
terahertz generation from water, but the mechanism is not clear and the
efficiency demands to be enhanced. We show experimentally that strong
single-cycle terahertz radiation with field strength of $\rm 0.2~MV cm^{-1}$ is
generated from a water line/column of $\sim 200 \mu m$ in diameter irradiated
by a mJ femtosecond laser beam. This strength is 100-fold higher than that
produced from air. We attribute the mechanism to the
laser-ponderomotive-force-induced current with the symmetry broken around the
water-column interface. This mechanism can explain our following observations:
the radiation can be generated only when the laser propagation axis deviates
from the column center; the deviation determines its field strength and
polarity; it is always p-polarized no matter whether the laser is p- or
s-polarized. This study provides a simple and efficient scheme of table-top
terahertz sources based on liquid water.
