  We demonstrate record conversion efficiencies of up to 0.9 percent for
narrowband (1 percent relative bandwidth) THz generation by strongly-cascaded
difference frequency generation. These results are achieved using a novel laser
source, customized for high efficiencies, with two narrow spectral lines of
variable separation and pulse duration (250 ps). THz generation in 5 percent
MgO doped PPLN crystals of varying poling period was explored at cryogenic and
room temperature operation as well as different crystal lengths. This work
addresses an increasing demand for high-field THz pulses which has, up to now,
been largely limited by low optical-to-THz conversion efficiencies.
