  We introduce a general technique to generate compressed broadband terahertz
pulses based on difference frequency generation of optical pump radiation using
aperiodically poled structures. The pump pulse format and poling of the
crystals conceived are such that the emergent terahertz pulse is compressed.
The approach circumvents pump pulse distortions that result from non-collinear
approaches and the need for external compression. It is particularly efficient
for the generation of pulses of intermediate durations of few to tens of
cycles. For instance, conversion efficiencies in the few percent range are
attainable in cryogenically cooled lithium niobate.
