  Highly-efficient optical generation of narrowband terahertz (THz) radiation
enables unexplored technologies and sciences from compact electron acceleration
to charge manipulation in solids. State-of-the-art conversion efficiencies are
currently achieved using difference-frequency generation (DFG) driven by
temporal beating of chirped pulses but remain, however, far lower than desired
or predicted. Here we show that high-order spectral phase fundamentally limits
the efficiency of narrowband DFG using chirped-pulse beating and resolve this
limitation by introducing a novel technique based on tuning the relative
spectral phase of the pulses. For optical terahertz generation, we demonstrate
a 13-fold enhancement in conversion efficiency for 1%-bandwidth, 0.361 THz
pulses, yielding a record energy of 0.6 mJ and exceeding previous
optically-generated energies by over an order of magnitude. Our results prove
the feasibility of millijoule-scale applications like terahertz-based electron
accelerators and light sources and solve the long-standing problem of temporal
irregularities in the pulse trains generated by interfering chirped pulses.
