  A highly efficient, practical approach to high-energy terahertz (THz)
generation based on spectrally cascaded optical parametric amplification
(THz-COPA) is introduced. The THz wave initially generated by difference
frequency generation between a strong narrowband optical pump and optical seed
(0.1-10% of pump energy) kick-starts a repeated or cascaded energy
down-conversion of pump photons. This helps to greatly surpass the
quantum-defect efficiency and results in exponential growth of THz energy over
crystal length. In cryogenically cooled periodically poled lithium niobate,
energy conversion efficiencies >8% for 100 ps pulses are predicted. The
calculations account for cascading effects, absorption, dispersion and
laser-induced damage. Due to the coupled nonlinear interaction of multiple
triplets of waves, THz-COPA exhibits physics distinct from conventional
three-wave mixing parametric amplifiers. This in turn governs optimal
phase-matching conditions, evolution of optical spectra as well as limitations
of the nonlinear process.
