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.
DOIMulti-cycle or narrowband terahertz (THz) frequency sources with simultaneously high pulse energies (>10 mJ) and peak fields (>100 MV/m) in the frequency range between 0.1 and 1 THz are of great interest for compact particle acceleration [1], coherent X-ray generation [3], linear and nonlinear spectroscopy and radar applications. Such a class of THz sources has the potential to achieve considerable reduction in the cost and size of current accelerators to enable unprecedented modalities in biomedical imaging, therapy and protein structure determination [2]. Among existing THz generation methods, photoconductive switches can be efficient [3], but are challenging to scale to high pulse energies.Vacuum electronic devices such as gyrotrons [4] are limited in their frequency of operation or peak powers while free-electron lasers are expensive large-scale facilities [5].Due to the rapid increase in laser pulse energies produced by solid-state laser sources, laser-driven narrowband THz generation methods based on difference-frequency generation (DFG) are attractive. However, an issue that needs to be addressed is the realization of high optical-to-THz energy conversion efficiencies (or conversion efficiencies), particularly at high pump energies. To generate tens of millijoules (mJs) of THz energy from Joule-class lasers, conversion efficiencies >>1% are necessary.Previous work on multi-cycle THz generation demonstrated conversion efficiencies in gallium arsenide (GaAs) of 10 -4 [6,7], in gallium phosphide 10 -6 [8], and organic materials 10 -5 [9]. In lithium niobate (LN), multi-cycle THz generation by interfering chirped and delayed copies of a pulse with tilted-pulse-fronts (TPF) was demonstrated [10]. However, TPFs have limitations due to group-velocity dispersion (GVD) induced by angular dispersion [11]. Such issues were circumvented by optical rectification in cryogenically cooled periodically poled lithium niobate (PPLN) crystals, but the conversion efficiency was only 0.1 % at 0....