Broadband terahertz generation by optical rectification of ultrashort multiterawatt laser pulses near the beam breakup threshold

We identify the physical factors that limit the terahertz (THz) yield of an optical rectification (OR) of ultrashort multiterawatt laser pulses in large-area quadratically nonlinear crystals. We show that the THz yield tends to slow its growth as a function of the laser driver energy, saturate, and eventually decrease as the laser beam picks up a spatiotemporal phase due to the intensity-dependent refraction of the OR crystal. We demonstrate that, with a careful management of the driver intensity aimed at keeping the nonlinear length larger than the coherence length, OR-based broadband THz generation in large-area lithium niobate (LN) crystals is energy-scalable, enabling an OR of multiterawatt laser pulses, yielding ∼10µJ/cm2 of THz output energy per unit crystal area. With a 27-fs, 10-TW, 800-nm Ti:sapphire laser output used as a driver for OR in large-area LN crystals, this approach is shown to provide a THz output with a pulse energy above 10 µJ and a bandwidth extending well beyond 6 THz, supporting single-cycle THz waveforms.