High-Energy Two-Color Terahertz Generation

A laser pulse composed of a fundamental and properly phased second harmonic exhibits an asymmetric electric field that can drive a time-dependent current of photoionized electrons. The current produces a near-single-cycle burst of terahertz radiation. Experiments using ~1-TW ultrashort laser pulses observe optimal THz energies (~10-mJ) when the “two-color” pulse undergoes filamentary propagation in low-pressure gas¹. Here we use simulations to investigate the optimal conditions for two-color THz generation driven by >100-TW ultrashort laser pulses. Simple scalings indicate that the number of photoionized electrons is independent of gas pressure. As a result, use of a low-pressure, small nonlinear refractive index, high-ionization-potential gas such as helium can mitigate multiple filamentation of the high-power pulse while strengthening the field experienced by electrons at the instant of ionization, thereby increasing the current and THz energy. A high-energy (~1-mJ), THz source driven by >100-TW pulses would enable access to a novel physics regime in which bound electron nonlinear optics and relativistic plasma physics coexist.

¹Y.-J. Yoo, D. Jang, and K.-Y. Kim, Opt. Express 27, 22,663 (2019).