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Terahertz Wave Generation From Noble Gas Plasmas Induced by a Wavelength-Tunable Femtosecond Laser

We investigated the effects of pump wavelength on terahertz wave generation via two-color laser-induced plasma in various gas targets including nitrogen and noble gases. The terahertz energy was measured as functions of pump wavelength, input pulse energy, gas species, and pressure. The results suggest that the plasmas in the heavier gases induced by relatively longer wavelength lasers are more likely to generate higher energy terahertz waves. However, the existence of phase slippage in the neutral gases causes distinct fluctuations in the terahertz energy as the gas pressure increases. Meanwhile, the terahertz waves produced in the heavier gas are more likely to saturate at relatively higher pressures or incident optical powers. We propose an ionization rate formula associated with the gas species and pump wavelength to theoretically explain the validity of the experimental results. As a result, the terahertz pulse energy approaches a maximum value of 0.099 μJ per pulse in xenon with a 1500-nm excitation laser, and the terahertz wave to optical pulse energy conversion efficiency reaches 5.6 × 10–3, which is an order of magnitude higher than that of a conventional 800-nm pump laser.