Enhanced third harmonic generation in air by two-colour ultrashort laser pulses

Abstract

We report on third harmonic generation in air in a non-filamentation regime using tightly focused, ultrashort laser pulses (1–2 µm wavelength). Enhancement in the third harmonic efficiency is observed from co-propagating laser pulses of two different wavelengths which emanate from the same source—an optical parametric amplifier—and are spatially and temporally overlapped. The third harmonic efficiency for signal wavelength (1.35 µm) is measured to be 4 × 10−3 %; in the presence of idler wavelength (2.09 µm), the corresponding value becomes 1.6 × 10−2 %—a fourfold enhancement in efficiency. The pulse duration of the generated third harmonic is measured to be 37 fs. We examine the possible role of plasma to account for the observed enhancement in third harmonic generation.