(a)–(e) Time derivative of oscillating current d J / d t with various frequency ratios in the case of peak THz electric field modulating plasma during the time of one fs-laser pulse. (f) and (g) Variation of time derivative of oscillating current Δ d J / d t in the peak and valley positions of incident THz wave with ω 2 / ω 1 = 2 and 3, respectively.