In the case of peak THz electric field modulating plasma during the time of one fs-laser pulse, (a) time derivative of plasma density d N e / d t and (b) variation of time derivative of plasma density Δ d N e / d t after subtracting the case without a modulation of THz wave with relative-phases Δ φ = 0 and π / 2 when ω 2 / ω 1 = 2 .