In the case of peak THz electric field modulating plasma during the time of one fs-laser pulse, (a)–(e) time derivative of plasma density d N e / d t (black line) and plasma density N_e (blue line) with various frequency ratios; (f)–(j) variation of time derivative of plasma density Δ d N e / d t after subtracting the case without a modulation of THz wave with various frequency ratios.