The evolution of terahertz(THz)waveform in air plasma driven by low-energy few-cycle laser pulses is investigated to improve the accuracy of the carrier envelope phase(CEP)determination.Based on the transient photocurrent model,a balanced spatial distribution of the Kerr and free-electron effects in the plasma is found at 109μJ input energy.THz inversion occurs only once at the initial CEP of 0.5π,in which high-precision measurement of the CEP of few-cycle laser pulses is achieved.
