A procedure to improve the sensitivity of thetwo‐colorZ‐scantechniqueis presented. The sensitivity of the technique as a function of the pump and the probe beam confocal parameters and their relative transverse dimensions is evaluated. The results show that, in the case of small nonlinear phase shift, the sensitivity of the measurements at the probe beam center may be a factor of 4 greater by the appropriate choice of the beams parameters. Results for arbitrary phase shift values and for detection at or out of the probe beam center are also reported. ©1995 American Institute of Physics.