An analysis of femtosecond optical heterodyne detected (OHD) birefringence and dichroism is presented within the framework of the Z-scan technique previously developed for the measurement of nonlinear refractive indices. In particular, the phase of the effective local oscillator field, in the language of OHD spectroscopy, is shown to depend on the sample position (z) and the spatially detected portion of the signal pulse in the far field for nonresonant pump-probe responses due to focused Gaussian beams. Consequently, this treatment shows how the contributions of OHD birefringence and dichroism, and the corresponding homodyne signals, can best be separated in this simple two-beam experimental configuration and provides a description of the previously reported technique called position-sensitive Kerr lens spectroscopy. The nonresonant third-order polarization electronic response of fused silica illustrates this apertured OHD description. Spatial and frequency filtering results in ultrafast Z-scan responses that are dominated by dichroic and birefringent responses, respectively. An analysis of apertured OHD spectroscopy which incorporates a description of the optical phase fronts of the signal fields is required in order to exploit this relatively simple experimental technique for quantitative determinations of polarization-specific electronic responses and Raman nuclear spectral densities. (C) 2001 American Institute of Physics.