in the exemplary embodiment of fig2 collimation is obtained with a 90 degree off axis parabolic mirror 40 as more clearly shown in fig4 the collimated thz beam 41 illuminates the target 42 placed on the stage 44 the interaction of the beam 41 and the target 42 results in a scattered emerging beam 45 on the other side of the target 42 the optical probe beam 20 is also expanded and collimated to form an expanded gaussian collimated optical probe beam 56 in the exemplary system shown in fig2 this is accomplished by a system of mirrors generally represented by mirror 48 used to direct the optical probe beam 20 along a desired path and a telescope beam expander consisting of a negative lens 50 followed by a positive lens 52 expanded optical probe beam 56 is transmitted through a polarizing crystal 54 and emerges as a linearly polarized beam 57 the polarized pulsed optical probe beam 57 and the pulsed thz beam 45 exiting the sample 42 are then directed along a coincident path and propagate co linearly as combined beams 59 in a preferred embodiment a tin doped indium dioxide thz mirror 47 is used to combine the paths of the scattered thz emerging beam 45 and the polarized optical beam 57 the combined beams 59 and the scattered thz emerging beam 45 propagate co linearly and impinge on sensor 58 the probe beam and the thz pulse propagate through the znte crystal at the same speed providing a measurement of the instantaneous thz field at the instant the probe pulse reaches the thz detector the polarization of the probe beam is measured by passing it through.