  A pump-probe polarimetric technique is demonstrated, which provides a
complete, temporally and spatially-resolved mapping of the megagauss magnetic
fields generated in intense short-pulse laser-plasma interactions. A
normally-incident time-delayed probe pulse reflected from its critical surface
undergoes a change in its ellipticity according to the magneto-optic
Cotton-Mouton effect due to the azimuthal nature of the ambient self-generated
megagauss magnetic fields. The temporal resolution of the magnetic field
mapping is of the order of the pulsewidth, whereas a spatial resolution of a
few microns is achieved by this optical technique. In addition, this technique
does not suffer from refraction effects due to the steep plasma density
gradients owing to the near-normal incidence of the probe pulse and
consequently, higher harmonics of the probe can be employed to penetrate deeper
into the plasma to even near-solid densities. The spatial and temporal
evolution of the megagauss magnetic fields at the target front as well as at
the target rear are presented. The micron-scale resolution of the magnetic
field mapping provides valuable information on the filamentary instabilities at
the target front, whereas probing the target rear mirrors the highly complex
fast electron transport in intense laser-plasma interactions.
