  Ultrafast imaging is essential in physics and chemistry to investigate the
femtosecond dynamics of nonuniform samples or of phenomena with strong spatial
variations. It relies on observing the phenomena induced by an ultrashort laser
pump pulse using an ultrashort probe pulse at a later time. Recent years have
seen the emergence of very successful ultrafast imaging techniques of single
non-reproducible events with extremely high frame rate, based on wavelength or
spatial frequency encoding. However, further progress in ultrafast imaging
towards high spatial resolution is hampered by the lack of characterization of
weak probe beams. Because of the difference in group velocities between pump
and probe in the bulk of the material, the determination of the absolute
pump-probe delay depends on the sample position. In addition, pulse-front tilt
is a widespread issue, unacceptable for ultrafast imaging, but which is
conventionally very difficult to evaluate for the low-intensity probe pulses.
Here we show that a pump-induced micro-grating generated from the electronic
Kerr effect provides a detailed in-situ characterization of a weak probe pulse.
It allows solving the two issues. Our approach is valid whatever the
transparent medium, whatever the probe pulse polarization and wavelength.
Because it is nondestructive and fast to implement, this in-situ probe
diagnostic can be repeated to calibrate experimental conditions, particularly
in the case where complex wavelength, spatial frequency or polarization
encoding is used. We anticipate that this technique will enable previously
inaccessible spatiotemporal imaging in all fields of ultrafast science and high
field physics at the micro- and nanoscale.
