Ultrafast pump-probe second harmonic generation - Always a reliable tool to study surface dynamics?

In this contribution, we demonstrate that a generalisation of optical secon d harmonic generation as a surface sensitive probe to study ultrafast dynam ics in a pump-probe arrangement must be handled with care. We show that, fo r non-collinear pump and probe beams, interference between both beams can r esult, because of the optical Kerr effect, in an induced index grating in t he irradiated volume. The grating lifts the bulk inversion symmetry in that region. Consequently, SHG becomes allowed in the full volume where the gra ting is present, and is no longer surface sensitive. This results in an enh ancement of the second harmonic signal. Since the grating appears and disap pears instantaneously upon correlation of pump and probe, it is very likely in this case, that one measures the dynamic properties of the grating and not of the surface.