
timeresolved nonlinear optical spectroscopy


The response of a medium to an incident electromagnetic wave E(r,t) can be described by the induced polarization P(r,t)
For strong incident intensities the higher terms in the expression should be taken into account. In particular, the nonlinear optical spectroscopy techniques take
advantage of the 2^{nd}order effects, whose intensity is correlated to the 2^{nd} order susceptibility tensor χ^{(2)}. Since the χ^{(2)}
is nonzero only for noncentrosymmetric materials or where the symmetry is broken, this method provides high surface and interface sensitivity. Moreover, it is also sensitive to "buried" interfaces
and does therefore not necessarily require UHV conditions.


In SFG, two incident photons with frequence ω_{1} and ω_{2} are combined to create a third photon with ω_{3} = ω_{1} + ω_{2}.
The SHG process is a special case of SFG, where ω_{1} = ω_{2} = ω and therefore ω_{3} = 2ω.
The created signal is detected with a photosensor or a spectrometer. Both processes can be resonantly enhanced if the involved photon energies match the energy gap between the initial, intermediate, and final level of an electronic transition.




We combine the nonlinear optical probe with another laser (pump) pulse to achieve femtosecond time resolution of the nonequilibrium charge carrier dynamics in our samples.


