The third-order nonlinear optical susceptibility of gold

We critically analyze reported measured values of the third-order nonlinear optical susceptibility χ(3)χ(3) of bulk gold. Reported values of this quantity span a range of more than three orders of magnitude. Much of this variation results from the use of different measurement procedures which are sensitive to different contributions to the nonlinear optical response. For example, values measured through use of third-harmonic generation or non-degenerate four-wave mixing tend to be significantly lower than those obtained from measurements of the intensity-dependent refractive index. We ascribe this behavior to the fact that the first two processes respond only to “instantaneous” nonlinearities, whereas the nonlinear refractive index has a contribution from the much stronger but much slower “hot electron,” or “Fermi-smearing” mechanism, which has a response time of the order of picoseconds. The data also reveal that the hot-electron contribution has a strong dependence on laser wavelength, because of the turn-on of the 5d to 6sp transition at about 550 nm. It is hoped that the compilation presented here will prove useful in establishing what value of χ(3)χ(3) is most appropriate for adoption under various laboratory conditions.