| Historically, all nonlinear optical effects with response times shorter than a picosecond were classified as "ultrafast", and considered to happen instantaneously. Recently, this classification has had to be revised. Today the shortest achievable pulse durations are shorter than the relevant time scale for any nonlinear response but that of bound electrons in a non-resonant interaction, and the sub-picosecond dynamics of nonlinear optical phenomena have become accessible. This work is concerned with determining these dynamics.;Time-domain measurements of a variety of dielectric materials, such as highly nonlinear glasses, laser gain media, and optical waveguide materials show the dynamics of the nuclear contribution to the optical nonlinearity, arising from the modulation of the polarizability through nuclear motion. The experimental data are in very good agreement with the nuclear dynamics expected from calculations using Raman scattering spectra. Nuclear contributions amount to ∼15% of the total refractive nonlinearity of all investigated glasses, independently of composition. In contrast, the fractional nuclear contribution in crystals is found to vary between 25% in Cr:LiSGaF and <1% in sapphire.;The capabilities and limits of spectrally resolved two-beam coupling (SRTBC) are investigated analytically, numerically, and experimentally. The technique is expanded to include pulses and response functions of arbitrary shape, and it is shown that SRTBC is capable of directly resolving the nuclear response of fused silica. Using this technique, we find that in the GeO 2-SiO2 glass system, both the total nonlinearity and the fractional nuclear contribution increase in proportion to germania content.;Two-beam coupling with chirped pulses is investigated numerically, in order to obtain predictions for the energy transfer due to a purely refractive non-instantaneous response. Experiments in fused silica show good agreement with the expected signals calculated using the real nuclear response function, in both shape and magnitude, verifying the model. In contrast to predictions made from analytical approximations, we find that energy transfer is possible with transform-limited pulses.;Purely electronic two-beam coupling in alkali-halides fails to conclusively establish a non-instantaneous electronic response, which from the calculation is expected to yield signal levels <10--5, beyond the sensitivity of the experiment. |
