Influence of an ultrafast transient refractive-index grating on nonlinear optical phenomena

With two interfering femtosecond pulses it is possible to produce an ultrafast transient refractive-index grating with relatively high contrast in a transparent material. The grating has a significant impact on nonlinear optical effects, it is responsible for a loss of surface sensitivity in second-harmonic generation from centrosymmetric materials, an enhancement of the generated third harmonic due to self-phase matching, and for the diffraction of the generated harmonics. The latter phenomenon is determined by the nonlinear equivalent of the diffraction law. In a nonresonant material, the refractive-index grating is a thin slide with the thickness of the spatial equivalent of the pulse duration, propagating through the crystal with the group velocity. The duration of the grating depends only on the pulse width, in our case 80--100 fs. Besides offering a fundamental point of view, this opens the way to a variety of interesting applications, in particular in telecommunications, such as any kind of beam manipulation, optical switching, or demultiplexing, all of them on a femtosecond time scale.