Abstract

The recent availability of Ti:Sapphire oscillators [1] directly delivering sub 10-fs pulses makes possible the study of the frequency-domain response of a material by coherent broad-band excitation. We show that, by using Fourier-transform spectral interferometry (FTSI) (2, 3), it is possible to measure in both amplitude and phase the coherent emission of a material subsequent to its excitation by a sequence of two ultrashort pulses. In analogy with two-dimensional Fouricr NMR [4], the data recorded as a function of time delay is Fourier-transformed to yield the second-order response function in two dimensions of frequency, ≡⁽²⁾SW(ω,σ), where ω is the sum frequency and σ is the difference frequency. The figure shows an experimental result obtained in the case of sum-frequency generation in a type-II KDP crystal [5]. The result then corresponds to a phase-matching map of KDP, the nonlinear susceptibility itself being frequency-independent over the range considered here. The experiment is in good agreement with a calculation based on the tabulated values of the refractive index of KDP.

© 1996 IEEE