THE KRAMERS-KRONIG RELATIONS IN TRANSIENT SPECTROSCOPY

A new method of femtosecond time-resolved interferometry is proposed which utilizes frequency-domain interference fringes. Breakdown and sign reversal of the Kramers-Kronig (K-K) relations were observed for the first time in time-resolved spectroscopy at zero and positive delay times of the probe pulse from the pump pulse, respectively, using a frequency-domain interferometer. The conditions for applying the K-K relations to time-resolved data were obtained theoretically and verified experimentally. In time-resolved spectroscopy, for negative delays the K-K relations always hold approximately, while for positive delays the relations with opposite signs hold in the case of transparent Kerr materials.