Abstract

We present a new simple method feasible for chirped femtosecond pulses temporal structure characterization in a spectral range from ultraviolet to mid-infrared. The method is based on phase-match insensitive cross-correlation measurements of group delay between spectrally limited pump pulse at central frequency ω_p and the corresponding spectral component of an analyzed (probe) pulse at central instantaneous frequency ω_i (t). A principle scheme is shown in Fig. 1. The cross-correlation is obtained due to two-photon absorption (TPA)¹ in a thin sample made from an appropriate material (e.g. water, fused silica, diamond, etc.). The band gap energy of the absorbing material E_g should satisfy the condition ω_p < E_g/ħ < ω_p+ ω_i, + ΔΩ_i, where ΔΩ_i is the spectral resolution corresponding to the pumping pulse bandwidth. Behind the sample any suitable spectral system can be installed to select the frequency interval ΔΩ_i centered at the chosen value . ω_i.Transmission of the sample at CO, depends on intensity of pump pulse, the corresponding value of the TPA coefficient β₂ (ω_p,ω_i) ¹ and both spatial and temporal overlapping of the interacting pulses. The extreme of the cross-correlation function (minimum of the transmission at the chosen ω_i) is shifted on time delay τ_D from the “real zero" position corresponding to the maximum of the autocorrelation function. Finally, one can plot the dependence of the group delay on instantaneous frequency τ_D (ω_i) Being combined with the data of the traasmission spectrum of the analyzed signal the information is enough for a characterization of the chirped pulse. One can also reproduce the intensity profile of the analyzed pulse in the sample if the dependence β₂ (ω_p,ω_i) has been carefully measured as well as spectral parameters of the used elements. The temporal resolution of the method is limited primarily by the pump pulse width. The spectral range of the method is limited from a blue side by the value E_g of the chosen optical material (down to 200 nm). The red boundary depends in major on the Boltzman broadening of the conductive and valence band boundary levels. For some materials it exceeds 10 µm. For some practical cases an object of the transient spectroscopy can be used as the two-photon absorbed material itself.

© 2001 EPS