CCD broadband detection technique for the spectral characterization of the inhomogeneous signal in femtosecond time-resolved four-wave mixing spectroscopy

The spectral analysis of the inhomogeneous signal obtained from femtosecond (fs) time-resolved degenerate four-wave mixing spectroscopy on iodine vapor with the help of CCD broadband detection is reported. It is shown with this spectral analysis that the broad band detection technique makes the simultaneous observation of femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS), coherent Stokes Raman scattering (CSRS) and degenerate four-wave mixing (DFWM) processes possible. The fs lasers employed in our studies possess spectral widths which allow not only for a DFWM process but also other forms of four-wave mixing (FWM) processes such as CARS and CSRS to take place simultaneously in the molecular sample. These processes show themselves at different spectral positions within the FWM signal. The separation of the FWM signal with help of a monochromator and subsequent broadband detection with a CCD camera allow all the spectral components of the signal to be detected simultaneously under identical experimental conditions. As a result, the different FWM processes can be compared with one another directly and the continuous transition between these processes can be observed. Furthermore, the high quantum efficiency innate to a CCD camera allows for an evident improvement in the quality of the recorded transient FWM signal in comparison with detection with a fast photomultiplier. Copyright © 1999 John Wiley & Sons, Ltd.