Femtosecond coherence spectroscopy using spectrally selective differential photodetection

We present a novel, spectrally selective, detection scheme for pump–probe femtosecond coherence spectroscopy (FCS) that attenuates the non-oscillatory background signal so that the coherent oscillations are more clearly revealed. The method is based on spectral dispersion of the probe beam after it has traversed the sample, followed by detection of spectrally specific portions with a differential detector containing two balanced photodiodes. By appropriately choosing the spectral windows detected by each photodiode, we show how vibrational oscillations can be selectively enhanced or suppressed. Information about the phase behavior of the vibrational modes is also revealed using this detection scheme.