Theodore Sjodin

Wavelength selective modulation in femtosecond pump-probe spectroscopy and its application to heme proteins

We demonstrate novel lock-in detection techniques, using wavelength selective modulation of ultrafast pump and probe laser pulses, to discriminate between vibrational coherence and electronic population decay signals. The technique is particularly useful in extracting low frequency oscillations from the monotonically decaying background, which often dominates the signal in resonant samples. The central idea behind the technique involves modulating the red and/or blue wings of the laser light spectrum at different frequencies, ΩR and ΩB, followed by a lock-in detection at the sum or difference frequency, ΩR±ΩB. The wavelength selective modulation and detection discriminates against contributions to the pump–probe signal that arise from degenerate electric field interventions (i.e., only field interactions involving different optical frequencies are detected). This technique can be applied to either the pump or the probe pulse to enhance the off-diagonal terms of the pump induced density matrix, or to selec...