Origin of spectral interferences in femtosecond stimulated Raman microscopy

In femtosecond stimulated Raman microscopy (FSRM), a spectrally narrow (Raman pump) pulse and a broad (Raman probe) laser pulse are employed to generate the Raman spectra of microscopic objects. The resulting spectra exhibit, in addition to the Raman bands, spectral modulations of comparable amplitude. Here a model is devised that attributes these modulations to a four-wave mixing (FWM) process. Two light fields of the probe pulse and one field of the pump pulse serve as input fields. The resulting FWM field experiences a heterodyne amplification by the probe field. Simulations based on this model reproduce the appearance of the spectral modulations. Furthermore, the amplitude of the modulations exhibits dependences on the energies of pump and probe pulses as well as on the nonlinear refractive index n(2), which are in line with the model. Copyright (C) 2011 John Wiley & Sons, Ltd.