Femtosecond pulse shaping in the mid-infrared generated by difference-frequency mixing: a simulation and experiment

We have examined phase- and amplitude-modulated femtosecond laser pulses in the mid-infrared (MIR) region (3-10 μm) generated by difference-frequency mixing both theoretically and experimentally. Transfer of the pulse shape from near infrared to MIR by a difference-frequency process was evaluated in detail for various spectra, linear chirps, phases, and optical delays of two pulses before the different frequency was compared and with experimentally obtained MIR shapes. In the experiment, the signal pulse of an optical parametric amplifier was shaped with an acousto-optic programmable dispersive filter and mixed in an AgGaS2 crystal with the idler pulse that was temporally stretched by passing it through a dispersion block to generate a shaped MIR pulse. The agreement between the theory and experiment was reasonable despite the complicated experimental procedure. It was demonstrated that the resultant MIR pulse shape could be completely different from the pulse shape before the difference-frequency generation. However, it is possible to reproduce any shape of MIR pulses by predicting the pulse shape using the present theoretical framework. This will allow us to manipulate rovibrational wave packets of real molecules for practical applications.