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Molecular phase modulation (MPM)

Top: Probe spectrum as a function of delay relative to the seed pulse from an initial delay of 3.9ns. Left middle, left lower: Line outs from the probe spectrum at 763nm and 839nm respectively. Right middle, right lower: Fourier spectra of the line outs with peaks at 586cm-1. The delay dependent oscillations and spectral peaks clearly demonstrate that the coherent molecular motion is phase-stable with respect to the fs probe radiation.
Molecular phase modulation (MPM) uses the rapid variation of refractive index in an ensemble of coherently vibrating or rotating molecules to spectrally modify radiation, allowing broadband radiation to be generated. Typically, coherent molecular motion is prepared using a rapidly changing pump field, or fields, which drive the dynamics. A key challenge is to control the phase of the molecular dynamics with respect to additional ultrafast optical sources. In a recent Physical Review Letter, researchers at the University of Oxford proposed and demonstrated a solution to this problem. The scheme involves preparation of high-coherence molecular dynamics which are phase-stable with respect to ultrashort pulses.
The scheme involves three stages. First, a fs pulse is divided into two parts, a weak seed pulse and a delayed probe pulse. The seed pulse impulsively excites a small molecular rotational coherence by impulsive stimulated Raman scattering: this sets the phase of the excitation. A much longer, more energetic, pulse from a separate phase-independent laser ("the pump") then amplifies the rotational excitation by stimulated Raman scattering, generating a Stokes field and transferring energy to the molecules in the process. This leaves a large rotational coherence in the medium with a well defined phase with respect to the seed pulse - despite the use of a completely phase-independent pump pulse. In the final step, the delayed probe pulse propagates through the coherently rotating molecules and is spectrally modified by MPM. The spectral shifting of the femtosecond probe pulse is delay dependent because the excitation was initially generated by the seed pulse which has a well defined optical delay with respect to the probe.
In the Physical Review Letter, the authors demonstrated phase-stable MPM of femtosecond probe pulses using rapidly rotating hydrogen molecules. The figure shows the 800nm probe pulse spectrum as a function of delay, relative to the seed pulse. Delay-dependent oscillations are clearly visible in the 2D spectrum and in the selected line outs from the probe pulse Raman sidebands. The oscillations show that the excitation is phase-stable with respect to the probe.
The scheme offers a general route to preparation of particular quantum electronic, rotational and vibrational dynamics with a well defined phase with respect to ultrashort pulses. As such, it will act as a vital enabling resource for time-resolved spectroscopy experiments and ultrashort pulse generation techniques, for example.