A frequency comb Raman amplifier of fast molecular dynamics

Measurement of vibrational dynamics in molecules is key to the understanding of chemical processes, such as the evolution of chemical reactions, and to the ability to control them. Current methods, such as molecular tomography or pump-probe techniques generally suffer from a very low signal per molecule (1 photon or less), limiting their sensitivity and applicability. I will describe a method to coherently amplify the signal per molecule using a coherent Raman oscillator that is synchronously pumped by an optical frequency comb, thereby improving the sensitivity of detection by orders of magnitude.

By placing the molecules in an optical cavity that is synchronously pumped by a frequency comb laser source, the emission from the molecular excitation of one pulse returns to the molecule in phase with subsequent excitations and can be amplified by stimulated emission. The cavity can therefore serve as a memory that maintains the coherent emission between the pulses, enabling amplification by stimulation of the emitted signal per molecule. As we have recently shown in extensive simulation and calculation, this coherent amplifier can easily cross the oscillation threshold, producing short bursts of strong optical pulses that directly represent the wave-packet dynamics initiated by the pump pulses in the excited electronic potential.

Such an oscillator will improve the signal to noise ratio of measurements by several orders of magnitude compared to standard techniques and may open avenues for analysis of chemical reactions in the femtosecond time scale. Since here, the cavity serves as the coherent memory and not the molecules themselves, the scheme operates well also when the molecular coherence times are short; i.e. with molecules in hot / room temperature conditions, and also in solution / on surface, which broadens the scope of applicability significantly.

I will describe our (so far theoretical) research of this unique Raman oscillator , its fascinating coherence properties and the experiments we plan to perform with it.