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Raman based source development
Methods based on coherently enhanced Stimulated Raman Scattering (SRS) [1,2] offer the potential to generate isolated sub-fs pulses that are complementary to those generated through High Harmonic Generation (HHG)techniques, being predicted to be of significantly higher energy (a few μJ), but longer pulse duration (500 as - 1 fs). At Imperial College, experiments have been conducted towards this goal, with the successful demonstration that a strong adiabatically prepared material coherence can be exploited to enhance the SRS of an ultrashort (~100fs) pulse. This resulted in the generation of sufficient bandwidth to support short trains of approximately ten 3 fs pulses, focusable to peak intensities > 1013 Wcm-2 [3,4].
Adiabatic generation of a vibrational coherence
The adiabatic generation of a strong material coherence for the application of sub-fs pulse generation by SRS was first pioneered by groups working at Stanford and Tokyo [5,6]. Recently, the Stanford group have demonstrated that the Raman sidebands generated from such a scheme have the correct phase properties to synthesize trains of sub-fs pulses in the time domain . When a coherence is prepared between two vibrational eigenstates of a molecular system, the third order polarisation in the sample is increased, resulting in enhanced efficiency of SRS. This leads to the generation of multiple Raman sidebands of the fields used to generate the coherence (narrowband "driving" fields) at low pressure and driving field intensities (as compared to that used for low-coherence SRS).
We observed the generation of 9 antistokes sidebands of our driving fields (intensity 0.4 GWcm-2 and 0.7 GWcm-2) in a sample of molecular D2 at pressures of ~200 mbar (see fig 1). The sidebands were observed to be generated collinearly with the driving beams: a characteristic of a coherently enhanced Raman process. The characteristic detuning properties of sidebands generated in a sample in which a coherence had been prepared by adiabatic techniques (the reduction in the sideband yield if the driving fields frequencies are chosen such that their difference exactly matches the frequency of the Raman transition) was also observed (see fig 2). We were therefore able to confirm that a strong material coherence had been generated in the sample.
Using the coherence to generate isolated sub-fs pulses
The generation of isolated pulses from such a scheme has been analysed theoretically , and requires the SRS of a pulse of duration shorter than the Raman transition frequency (11 fs for the D2 Q(0) transition). However, such pulses are unsuitable for the adiabatic preparation of a strong coherence due to their broad bandwidth. Therefore, we introduced a probe pulse of duration 130 fs to the sample that had been coherently prepared by the narrowband driving fields (see fig 3 for experimental set-up). This technique has the advantage that the enhancement in SRS efficiency provided by the coherence allows a low intensity ultrashort pulse to be used (10 GWcm-2), and thus the problems associated with unwanted nonlinear effects are avoided. We successfully demonstrated the efficient SRS of the probe pulse, generating two Stokes and two anti-Stokes sidebands with an efficiency > 5% [3,4]. This is a promising result, indicating that the generation of isolated, high power, sub-fs pulses by this technique is a realisable goal for future work.
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