Amplification of Impulsively Excited Molecular Rotational Coherence

We propose a scheme for preparation of high-coherence molecular dynamics which are phase stable with respect to ultrashort pulses. We experimentally demonstrate an example of this scheme using a phase-independent, nanosecond-duration, pump pulse to prepare a rotational coherence in molecular hydrogen. This rotational coherence is made phase stable with respect to a separate source of ultrashort pulses by seeding. The coherence is used to generate spectral broadening of femtosecond probe radiation by molecular phase modulation.

Figure 1

Diagram showing the experimental setup (a) and field-matter interactions (b,c,d) for phase-stable MPM. A Ti:sapphire chirped pulse amplifier laser produces a weak seed pulse and a delayed probe pulse; a Nd:YAG laser generates the pump pulse. (b) The seed excites a rotational coherence by ISRS. (c) The coherence is enhanced using a phase-independent pump pulse. (d) The probe pulse is spectrally broadened by MPM.

Figure 2

Rotational-Stokes 1 growth at 549 nm in hydrogen at 760 torr as a function of pump pulse energy for seeded (blue circles) and unseeded (red crosses) cases. Stokes growth is more efficient when a $12\mu \mathrm{J}$ seed is used.

Figure 3

The 800 nm probe spectrum after propagation through the seeded hydrogen gas at 1500 torr with (solid green line) and without (dash-dotted blue line) the pump. MPM broadens the probe spectrum when the pump is on. The theory plot (dashed red line) is generated by numerical solution of the equations describing the one-dimensional Raman field-matter interaction. Inset: 400 nm probe spectrum with (solid green line) and without (dash-dotted blue line) the pump.

Figure 4

(a) Probe spectrum as a function of delay relative to the seed pulse from an initial delay of 3.9 ns. Line outs from the probe spectrum at $\lambda =763\mathrm{nm}$ (b) and $\lambda =839\mathrm{nm}$ (c). Peaks at $\Omega$ in the Fourier spectrum of the line outs (d), (e). The delay-dependent oscillations at $\Omega$ demonstrate phase coherence of the excitation with respect to the probe pulse.