Coherent control of molecules for pulse compression
14 May 2011
Andrea Schiavi, Dr Adam S Wyatt, Professor Ian Walmsley
A key challange in the study of matter is the ability to actively control the dynamics of atoms, molecules and solids. The interaction of light and matter is governed by the different response of electrons and nuclei to an applied external electric field. Electrons, with a smaller mass, react almost instantaneously to the applied filed; nuclei, on the other hand move much more slowly. The effect is the creation of an electric dipole, which drives the light-matter interaction. In molecular coherent control, one would like to control the vibrational and rotational states, such that we can place the molecule in a particular state and at a particular time.
Recently we have demonstrated a new ability in this field: firstly we use a weak pulse to set the molecules in motion, and then amplify this motion using a high power inexpensive laser. This molecular or vibrational motion then acts as an extremely fast temporal "phase modulator", thus enabling temporal shaping of ultrashort optical pulses over a variety of spectral ranges, such as in the ultraviolet or the infrared. This temporal shaping could be used for example to temporally compress optical pulses to even shorter pulse durations, which can then be used to study and control the dynamics in atoms, molecules and solids on a femtosecond timescale. The molecular state preparation is performed through Raman scattering among the rotational or vibrational levels of the molecules, using a first ultrashort pulse to start the motion with an initial "kick" and a second strong pulse to spread this movement over an ensemble of molecules, keeping the same phase relation (i.e. timing) with the first pulse.
The refractive index of the medium, and thus the speed of propagation of light, depends on the orientation or compression of the molecules, and thus varies sinusoidally as the molecules rotate or vibrate in unison. Thus when a third, probe pulse, propagates through the medium with the correct timing, the leading edge of the pulse can be caused to slow down and the trailing edge to speed up relative to the centre, thus shortening the pulse duration. Further developments will enable higher efficiencies and different wavelengths to be used, thus opening the way for a tunable broadband intense source of laser pulses with durations only a few optical cycles long.
 P. J. Bustard, B. J. Sussman, and I. A. Walmsley. Amplification of impulsively excited molecular rotational coherence. Phys. Rev. Lett., 104(19):193902, May 2010.