Laser pulses can be generated such that their shape and state of polarization change on the scale of a few femtoseconds, adding a new twist to the control and manipulation of molecules.
When driving a car, steering is crucial for getting where you want to go. Imagine driving a car with no steering-wheel in Manhattan: you could go to some places, along north–south avenues or east–west streets, but you would be very limited in your choice of destination. Laser scientists faced a similar limitation when trying to drive quantum systems, such as atoms or molecules, into desired states. The linearly polarized laser pulses they used could drive the system along one axis only; full control would require more complex light signals. But a series of advances in ultrafast optics — the latest reported by Suzuki et al.1 and Brixner et al.2 in Physical Review Letters — has now put a steering-wheel in their hands.
When a light pulse is short enough, it interacts with atoms and molecules before they can be affected by their environment. The interaction is then described by simple quantum mechanical rules. By carefully tailoring the shape of the optical pulse, it is possible to manoeuvre the system into desirable final states, particularly those that are hard to reach through simple thermodynamic processes. For example, it might be possible to break a certain bond in a molecule while leaving other, perhaps weaker, bonds intact. The general approach is known as quantum coherent control, a field that developed as a theoretical exercise in the mid-1980s, but which has seen intense experimental effort in recent years3.
Coherent-control experiments start with light pulses that last typically a few tens of femtoseconds (one femtosecond is 10−15 seconds). Such pulses are now routinely produced by commercial lasers. The pulses are sent through an optical set-up known as a pulse shaper, which can be programmed to generate complex temporal shapes. The shaper acts as a frequency-domain synthesizer, separating a short pulse into many frequency components. The phase, and possibly the amplitude, of each component can be tweaked individually. The result is a longer pulse with an internal structure that can be defined with great precision.
It used to be the case that all quantum control experiments with shaped pulses used linearly polarized light — light whose electric-field vector is confined to a single direction. The optical field of a linearly polarized pulse puts a force on the charges in the system — be they electrons or ions — along only one direction. Now, it is quite easy to convert this linearly polarized light into other polarization states, say circular or elliptical ones, by placing simple polarization converters in the beam. Yet this modifies the polarization of the entire pulse uniformly. Stretching the analogy made earlier, this is like driving a car whose steering-wheel is stuck, forcing it to turn constantly to one side — not a much better situation.
What is needed is the ability to change the driving direction continuously — that is, to modify the polarization direction within the optical field. This was recently realized by Brixner and Gerber4: in their polarization pulse shaper, not only the amplitude and phase but also the polarization state of the different frequency components can be changed, creating pulses with complex, twisted polarization structures. In such a pulse, the polarization direction may change on the scale of a few femtoseconds.
The polarization shaper was first used to perform a relatively simple task. By rotating the polarization of a narrow band of frequencies within the broader pulse spectrum, my group5 was able to generate two synchronized pulses whose polarization was at right angles to each other. This was used in a technique for nonlinear molecular spectroscopy known as CARS, which usually requires two laser sources. However, the main interest in polarization-shaped pulses stems from their ability to impose and control rotations. Light pulses with varying polarizations can transfer angular momentum and therefore either drive a system into a final rotating state or use rotational states as intermediates in more complex interactions. Both goals have been demonstrated in atomic and molecular systems.
In atoms, because of their symmetry, the situation is somewhat simpler than it is in molecules. Coherent control of the angular-momentum states of atoms using polarization-shaped pulses has been demon- strated by Dudovich et al.6: we showed that careful crafting of the polarization and phase of the pulse can excite, via multi-photon absorption, particular states that cannot be recognized by linearly polarized control. Controlling angular momentum in molecules is a greater experimental challenge. To start with, although molecules have preferred directions, in most experimental situations they are oriented randomly in space. In addition, the energy-level structure that is quite simple in atoms becomes more complex, as electronic states are combined with vibrational and rotational molecular states. This complexity demands more elaborate schemes for control that cannot rely on perfect knowledge of the system at hand.
Now the potential of polarization shaping for controlling molecular processes has at last been demonstrated. Suzuki et al.1 and Brixner et al.2 have investigated the ionization of diatomic molecules that were pre-aligned (to make sure they respond uniformly) and then irradiated with polarization-shaped pulses. These molecules — iodine1 and potassium2 —are as simple as molecules can be, but they are already too complex to theoretically design an optimized polarization structure that would maximize ionization. Therefore, both experiments relied on self-learning techniques, in which the optimal pulse shape and polarization structure (Fig. 1) are found through an iterative optimization procedure. Both groups have shown conclusively that pulses with complex polarization structures ionize these molecules more efficiently than pulses with a uniform polarization.
Further work will surely follow, using polarization-shaped pulses to tweak atomic and molecular systems with greater precision. Several proposals have already been made and await experimental tests, including the alignment of molecules in a gas phase, the manipulation of chiral molecules and the control of attosecond pulse formation. Expect laser scientists to steer towards even more uses for these twisted pulses of light.
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Scientific Reports (2015)