Archive Optical Centrifuge for Molecules Joanna Karczmarek, James Wright, Paul Corkum, and Misha Ivanov Phys. Lasers can now routinely be used to nudge, trap, and stir atoms to perform Nobel-prizewinning feats, but so far molecules have avoided being subject to the same level of control. Now a paper in the 26 April PRL is the latest suggesting this might soon change. The paper describes how to use short laser pulses to create large rotational forces on molecules, causing them to rapidly spin around at speeds that can be extremely well controlled.
Atoms are amenable to manipulation by light because they are simpler than molecules.
It is possible to isolate just two atomic energy levels and tune lasers to the energy difference between those levels, which corresponds to a particular, or resonant, frequency. Being a resonant frequency of the atom, even a weak laser pulse then produces forces strong enough to control the atom, much like pushing gently on a swing at the same frequency it naturally moves will amplify the amplitude. But even the simplest molecules have a complicated pattern of energy levels that are usually very close together, so photons from even a finely tuned laser will usually end up being absorbed by levels that are not supposed to absorb them. In the past few years, however, researchers have started learning how to use nonresonant laser pulses to produce forces sufficient for controlling molecules. Joanna Karczmarek and Misha Ivanov, physicists at the Steacie Institute for Molecular Sciences in Ottawa, Canada, and their colleagues, have found a way to fashion a nonresonant infrared laser pulse to rotate molecules.