1. Seeing bonds shake on single molecules
Eric O. Potma, University of California, Irvine, CA
Can one follow the motion of molecular bonds on individual molecules? This is an important question, as seeing molecules shake would enable an arsenal of fundamental molecular investigations that have hitherto been impossble. The experimental techniques suitable for capturing the oscillatory motion of molecules, so-called nonlinear optical spectroscopy methods, have never been used to probe systems at the level of individual molecules. It is a challenging quest which pertains to the limits of our current technical capabilities.
First nonlinear coherent microscopy images of an individal carbon nanotube. Shown on the left is the tube when driven with the polarization aligned with the major optical transitions in the tube, on the right when the polarization is orthogonal.
Experiments performed in this program has brought us one step closer to the goal of measuring single molecular bonds. In the Figure above, we have used three photons to induce an optical signal from a single molecular compound: a carbon nanotube. These so-called four-wave-mixing experiments are exactly the kind of nonlinear optical interactions that would enable seeing the motion of a bond vibration. These images show that such nonlinear optical properties can be convienently attained from individual molecules, which have opened doors to single molecule nonlinear spectroscopy.