Quantum mechanics is a powerful theory that explains much of the world at the level of atoms and molecules. Its predictive and explanatory powers are only matched by relativity and evolution. There are, however, gaps. These gaps are not the sort that you can see in everyday experiments—for instance, if you shine the right color of light on an atom, an electron might be excited from one state to another, and quantum mechanics predicts this perfectly. But there are some questions it doesn't handle so easily. How long does it take the electron to enter an excited state? How does the the probability of finding an electron in a region of space around an atomic nucleus change as the transition takes place?
To measure events like this, you need to probe the atom with light pulses that are shorter in duration than the time it takes the electron to make the transition. That means pulses of light that are less than a single femtosecond in duration, made by a process called high harmonic generation (HHG). Now, in a follow-up to earlier research, a group from Korea has shown how to do HHG in a tiny tube, opening up all sorts of experimental possibilities.
HHG experiments are easy to conceive, hard to pull off. HHG generates short X-ray pluses through the interaction between high-intensity laser pulses and gas atoms. The laser light rips an electron from an atom and accelerates it along an elliptical trajectory that terminates when the electron collides with the atom again. During the collision, it releases all the energy it gathered during its trip as a single, high-energy photon. If you set it up correctly, all the released photons have the same phase—that is, the peaks of their waves all line up together. This gets you a nice, bright, and very short light pulse.
But, the equipment is... yeah, it's not pretty. A typical example of a high harmonic generation setup involves a laser emitting light pulses of 50-100fs, amplified so that, when focused, the light pulse reaches intensities of 1TW/cm2. Just the mention of high harmonic generation will make your local Coherent sales representative grin in anticipation of dining out on your scientific ambitions for months to come.
Even with all of this equipment, you are still left with X-ray pulses that need to be coupled to an experiment. This is so difficult that almost all the experiments on electron trajectories and density distributions have been performed on gases. These results average over many different atoms with different orientations and with different starting points. Because of this averaging, interpreting the data is very difficult if the atom is replaced by something more complicated, such as organic molecules.
It would be nice to look at just a few atoms or molecules with a fixed orientation. To do that, you need your attosecond (10-18s) pulses to be highly localized in space. What better way to localize light than the combination of surface plasmon polaritons and near field optical techniques (see side bar)? This is exactly what this latest Nature Photonics paper demonstrates.
The new experiment is deceptively simple. Take very short light pulses from a commercial laser system and focus them into a very fine tube made of silver. The tube itself is cone shaped so that it ends in a sharp point. As the light travels down the tube, it is partially absorbed by the silver to generate a surface plasmon. The plasmon travels slowly, so the wavelength shortens up, condensing the energy of the light field into a smaller volume. The natural narrowing of the tube further increases the intensity until the relatively weak initial laser pulse has an intensity sufficient to generate high harmonics. The harmonics are generated from Xenon atoms that are injected into the tube along with the laser light.
I say deceptively simple because the tube took some skill to manufacture. The researchers took a hollow atomic force microscope tip and filled that with silver. They then drilled the silver out by shooting it with high energy ions. How they managed to get a hole nine micrometers deep with a taper of precisely 28 degrees I will never know, but perseverance pays off, I guess.
Then there was the problem of analyzing the light. The harmonics are generated at the very end of the tip and exit from what looks very much like a point. After such tight confinement, the light expands rapidly. And because the wavelengths are so short, you can't use lenses to corral it, so it becomes an exercise in capturing what signal you can and analyzing it a number of different ways to figure out what actually is happening at the tip.
The researchers looked at how the light spread out, and its spectrum, to resolve what harmonics were present in the spectrum. Understanding both was necessary because the resolution of their wavelength-measuring equipment is reduced by the way the light is emitted from the tip.
In the end, they concluded that they had successfully generated a spectrum that contained wavelengths as short as 18nm (visible light is in the range 400-650nm) from a starting wavelength that was on the other side of the visible spectrum at 780nm. But the big thing is that they don't need a very complicated laser for this. They used something that is almost off-the-shelf. Indeed, the laser was not amplified, allowing it to be operated with a repetition rate in the MHz range instead of the kHz range. These are all important considerations when thinking about how long it takes to perform an experiment.
The big advance here, however, is that tip, which does two things that are quite exciting. First, it cuts out longer wavelengths so you can use the tip design to choose your emission spectrum (to a certain extent). And in the vicinity of the tip, the light is quite intense and confined to a volume of just a few nanometers cubed. This means that you can scan the tip over a sample and just probe a tiny region containing a very small number of molecules. This means that, with appropriately arranged samples, one could look at the electron distributions of single molecules at a fixed orientation.
That is exciting. Apart from the coolness factor of being able to tell someone that you know where their electrons are, in computational chemistry the structure and behavior of molecules is all based on calculating electron distributions. Being able to directly compare these calculated distributions to real distributions could make computational techniques more accurate. That would cascade on to making the search for new and useful materials more efficient.
A final word about the researchers who did this work. In 2008, they demonstrated plasmonically enhanced HHG in a very different manner. Everyone in the field was (and still is) very excited about that. But in the following three years, no one has managed to replicate that result. Indeed, in talks Kim is quite open about how difficult they found the experiment—the delicate metal structures they used tend to melt. This new result seems a lot more robust, so perhaps replication will be a lot faster. Indeed, the research possibilities make it more attractive to replicate this work than their previous work.
Nature Photonics, 2011, DOI: 10.1038/NPHOTON.2011.258