Shorter, faster

When examining the length of the visible light waves, it becomes clear that no technological invention could use them to create flashes shorter than a femtosecond.
The key to overcoming this limitation lies in the interplay between the laser light and the material.

Scientists, like athletes, test the limits of human ability. How fast can a person run? how far? How much effort can he concentrate in a short time? And on the other hand, to what level is it possible to document and understand very fast processes?

When, for example, an electron is "torn" from a molecule, the remaining electrons reorganize to maintain the balance of the electric charges in the molecule. How is the reorganization carried out? How long does it take? In the past, scientists solved these questions by saying that after an electron is "removed" from a molecule, the remaining electrons "immediately" create a new equilibrium. The expression "immediately" reflected the fact that the monitoring systems available to the scientists showed the immediate formation of a new electron array, without intermediate stages. But this fact is just another example of the way in which the limitations of technology also limit scientific research.

To follow fast processes in molecules, such as a reaction in which one molecule breaks and splits into two smaller molecules, scientists use fast laser flashes, which serve as a kind of "cameras" that capture and "freeze" the molecular processes, just as sports photographers' cameras freeze basketball players floating in their path For the "ring" a ball in the basket, or as technical photographers record the impact of a rifle ball in a glass cup. As the natural process under investigation takes place more quickly, the laser "camera" required for the purpose of "freezing" the stages of the process must produce shorter and faster laser flashes. For several years, the fastest lasers available to scientists were able to produce flashes lasting several millionths-billionths of a second ("femtoseconds").
These are very fast flashes, which make it possible to study molecular reactions. But the movement of electrons in an atom or molecule takes place in even shorter periods of time. To investigate and "freeze" such movement, faster "cameras" were required. And here, more or less, the business stalled for several years. "The limit of the capabilities of the lasers stemmed from a basic barrier," says Dr. Nirit Dudovich, who joined the Department of Physics of Complex Systems at the Weizmann Institute of Science about two years ago. "The shortest laser flash that can be produced is limited by the length of one cycle of the light wave. When you look at the length of the visible light waves, it becomes clear that no technological invention could use them to create flashes shorter than a femtosecond."

The key to overcoming this limitation lies in the interplay between the laser light and the material. The experiment begins by activating powerful laser flashes lasting a few femtoseconds. Such radiation, which is sent towards certain molecules, may "tear off" one of the moving electrons in the molecule, which can allow the electron to "escape" and go on a "short trip" - a quantum phenomenon called "tunneling". As it happens in many other cases in life, the "short trip" leads the electron back to its parent molecule. When the electron that went on a "trip" returns and enters the molecule, it causes the emission of a photon (particle of light) with a wavelength much shorter than that of visible light. The whole process takes place during a time shorter than the cycle length of the light wave. This method enabled the production of lasers that "fire" flashes with a length of tens of "atto-seconds" (an atto-second is a billionth-billionth of a second). Such lasers are able to photograph and "freeze" the movement of electrons in atoms or molecules, which has created a new field of research where, through a rapid series of photographs, it is possible to follow changes in the position of electrons in different systems. For example, recording different stages in the movements of electrons in the process of breaking down a certain molecule.

But Dr. Dudowitz took her research question one step further. "I thought", she says, "why use a laser to track other molecules, if I can study the process of light emission from the molecule on which the laser itself is based". In other words, it is a kind of self-portrait. The molecule that emits the fast photons films the process in which its own electrons act and move during the emission process. The question before Dr. Dudovich and the members of her research group is, how can the "torn" electrons that "go on a trip" be made to come back and hit the molecule or atom from different directions (something equivalent to photographs from different angles showing different sections of the molecule)?

The answer, which they describe in an article recently published in the scientific journal Nature Physics, is based on the ability to control the trajectory of the electron before it returns to the atom. By changing the polarization of the radiation that "tears" the electron from the atom, the scientists were able to control the directions from which the electrons returned from their "trips", which means taking a picture of the atom emitting the laser light from different angles, during the emission itself. This method is somewhat similar to the operation of various medical imaging systems. In this way, Dr. Dudovich and her group members were able to characterize the distribution of electrons in the atom. "In the future," she says, "we aim to measure time as well, and combine it with measurements of the position of the electrons. Thus, instead of a frozen image, we will get a kind of film that records the movement of electrons in different processes of different chemical reactions."