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About I-RITE

Disciplining Molecules

Miro Shverdin
Department of Applied Physics
Stanford University
June 2002

Special techniques are often used to observe events that happen faster than the eye can see. In a race, for example, a photo finish is used to discern who crossed the line first. I am interested in observing and controlling processes that occur during the time it takes for an electron to move around an atom: a very short time! My research group has developed methods for producing pulses of laser light shorter than anyone has ever created, and then using these pulses to observe and control chemical reactions as they happen. What we hope to gain is a greater understanding of these very fast processes and an ability to create new types of light-matter interactions.

Creating very short laser pulses is an important scientific and technological challenge. As laser pulses have been made progressively shorter, they have greatly benefited fields of medicine, materials processing, communications, and precision standards. Some specific examples in which very short laser pulses are employed include laser surgery, laser welding and cutting, and exact time measurement. As laser pulses are made ever-shorter, we are able to observe and control ever-faster events. A molecule is an example of something that is very small and moves and reacts very rapidly. Since molecules represent building blocks of matter, an ability to understand and control their interactions is interesting for both practical and academic purposes.

We have developed a novel technique for generating short laser pulses. After applying very intense laser beams to molecules, the molecules begin to move in unison and display the same behavior. This process involves driving the molecules with a specially chosen combination of laser colors and intensities. When the molecules are "excited," they produce over one hundred different colors, ranging from ultraviolet to deep red. These colors, however, differ significantly from those produced by the sun or a light bulb in that all of the beams produced are coherent. We can think of coherence as being like waves in the ocean; just as we can distinguish where we are on the ocean wave (top or bottom), we can talk about where we are on the light wave when light is coherent. We can think of a molecule controlled by a laser pulse as being like a boat controlled by an ocean wave. If light is not coherent, then the situation is similar to a boat on the water experiencing waves in every direction at the same time; there is no telling where it will go or what will happen to it. After we create many coherent beams of different colors, we can combine these beams together. If we think of two ocean waves colliding, they produce many different patterns. A wave can become chopped in some places and become higher in others. Similarly, when we combine laser beams together, we can create many different shapes depending on how much we delay one pulse with respect to the others. By choosing the appropriate delays for all of our generated laser pulses, we create an output of extremely short laser pulses.

Experimentally, controlling the delays between the different pulses is a complicated problem that we are currently trying to resolve. Another difficulty is trying to observe the ultra-short pulse once it has been generated. Since it is shorter than what has been previously done, we need to develop our own measurement technology. This is done by observing how the created pulse interacts with other molecules. We use different types of molecules to create pulses, delay them, and measure the obtained ultra-short pulse.

Manipulating molecules and atoms is one of science's key objectives. We therefore hope to further advance and enhance people's scientific ability to do so. Molecular and atomic interactions are very rapid, and short pulses allow us to take snapshots of chemical reactions as they happen. Once we better understand how certain processes occur, we can also figure out ways to modify them. This could lead to the more efficient production of certain useful compounds, or even the creation of molecules that would not otherwise occur. Best of all, because this new tool provides us with the capability to observe new regimes of nature, the potential for valuable discoveries is significant.