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What’s in a Femtosecond of Laser Light? A Map of Electron Energy


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Illuminate a piece of metal, such as copper or silver, and the electrons get excited. These excitable particles in turn alter the electromagnetic fields that give rise to many of the properties technologists exploit, such as copper’s excellent performance as a conductor of electricity.

Efforts to observe electrons have become easier in recent years, thanks to advances with incredibly short laser pulses, despite the foundational principles of quantum mechanics that hold sway at this scale. Quantum mechanics and its wave functions suggest one can observe an electron’s motion but not without introducing uncertainty about its position, for instance, among other facts one might want to ascertain about an electron. And, it’s far more common to observe an electron losing energy than to observe one gaining it. But a better understanding of what happens to electrons excited by incoming light might allow for better photovoltaics or better design of electronic systems that employ light, such as some advanced computer chips.

Now, a team of researchers at the California Institute of Technology has observed electrons in action, creating maps that show the energy fields of excited electrons over time on silver and copper surfaces. Using an electron microscope, the scientists focused the beam on a silver nanoparticle with a larger graphene backing for a femtosecond (one millionth of one billionth of a second, or really, really, *really* short). The energy gain (or loss) is then calculated from the time delay between the pulses of laser light and electrons. The researchers call the technique “ultrafast spectrum imaging,” which actually fails to capture in language how fast it is.

The idea is to map where energy is gained and lost in the electrons of a specific elemental compound. Such a map shows the likely position of excited electrons (and even the amount of energy gained) without revealing other properties (thus keeping the finding in line with the Heisenberg uncertainty principle). So, for example, the new research shows that the triangular silver nanoparticle gained the most energy along its left border and lower right hand corner (for reasons of the particle’s thickness and edge sizes, which are smaller than the wavelengths of the incoming photons). The ultrafast technique could next allow scientists to see the interactions of molecules, the properties of particles and, ultimately, the inner workings of cells. Not bad for a super short pulse of laser light.

Image: © iStockphoto.com / Tamer Yazici

 

About the Author: David Biello is the associate editor for environment and energy at Scientific American. Follow on Twitter @dbiello.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. jtdwyer 10:39 pm 01/6/2012

    Fascinating! This can hopefully be used to reveal some key elements of atomic structure and interaction.

    Link to this

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