Since first invented, the effort to make lasers that can produce shorter and more powerful pulses of light has been a very active one. One driving force is that if you want to take a picture of something occurring very rapidly, you need a very short pulse of light to prevent the image from blurring. The first ruby laser produced microsecond pulses of light, but more recently femtosecond optical pulses a billion times shorter have become common. Still shorter pulses belong to the attosecond regime - the regime wherein a University of Central Florida research team is creating optical pulses sufficiently brief to stop quantum mechanics in its tracks.

An attosecond is a millionth of a picosecond, or 10−18 of a second. Light travels 0.3 nanometers in an attosecond, which is roughly the spacing between atoms in a solid. To put this another way – an attosecond is to a second as a second is to twice the age of the Universe.

A one attosecond pulse of light would mostly be made of x-rays, as less energetic photons have wavelengths too long to fit in the pulse. Despite these amazing properties, University of Central Florida Physics Professor Zenghu Chang has managed to develop methods and apparatus for generating attosecond pulses of light one at a time. His shortest pulse is a mere 67 attoseconds. So fast that Prof. Chang needed to create a new sort of camera just to measure the pulses.

The key to attosecond technology is extremely nonlinear optical interactions. When a noble gas atom is hit by a laser pulse having an electric field strength of about the same magnitude of the atom's own field, an outer electron is removed, leaving an ion and a free electron. The free electron is then accelerated by the electric field of the light. As the direction of the light's electric field oscillates back and forth, so too the electron will move back and forth in response to that field. First the electron moves away from the ion, and then returns to the ion as the electric field changes direction. When the electron recombines with the ion, the resulting atom is in a very highly excited state, owing to the kinetic energy the electron gained from its interaction with the electric field. The atom then emits its excess energy as a photon. Because of the acceleration of the electron, however, the photon emitted has far more energy than does a photon from the incoming beam.

Simplified schematic of Chang's attosecond optical pulse generator. Nonlinear interaction of the focused femtosecond pulses with the inert gas in the cell shortens the input pulses by a factor of a thousand.

In Prof. Chang's work, attosecond pulses are generated by interacting intense femtosecond lasers with noble gases, typically krypton. His laser forms pulses of light with a wavelength of 800 nm having a duration of 35 femtoseconds (1000 times longer than an attosecond) and an energy of 11 mJ per pulse. This corresponds to an optical power of about 0.3 gigawatts, which is more than enough to power nonlinear optical interactions.

The shortest pulse generated in Chang's lab has a duration of 67 attoseconds, corresponding to a length of some 20 nm. The pulse contains photons having a range of energies from about 55 to 130 eV, which means the attosecond pulse contains photons having energies more than 100 times those of the original photons.

Data contrasting the pulse characterization of Chang's PROOF technique with the standard technique

Once you have created a brief pulse, it would be nice to know just how short it is. To accomplish this Prof. Chang created a very fast camera, the Phase Retrieval by Omega Oscillation Filtering (PROOF). The PROOF technique is highly complex, made difficult by the need to characterize pulses whose bandwidth is greater than their peak frequency. It requires further refinement, but the figures above demonstrate that PROOF is more accurate on such pulses than conventional streak camera methods (labelled CRAB).

“Dr. Chang’s success in making ever-shorter light pulses helps open a new door to a previously hidden world, where we can watch electrons move in atoms and molecules, and follow chemical reactions as they take place,” said Michael Johnson, physicist and Dean of the UCF College of Sciences. “It is astounding to imagine that we may now be able to watch quantum mechanics in process.”

Chang's new laser system which will generate 5 femtosecond pulses with 150 millijoules energy per pulse

A large proportion of the quantum mechanical phenomena that control our world, such as electrons moving to form or break chemical bonds, occur at extremely short time scales, as small as 100 zeptoseconds (a zeptosecond is a thousandth of an attosecond). To see what is going on, you need a flash short enough to freeze the action. Prof. Chang is confident that his methods can be extended to generate zeptosecond pulses with an energy of a microjoule, yielding a power of ten trillion watts.

This means that Prof. Chang's research is quickly approaching the point of directly observing phenomena which we have only understood from indirect evidence, an exciting prospect for scientists, engineers, and the rest of us who will benefit from new opportunities opened by attosecond instrumentation.

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