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Researchers profile femtosecond laser pulses

08 Dec 2006

A breakthrough at Imperial College, London, exploits attosecond X-ray pulses to characterize femtosecond laser pulses with unprecendented accuracy. In an interview with optics.org, researcher John Tisch explains its implications for research into high-speed atomic and molecular processes.

Scientists have perfected a technique for more accurately controlling and measuring the electromagnetic waves in some of the shortest femtosecond laser pulses ever generated. Being able to fully understand and control these laser pulses represents an important step towards using them to track and manipulate electrons in atomic and molecular research.

The study, published in Nature Physics, focused on extremely short laser pulses, of less than 10 fs. These laser pulses can allow scientists to move and control the electrons in atoms and molecules, and to understand, for example, how molecules are formed.

To achieve this reliably, the pulse of electromagnetic waves emitted from a femtosecond laser must be controlled and measured with a precision which has previously been difficult to achieve. The research project, which commenced in 2003 and involves several UK rsearch institutions, has been funded by a £3.5 million ($6.8 million) Basic Technology Program grant from RCUK.

The team of physicists from Imperial College, London, attained measurements of an unprecedented level of accuracy by firing the femtosecond laser pulse into a sample of gas, which responds by emitting an X-ray pulse up to 10 times shorter than the original laser pulse. The researchers found that the spectrum of the attosecond (10-18) X-ray pulse has encoded within it all the information necessary to precisely reconstruct the waveform of the original femtosecond laser pulse.

Through careful measurements and algorithms designed specifically for this purpose at Imperial, the researchers have been able, for the first time, to measure the waveform of individual femtosecond pulses.

"This measurement technique is so accurate that we can determine the position of a peak in the pulse of electromagnetic waves from the laser with a precision of a mere 0.05 femtoseconds - in other words, 50 attoseconds," said John Tisch, research leader. "The measurement can be made on individual pulses rather than by looking at the average properties of many pulses, so this is an important step forwards."

"We measure the spectrum of the X-ray pulses using an imaging spectrometer which gives us spatial and spectral data. To do this we developed an algorithm."

Tisch explains that not only will this new technique lead to a greater ability to use short laser pulses for accurate sub-atomic level research, but it also sheds new light on the extremely short X-ray pulses emitted in response. "The X-ray pulses we used in the measurement process of our research are of great interest in their own right. They are on the attosecond timescale," he said. "Attoseconds are an exciting new tool for scientists to probe even faster motion than the femtosecond pulses that triggered them."

John Tisch explains

"The femtosecond laser is a research tool of growing importance. It has helped us to understand fast dynamics in molecular interactions over the past 10 years or so. As the laser pulses have been getting shorter - and the limit up until now has been around 5 fs - pulse characterization has posed a problem. For example if we want to know more about electric field oscillations in a pulse, these have been difficult to characterize.

"Our method is not to concentrate on the infrared femtosecond pulse itself but to look at the effect of the high harmonic generation in an inert gas, such as helium or neon. Such emission is in the X-ray region; the pulses are even shorter and what is useful about the attosecond emission is that it carries a history of the original femtosecond pulses' electric field oscillations.

"High harmonic generation has been studied arund the world for some time, but making the connection between the harmonics and the femtosecond pulses has been a problem.

"One of the advantages of short femtosecond pulses is they generate the short X-ray pulses. The reason that researchers are interested in shorter X-ray pulses is that they offer better time resolution which means that rapid processes can be monitored at the attosecond level.

"To extract the information we developed an algorithm that incorporates a detailed simulationof the way the femtosecond pulses interact with the gas sample to generate the X-ray pulses.

"Attosecond pulses can monitor rapid reactions such as proton or electron transfers, which are much faster events than molecular movements that are comfortably monitored using femtosecond pulses.

Experimental set up

"Femtosecond pulses are generated by a laser system that we have designed with a combination of commercial and custom-built components mounted on a 12x5 foot optical table. Imperial's equipment comprises a laser oscillator, amplifier, optics and compression device.

"The initial pulse generated measures about 30 fs which is compressed down to 6 fs. A Ti:Sa oscillator-amplifier sends the laser pulses into a hollow fiber pulse compressor, which increases the bandwidth to allow the pulse to be compressed to sub-10 fs. It operates at a 1 kHz repetition rate and the pulses typically contain 0.5 mJ energy However they have a high peak power of 1011W(100,000 MW) which is required for the high harmonic generation process.

"In the measurement of the X-ray pulses, the pulses pass through the X-ray spectrometer/channel plate to give us the image. We can also look at the photo-electrons that are produced when the X-rays go through the sample gas. We tend to use noble gases because of their high ionization potentials, which make their atoms resistant to ionization, even in a strong laser field.

Atto-world developments

"Europe has the lead in this area of research; there has been some pioneering work in Austria and Germany already and at Imperial we are putting the UK on the map. Funding for this research is by the Research Councils UK.

"At Imperial we are spearheading the project but we have partners in the work including universities of Oxford, Birmingham, UCL, Reading and Rutherford Appleton Laboratory. The main goals of the project are to generate and characterize attosecond X-ray pulses and find ways of applying them to new classes of experiment and to perform new high impact science. We are about three-quarters of the way through the project which started in 2003."

Next research project

The research team has recently received a four-year £2.5 million grant from the EPSRC to take this research to the next stage. Professor Jonathan Marangos explains: "Now we've perfected this technique, we are going to look into using our accurate measurements and control of these lasers to manipulate electrons and control quantum processes."

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