December 4, 2006, London, England--Scientists from Imperial College London have perfected a technique for very accurately measuring and controlling the electromagnetic waves within some of the shortest laser pulses ever made. Being able to fully understand and control these laser pulses represents an important step towards using them to track and manipulate electrons in leading-edge research at the sub-atomic level.
The study, published in Nature Physics, focused on extremely short laser pulses--less than 10 femtoseconds long. 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 the laser must be controlled and measured with a precision which, until now, has been very hard to achieve.
The team of physicists attained what they say is an unprecedented level of accurate measurement by firing the femtosecond laser pulse into a sample of gas, which responds by emitting an x-ray pulse which is even shorter in duration--up to 10 times shorter than the original laser pulse. The researchers found that the spectrum of the x-ray pulse has encoded within it all the information necessary to precisely reconstruct the waveform of the original laser pulse. Through careful measurements and intelligent software designed specifically for this purpose, the researchers were therefore 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 (50 attoseconds)," said John Tisch, a member of the research team. "Also, 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."
Tisch said 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," he said. "They are on the attosecond timescale, which is even shorter than a femtosecond--one billion-billionth of a second. They are a new tool for scientists to probe even faster motion than the femtosecond pulses that triggered them."
The research team recently received a four-year, £2.5 million grant from the EPSRC to take this research to the next stage: using the accurate measurements and control of these lasers to manipulate electrons and control quantum processes.