16 March 2016

by Hayley Dunning

Imperial and Oxford researchers have demonstrated a novel method for measuring the evolving waveforms of laser pulses just a few femtoseconds long.

Femtosecond laser pulses, measured in terms of quadrillionths (10^-15) of a second, are used in a wide range of scientific and technological applications from the study of ultrafast processes in atoms and molecules to the precision laser machining of materials.

Using these pulses effectively often requires that their waveforms are tailored to the application. While the technology for controlling lasers to achieve such ‘waveform engineering’ has advanced significantly, the methods for accurately measuring those laser pulses has lagged behind.

Now, researchers from Imperial College London and the University of Oxford have created a new method that can measure extremely complex waveforms of femtosecond laser pulses as they evolve over time. The research is published today in the journal Optica.

Laser waveforms can have complex structure, for example their amplitude and frequency can change during the pulse. Knowing exactly how the waveform behaves during the pulse will allow researchers to fine-tune the pulses for experiments and enable more accurate simulations to be performed.

Study co-author Professor John Tisch from the Department of Physics at Imperial said: “The technique should lead to enhanced use and control of femtosecond laser pulses, especially complex pulses that can’t be measured using other methods. It could perform in near real-time, giving researchers up-to-date information on what their laser pulse is doing during an experiment.”

Finding the rhythm

In order to ‘fingerprint’ a femtosecond pulse, the researchers had to invent a method that worked with attosecond resolution – sampling the pulse every few quintillionths (10^-18) of a second.

“Imagine the femtosecond waveform is a jazz trumpet track. Some basic measurement techniques can just tell you how long the track is, or if you’re lucky, that a trumpet is playing. Our method is able to follow the melody note by note, with the kind of precision that would probably enable you to identify the artist,” said Professor Tisch.

The method works by firing two laser pulses into a neon gas target. One pulse has a known waveform and the other is the pulse that needs to be measured. When the pulses interact with the neon atoms they create X-rays. Comparing the X-rays emitted from the known pulse to those detected from both pulses combined allows the waveform of the unknown pulse to be retrieved.

Cracking new problems

The team have tested their method on laser pulses in the near-infrared and the ultraviolet at opposite ends of the visible spectrum – proving that it works over a range of wavelengths. Professor Tisch already has plans to use the technique in his own lab to characterise some ‘exotic’ laser pulses.

Professor Tisch said: “The initial idea came from Dr Adam Wyatt, then in the group of Ian Walmsley at the University of Oxford, now at the Rutherford Lab. My colleague Professor Jon Marangos and I have been collaborating with the Walmsley group for a while and this was a great opportunity to do another experiment together.”

He added: “Hopefully the directness and relative ease of implementation of this technique will be as appealing to other groups as it is to us. I’m looking forward to it being used to crack some difficult research problems or in new applications of femtosecond pulses.”

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'Attosecond sampling of arbitrary optical waveforms' by Adam S. Wyatt, Tobias Witting, Andrea Schiavi, Davide Fabris, Paloma Matia-Hernando, Ian A. Walmsley, Jon P. Marangos, and John W. G. Tisch is published in Optica.