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Surfaces and interfaces

Surfaces and interfaces

New optical fibre shortens laser pulses the easy way

04 Feb 2015
Blink and you will miss it: the visible portion of an ultrashort pulse

A simple and efficient way of creating ultrashort infrared laser pulses has been unveiled by an international team of physicists. The technique reduces the length of a pulse by simply passing it through a specially structured, hollow glass fibre filled with a noble gas. The researchers say that the new method should make it easier for laboratories to produce pulses for studying chemical reactions on very short timescales.

The details of chemical reactions are often studied by “pump–probe spectroscopy”. This involves firing an attosecond-long (10–18 s) “pump” X-ray pulse at a sample to activate a reaction, followed a tiny fraction of a second later by a second, “probe”, pulse. By measuring the interaction between the probe pulse and the reacting chemicals, researchers can gain insight into the state of the reaction at the time it was hit by the probe. By varying the time gap between the pump and probe pulses, the progression of the reaction can be mapped out in time. Similar attosecond pulses can also be used to measure interactions between electrons in a molecule.

Attosecond pulses are produced in the lab by a process called high-harmonic generation. This involves firing an intense infrared laser pulse into a gas, where its powerful oscillating electric field drags electrons away from their atoms. The electrons then snap back to the atoms, producing an attosecond X-ray pulse.

More bandwidth needed

The process of high-harmonic generation requires ultrashort infrared pulses that are no longer than one cycle of the average frequency of the infrared light – longer infrared pulses will produce ill-timed sequences of attosecond pulses. This means that the infrared pulse needs to be less than about 5 fs (5 × 10–15 s) in duration. Such extremely short pulses cannot be produced directly by conventional lasers because confining light energy into such a narrow time interval requires a source that delivers light over a wide range of frequencies – something a laser cannot do. Instead, sophisticated and expensive optical equipment is used to shorten pulses from an infrared laser.

Now, scientists at Vienna University of Technology, together with colleagues in France, Germany, the UK and Russia, have demonstrated a simple and ingenious technique for compressing the energy in an infrared pulse into a single cycle. Ironically, the technique works because of dispersion, which usually causes pulses to spread out in time. In most materials, dispersion causes light at low frequencies to travel faster than higher frequency light. However, in materials with “anomalous dispersion”, light at higher frequencies travels faster than its low-frequency counterpart.

Basket weaving

The researchers took advantage of these two types of dispersion within an intricate “kagome optical fibre”. The central portion of the fibre is a void that is filled with a noble gas such as argon or xenon and has normal dispersion. This region is surrounded by a delicate kagome structure that resembles woven fabric – kagome is a traditional Japanese basket weave – and has anomalous dispersion. The combined effect of both regions on an 80 fs infrared pulse passing along the fibre is to squash all its energy into just 4.5 fs. Indeed, using the kagome fibre, the team could produce an output pulse with peak power of more than 1 GW, which is enough to pull electrons away from atoms.

The researchers focused their shortened pulses on a sample of xenon gas and used an interferometry technique to observe the movement of the individual electrons as they were torn away from the atoms. From these measurements the team was able to show that the pulses contained only one cycle of the infrared field. “By making the peak positive or negative, we were able to steer electrons in one direction or the other,” explains team member Tadas Balciunas. “That’s like an ultrafast electric switch.”

Many applications

The researchers now believe they can develop and simplify the technique even further. Their first priority is to demonstrate definitively that they can generate X-ray pulses using their technique. After that, says Balciunas, “there are many applications that can be tried”.

John Travers of the Max Planck Institute for the Science of Light in Erlangen, Germany, who was not involved in the research, is impressed with the new technique. “While lots of previous work, including work by me and my group, has often shown evidence that this self-compression is occurring,” he says, “this is significant because it shows that what we’ve been saying for some years is actually correct.”

The research is published in Nature Communications.

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