Tobias Brixner, a physicist at the University of Würzburg in Germany, has long been fascinated by the idea of exercising control over the way light interacts with matter. Over the years he has developed ways to control pulses of laser light in order to steer chemical reactions. On page 301, Brixner and his colleagues take this technique into the realm of nano-optics.

The move into nano-optics was suggested by Walter Pfeiffer of the University of Bielefeld, who had heard with interest about Brixner's work with lasers. Together, the two set about assembling a team — including Martin Aeschlimann, Michael Bauer and Javier García de Abajo — to realize their ideas. “It was a truly wonderful cooperation between colleagues who all have their expertise in different fields,” says Brixner. “We have found an ideal combination, and we will continue to cooperate in the extension and development of this project.”

The results that the team hopes to extend have seen the researchers control the interaction of ultrashort laser pulses with matter on a length scale of nanometres. “Roughly speaking, we could 'encode' into the shape of the laser pulse the position at which to deliver an electromagnetic energy 'packet', so that it would automatically arrive in the right place once it reaches the sample, without requiring any processing time,” says Brixner.

To do this, the group had to overcome the 'diffraction limit'. Diffraction — the way in which light waves bend and spread as they pass an object — limits the spatial resolution of a light beam, just as a drill cannot make holes smaller than the size of the drill head.

The collaborators knew that electromagnetic waves behave differently when they are close to an antenna. In the 'near field', electromagnetic properties can change on a scale much shorter than the radiation wavelength so, in principle, it should be possible to exert control at resolutions below the light's wavelength. By irradiating a sufficiently small antenna with pulses of light femtoseconds (10−15seconds) long, the team hoped to use interference effects to control the light fields with ultrafine spatial precision.

But getting the appropriate kind of antenna was not going to be easy. Radio waves, for example, would require an antenna tens of centimetres long — and the researchers needed antennas for optical fields with wavelengths that were hundreds of nanometres long. They achieved this by creating a suitable arrangement of silver disks, each with a diameter of 180 nanometres.

Having built the antennas, the team then had to find a way to map changes in the electromagnetic properties of the light pulse within these nanostructures. “We wanted to show that optical fields can be manipulated below the diffraction limit, but how could we possibly measure and prove this?” says Brixner. “If we were to use any optical method to detect the changes, we would again be limited by diffraction.” So the researchers used photoemission electron microscopy, which allowed them to detect optical fields with a resolution of 50 nanometres.

By measuring electromagnetic changes in the nanostructures, the team could adjust the shape of the femtosecond laser pulses to beat the diffraction limit and achieve subwavelength nano-optical field control. “It was hard work with a very specific goal. As we had already done the theory and calculations, we were very much hoping to see an experimental success,” says Brixner. “When it finally worked, we were happy to see that our previous claims were justified and not some unrealistic fantasy.”