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Physicists control light at the nanoscale

19 Mar 2007

A new experiment shows that the field intensity distribution of a laser can be controlled at dimensions that are much smaller than the wavelength of light.

"This is the first time that pulse-shaping techniques have been successfully combined with near-field optics to tailor light at the nanometre lengthscale," one of the researchers, Walter Pfeiffer of Bielefeld University, Germany, told optics.org. The achievement heralds a big step forward for nanophotonics, since it could allow lasers to be used as flexible tools to manipulate matter on a very fine scale.

The idea behind the experiment dates back to 2002, when Mark Stockman, a theoretical physicist at Georgia State University in the US, suggested that pulse shaping could have significant applications in nano-optics. Such pulse-shaping techniques have traditionally been used to achieve spatial and temporal control in dimensions that are around the wavelength of the light pulse (in the order of micrometres).

Pfeiffer, together with Tobias Brixner of the University of Würzburg, Germany, and Javier García de Abajo of the Instituto de Optica in Madrid, Spain, then extended Stockman's theory to find that polarization pulse shaping – which makes it possible to control the temporal evolution of the polarization state of a light pulse – is the key to perfecting spatial and temporal control on the nanoscale. To demonstrate the effect in the lab, they enlisted the help of two experts in photoemission electron microscopy (PEEM): Martin Aeschlimann of the University of Kaiserslautern and Michael Bauer, now at the University of Kiel.

In the experiment, a femtosecond laser source, fitted with a polarization pulse shaper developed at the University of Würzburg, was used to fire regular pulses (790 nm) at a specially designed nanostructure (Nature 446 301). The nanostructure consists of six tiny silver disks fabricated in a thin film of indium-tin oxide (ITO), which were arranged in three pairs and placed next to each other in a Y-formation. The disks each have a diameter of 180 nm with an inter-disk spacing of 10 nm, with the entire structure measuring 800 nm across.

Firing the laser at the nanostructure generates near-field interference effects that can be used for ultrafine spatial control. "By using the phase difference between the two polarization components in the far field, the spatial and temporal distribution of the field in the vicinity of the nanostructure can be controlled," explained Pfeiffer.

This is because the two components of polarized light have orthogonal polarizations in the far field, while the local fields generated by these two components in the vicinity of the nanostructure are no longer orthogonal – and this leads to interference. "Careful control of the polarization of the femtosecond laser pulses together with the near-field interference effect provides a versatile tool for spatial control on a sub-wavelength scale," Pfeiffer added.

The researchers used a two-photon photoemission electron microscope to capture images of the lateral field that was generated by the pulses. The images were then input to an adaptive pulse-shaping system that fine tuned the pulse parameters until the desired emission pattern was obtained.

Experiments were devised to contrast the photoemission yields across two different regions of the nanostructure. As a means to prove the efficiency of the process, the team conducted two experiments for maximizing and minimizing the contrast ratio. In the first case, the adaptive algorithm worked to maximize the intensity in the upper region (comprising the upper arms of the Y-shaped nanostructure). In the second case, the same process was applied to focus the intensity on the lower region.

Pfeiffer now wants to push the limits of the technique even further, and the team now intends to tackle the challenge of simultaneous spatio-temporal control.

According to the team, manipulating the temporal and spatial properties of light on a nanometer lengthscale could be used in a range of applications, including new space- and time-resolved spectroscopic methods, steering of nano-mechanical processes in optical traps, control of chemical reactions in large molecular aggregates, and new schemes for quantum computation. "The experiment presented in this letter to Nature represents a first step towards these fascinating applications and will probably trigger increased interest for the emerging field of ultrafast nano-optics."

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