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Nanohole array brings light into focus

12 Mar 2007

Optical physicists in the UK and Spain have fabricated an array of nanoholes that can focus light into a spot that is smaller than the wavelength of light used.

The research team, led by Nikolay Zheludev at the University of Southampton and also including co-workers at the Rutherford Appleton Laboratory in Oxfordshire and the Instituto de Optica in Madrid, say that the nanohole array can focus light at distances of up to several tens of wavelengths from the screen. Possible applications include high-density optical information storage, photolithography, and making a small "light pen" that can image inside cells and other microscale objects.

The researchers obtained their result using a so-called Penrose-like quasiperiodic structure, a complex pattern of nanoholes that is neither periodic nor completely random. Electron-beam lithography was used to make the holes in a 100 nm aluminium film deposited on a silica substrate (figure 1). The finished array, which contains about 14 000 holes that are each about 200 nm across, measures about 0.2 mm.

Light beams passing through different holes in the array interfere to create foci, or bright "hotspots". Using coherent light at 660 nm, the Southampton researchers observed bright hotspots around 290 nm across, at a distance of about 12.5 µm from the array. Smaller hotspots, with diameters of around 200 nm, were also observed closer to the array (figure 2).

The nanohole arrays could be used in a variety of applications, including lithography and imaging. Moreover, the physicists say that a single hotspot, isolated with a mask, could be used as a light pen for subwavelength imaging by scanning the object under investigation across the focal spot.

The array might also be used to focus inside cells and other small objects. The relatively large gap between the array and the object will allow for much faster scanning than is possible using conventional near-field optical microscopy techniques.

And, since the focus harvests light from a large number of holes in the array, the hotspot may be brighter compared to those obtained using even the most sophisticated, single subwavelength aperture lens devices available today. This last advantage could be put to good use in high-density optical information storage and optical trapping.

The researchers reported their work in Appl. Phys. Lett.

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