Nanoholes and arrays of nanoholes in metal screens have many intriguing optical properties, including high transmission of light. Now, a group led by Nikolay Zheludev at the University of Southampton, and co-workers at the Rutherford Appleton Laboratory in Oxfordshire and the Instituto de Optica in Madrid, have shown that a quasicrystal array of nanoholes in a metal screen can focus light at distances up to several tens of wavelengths from the screen.

The researchers obtained their result using a so-called Penrose-like quasiperiodic structure, a complex pattern of nanoholes that is neither really periodic or random. Light beams that penetrate through the different holes of the array interfere to create foci, or bright "hotspots". The array contains about 14 000 holes that are 200 nm in diameter. The overall size of the array is around 0.2 mm and it was made using electron beam lithography in a 100 nm aluminium film deposited on a silica substrate (figure 1).

Using coherent light at 660 nm and scanning aperture optical microscopy, Fu Min Huang of Southampton and colleagues 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..