Focusing of Light by a Nano-Hole Array

We demonstrate a conceptually new mechanism for sub-wavelength focusing at optical frequencies based upon the use of nano-hole quasi-periodic arrays in metal screens. Using coherent illumination at 660 nm and scanning near-field optical microscopy, ~290 nm “hot spots”, were observed at a distance of ~12.5 μm from the array. Even smaller hot-spots of about 200 nm in waist were observed closer to the plane of the array. The spatial resolution when imaging or patterning small objects with light is limited by diffraction to about half the wavelength. This is of primary importance in microelectronics and imaging as it translates directly into the size of the features that can be manufactured on a microchip or resolved by a microscope. Nano-holes and arrays of nano-holes in metal screens show a number of intriguing optical properties, most notably the extraordinarily high transmission of regular and quasi-periodic hole arrays and unusual polarization-sensitive effects. Here we show for the first time that a quasicrystal array of nano-holes in a metal screen can “focus” light at distances up to several tens of wavelengths from the screen. It produces bright foci of energy concentration, harvesting energy from a large number of holes in the array. The foci are sparsely distributed in the “focal plane”. Using coherent laser illumination at a wavelength of 660 nm and scanning near-field optical ♣ Presented as post-deadline paper at the Frontiers in optics Conference 2006,Rochester, New York, USA October 8-12, 2006