Credit: © 2007 ACS

By bombarding the tip of a tapered optical fibre with ions, European scientists have succeeded in crafting a nano-antenna that operates at optical wavelengths and can efficiently 'pick-up' green light (Nano Lett. 7, 28–33; 2006). Such optical antennas may ultimately prove useful for subwavelength microscopy and integrated optoelectronic devices, but, for now, they show how a well-known object can be reduced to the nanoscale to create fascinating tools for the future.

Wireless technology literally surrounds us with information, and the concept of the antenna has a crucial role to play. By converting free-space electromagnetic fields into guided waves, or vice versa, antennas act as either receivers or transmitters. The wavelength at which antennas operate is intrinsically related to their size and shape: for a simple antenna, the height required is approximately one quarter of the wavelength.

For an antenna to operate in the optical regime, its dimensions must be on the 100-nm scale. This has now been achieved by scientists in Spain and The Netherlands.

Starting with the flat end of a single-mode optical fibre, Tim Taminiau and colleagues create a sharp glass tip by so-called heat-pulling — applying tension to hot, soft glass. This tip is coated with a 150-nm-thick layer of aluminium and is then shaped by bombarding it with high-velocity ions. The result is a nano-antenna that is just 50 nm in diameter and has a height of between 30 and 140 nm. By positioning it on the edge of an aperture into the optical fibre, the local field effectively drives the antenna, replacing the transmission lines in the radio-wave equivalent. Simulations of the fields around the structure show that it behaves in the same way as a standard radio-frequency monopole antenna, enhancing the localized field near the apex at a resonant wavelength dependent on the height: a 75-nm tall antenna is resonant with green light with a wavelength of 514 nm. The device could also act as a receiver when driven by far-field illumination.

To demonstrate the potential of their antenna, the team have used it to perform near-field scanning optical microscopy on fluorescent molecules suspended in a polymer film. Laser light at 514 nm is passed along the optical fibre to excite molecules and the fluorescence is collected in the same way. The sample is scanned beneath the antenna to produce a two-dimensional image and it is here that the effect of the antenna can be seen. The molecules can be resolved with a resolution of 26 nm, three times smaller than the patterns associated with the aperture. This result demonstrates the tight confinement of the enhanced field at the end of the antenna.

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