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  • Nanoantennae Direct Light from Molecules

Photonics Spectra
May 2008
Study reveals how to borrow designs from radio and microwave antennae.

David L. Shenkenberg

Radio and microwave antennae enable modern conveniences such as listening to the radio, watching television, mobile phone communication and wireless Internet access. Relatively recently, antennae that receive and direct visible light have been created by scaling them down to sizes in the nanometer range.

Figure 1. A nanoscale monopole optical antenna juts out ∼80 nm from an aperture probe. It causes a molecule to emit light.

Such nanoscale antennae not only direct laser light onto single light-emitting molecules, such as fluorophores, but also cause the molecules to release more light by surface plasmon resonance. Because nanoantennae have these abilities, they could have broad applications in photonics and biophotonics. For instance, they could enable brighter displays or brighter fluorescence for highest-resolution biological detection and imaging.

To build nanoantennae, it would be ideal if the numerous designs for radio and microwave antennae could be borrowed, instead of “reinventing the wheel.” However, further research is required to determine how these designs will behave physically on the nanometer scale. For this reason, researchers at ICFO — Institut de Ciencies Fotoniques in Barcelona, Spain, have fabricated and applied an optical nanoantenna with a monopole design.

They made their antenna by pulling apart an optical fiber until it broke and formed a point, and then by coating it with chromium and aluminum. Finally, they focused an ion beam to mill away a hole in the tip and an approximately 80-nm-long linear antenna adjacent to the hole (Figure 1).

Using fluorescent carbocyanine molecules as model light emitters, they ascertained whether the emission is determined by the design of the antenna or by the molecular orientation. To do so, they performed polarity measurements. A 514-nm polarized laser beam traveled through the aperture of the nanoantenna and onto each fluorescent molecule. The emission traveled to a beamsplitter that separated the 570-nm emission by polarity. Positively polarized light went to one avalanche photodiode, and negatively polarized light went to another.

Figure 2. The nanoantenna directs emitted light from a single molecule in this pattern.

The researchers discovered that coupling and uncoupling of the nanoantenna to the fluorescent molecule dramatically changed the polarization of its emitted light. When the nanoantenna tip picked up a single fluorescent molecule, the nanoantenna design determined the direction of the emission. However, when the nanoantenna did not touch the molecule, the direction of the emission was highly erratic because it was determined by the molecular orientation.

These experimental results agreed well with theoretical predictions.

Nature Photonics, April 2008, pp. 234-237.

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