03 Apr 2008
An optical antenna that redirects the light emitted by nearby molecules could benefit sensing and analytical applications.
The redirection of visible light has been achieved for the first time using an antenna operating in the optical range, which was fabricated by researchers at the Institute of Photonics Sciences (ICFO) in Spain. The team hopes that the antenna could lead to simpler and more efficient biosensing systems, and that the design could be further improved to create single-photon sources for use in quantum communication devices (Nature photonics doi:10.1038/nphoton.2008.32).
"Optical antennas offer unique potential to control light at the nanometre scale," Niek van Hulst, a researcher from ICFO, told optics.org. "They allow us to localize, enhance and redirect light, and are considered the main tool for developing nanophotonics."
Although antennas are commonly used to transmit radio, television and audio signals, researchers are only now extending this concept to optical frequencies. All types of antenna are based on the principle of oscillating charges along the antenna, which means that the size of the antenna must fit to a resonant mode for the wavelength of the radiation it supports. To make an antenna work at optical frequencies, it must be scaled down to nanometre dimensions.
"Conventional radio antennas are not resonant for visible light," explained van Hulst. "One needs to scale the antenna down to the order of 100 nm dimensions to 'focus' light down to the nanoscale."
However, it would be wrong to think that optical antennas are simply scaled down versions of radio antennas, because at optical frequencies metals such as gold and silver exhibit plasmonic resonances that intensify the field enhancement. "Optical antennas demonstrate plasmonic modes that can be tuned to be resonant to electronic transitions in nearby molecules," said van Hulst.
It's these plasmonic modes that make the coupling between the light emitted by the molecule and the antenna strong enough for the antenna to control the direction of light emission. In particular, the angle of emission depends on the dominant antenna mode, which in turn depends on the antenna design.
"In our experiment, the molecule is the light source, and the antenna acts as the emitting element that determines the emitted radiation pattern," said van Hulst. "By choosing the right type and orientation of an antenna, one can direct the light in any direction we wish."
To demonstrate this idea, the researchers brought a simple optical antenna — made of aluminium and just 80 nm long — close to a single light-emitting or fluorescent molecule. By varying the coupling to the antenna, they showed that the light emission from the molecule can be redirected over a full 90°.
For the basic monopole antenna used in this experiment, the molecules need to be close to the antenna for the coupling to be strong enough for the antenna to direct the light emission. More complex antenna geometries would enable the light emission to be controlled over longer distances, but creating such nanoscale structures poses some serious fabrication challenges.
"At optical frequencies the nanostructures must be fabricated with an accuracy down to 10 nm, which requires e-beam lithography and/or ion-beam milling," said van Hulst. "The next challenge is to optimize the geometry for the application."
• Two other research groups have reported new results relating to optical antennas. In the first, Rudolf Bratschitsch and colleagues at the University of Konstanz have mechanically tuned a gold "bow-tie" antenna by controlling it with the tip of an atomic force microscope. The researchers say that the technique could open the way to new nano-optomechanical devices, in which mechanical changes on the nanoscale control the optical properties of artificial structures.
"In contrast to previously reported experiments on metal nanoantenna structures, we were able to study how the optical properties of the nanoobjects evolved as a function of its position," Bratschitsch told nanotechweb.org. "This fact renders our experimental findings significantly more quantitative than earlier results." For more details, see AFM tip controls optical antenna on nanoscale.
In the other work, Liang Tang and colleagues of Stanford University have constructed a photodetector with a germanium active element with a volume of just 0.00072 m3, two orders of magnitude smaller than existing detectors at this wavelength and far smaller than the limit imposed by the diffraction of light. In the Stanford design, a gold dipole antenna collects light from a large area and concentrates it into this subwavelength detector, leading to a more sensitive device.
All of these results were reported in the April issue of Nature Photonics.