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Posted: Jul 31, 2012
DNA-templated nanoantenna captures and emits light one photon at a time
(Nanowerk Spotlight) The emission of light by a single molecule is a cornerstone of nano-optics that will enable applications in quantum information processing or single-molecule spectroscopy. However, a key challenge in nano-optics is to bring light to and collect light from nano-scale systems.
In conventional electronics, the interconnect between locally stored and radiated signals, for example radio broadcasts or mobile phone transmissions, is formed by antennas. For an antenna to work at the wavelength of light it is necessary to downscale the structure by the same factor as the wavelength or the frequency of the wave, i.e. roughly by a factor of 10 million. So at the nanoscale, a simple antenna like a dipolar TV antenna would be about 100 nm in size and made of two polarizable elements (the metallic rods of a TV antenna) and a feed element (an emitter-receiver).
Once the nanofabrication issues are sorted out, nano-optical antennas could become ubiquitous in all applications based on light-matter interactions such as sensing, light emission (e.g. LEDs) and detection, as well as light harvesting, i.e. for solar cell applications.
Back in 2005 it already was proposed to make optical antennas to amplify the interaction between light and matter (see paper in Science: "Nanoantennas for Light Emission"). Although optical cavities could be used for this but they only work at a very specific wavelength and can only interact with emitter-receivers at very low temperatures (at room temperatures, typical emitters like molecules and semiconductors are broadband). On the other hand, antennas can function for a broad range of frequencies.
In the above-mentioned Science paper, it was proposed to build a basic optical antenna by putting a quantum emitter – the optical equivalent of the antenna feed element – between two gold or silver nanoparticles that replace the metallic rods of the radio wave antenna.
"But to do this" says Sébastien Bidault, a CNRS research scientist at the Institut Langevin's Optical Antennas and Sensing group, "there is a very complex technical issue: the position of the feed element and the particles must be controlled at the 1 nm scale and a lot of theoretical papers have since discussed this. This task is far beyond what conventional top-down lithography techniques, used in the fabrication of transistors, can do.
Schematic representation of a nanoantenna formed of two gold nanoparticles linked by a DNA double strand and supplied by a single quantum emitter. (Image: Mickaël P. Busson, Brice Rolly, Brian Stout, Nicolas Bonod, Sébastien Bidault)
Through self-assembly, they grafted the gold nanoparticles and a fluorescent organic dye onto short synthetic DNA strands which are only 10-15 nm long. These nanostructures are the equivalent of a dipolar TV antenna (rabbit ear antenna) downscaled by a factor of 10 million. The fluorescent molecule acts as a quantum source, supplying the antenna with photons, while the gold nanoparticles amplify the interaction between the emitter and far-field light.
The researchers produced in parallel several billion copies of these pairs of particles (in a purified water suspension) by controlling the position of the fluorescent molecule with nanometer precision, thanks to the DNA backbone.
"The need to have only one linking DNA molecule is to make sure we will only add one fluorescent molecule to drive the antenna," Bidault explains to Nanowerk. "In our most recent work, we measured several hundred molecule-driven antennas – using single molecule fluorescence measurements – and looked at their fluorescence lifetime. Comparing these measurements to electromagnetic theory, we obtained a quantitative agreement and were able to show that the position of the molecule is indeed controlled at the 1 nm scale. Also, since the molecule is a quantum emitter and the dimer is a passive element, the overall antenna is also a quantum emitter – it only emits one photon at a time. Both results had never been obtained experimentally before."
"We chose to study the simplest antenna geometry (dipolar antenna) to fully characterize its optical response and compare it to classical electromagnetic theory," says Bidault. "Since our systems are quantum emitters, they could be useful for ultrafast quantum communication, thanks to the short fluorescence lifetimes. An even more interesting aspect of our antennas is that they are made with DNA and are obtained purified as several billion copies in water. Since the antenna response strongly depends on the DNA linker, we can develop new types of biochemical sensors with these nanostructures."
He points out that there are still numerous issues to optimize in these systems: "In particular, our antennas have a lot of ohmic (heat) losses, so the antenna efficiencies require optimization, for instance by using silver instead of gold or rods instead of spheres for the particles. More importantly, the antennas we made are the simplest (dipolar) geometry."
Other groups have already described more complex antennas in which the position of the feed element was less controlled but that had other desirable properties: in particular directivity (see paper in Science: "Unidirectional Emission of a Quantum Dot Coupled to a Nanoantenna"). So, Bidault and his collaborators have their work cut out for them to make bright directional antennas for light.