Ultrasmall antenna controls light beam at nanoscale

An extremely small antenna can control light at the nanoscale. Researchers from FOM Institute AMOLF have demonstrated this in practice for the first time. They even developed a new measurement technique for this purpose. This breakthrough is vitally important for the design of small efficient antennae for future use in solar cells, ultrasmall sensors, optical computer chips and quantum computers. The researchers published their results today in the renowned journal Nano Letters.

Antennae are all around us to efficiently transmit and receive electromagnetic waves: on roofs and balconies, in radios, laptops, navigation systems and mobile phones. These antennae are designed for radio waves and microwaves with wavelengths varying from metres to millimetres. The antennae have a size compatible to the wavelength of the waves that they can transmit and receive. Visible light has a far shorter wavelength. This means that a good antenna for visible light must be 1000 to 100,000 times smaller than conventional antennae.

TV antenna
The antenna constructed by PhD student Toon Coenen and his colleagues at AMOLF consists of five extremely small gold nanoparticles that have been carefully positioned in a row. Each nanoparticle is highly sensitive for visible light, which is expressed in the form of resonance patterns caused by the oscillation of free electrons in the metal. The strong interaction of such resonance patterns with light has been known for centuries: these cause the deepest red colour in stained glass windows. With the aid of nanomanufacturing techniques, researchers can now arrange such particles so precisely that they together form an antenna, which is one million times smaller than a TV antenna but which works in exactly the same manner.

Cathodoluminescence
The nanoparticles that together form the antenna, are so incredibly small that it is impossible to see or control them individually using an optical microscope. The researchers therefore used a five nanometre wide electron beam from an electron microscope to control the antenna with extreme precision. The electron beam brings the particles into their excited state as a result of which they subsequently emit light. This measurement technique, also referred to as cathodoluminescence spectroscopy, was adjusted by the researchers such that they could, for the first time, measure and control the direction in which the light was emitted. As the electron beam can be focused very accurately, it is possible to excite a single nanoparticle in the antenna. Interaction with the other particles ensures that the light is subsequently emitted in a certain direction. This direction is strongly dependent on the wavelength of the light and the position of the particle that is excited. It is comparable to the controlled transmission of radio beams by antennae, which is achieved by selecting which elements receive current. With this approach, the scientists have demonstrated that they can influence the direction of the light beam at a scale far smaller than the wavelength of the light. This is a breakthrough, and this new method opens up possibilities for very small optical systems.
The research was carried out by FOM researchers Toon Coenen, Ernst Jan Vesseur, Albert Polman and Femius Koenderink and was made possible by funding from the FOM Foundation and NWO’s Innovational Research Incentives Scheme.

Reference
Directional emission from plasmonic Yagi-Uda antennas probed by angle-resolved cathodoluminescence spectroscopy
T. Coenen, E.J.R. Vesseur, A. Polman & A.F. Koenderink
Nano Letters (2011). DOI: 10.1021/nl201839g

For further information please contact:
Toon Coenen, tel. +31 6 1329 7673, email: coenenATamolf.nl
Femius Koenderink, email: f.koenderinkATamolf.nl

Figure 1: The researchers constructed an antenna of five gold particles in a row with a total length of 650 nanometres, about the wavelength of visible light. This antenna is one million times smaller version of the antenna developed by the Japanese scientists Yagi and Uda in the 1920s for radio waves. In the experiment, each particle can be probed separately by an electron beam, after which the antenna emits light.

Figure 2: If the antenna is probed by the electron beam (far left) it emits a highly directed beam. As the measurement set-up (centre) and cross-section through the measurement set-up (right) reveal, the beam is directed towards the right for blue light (500 nm). For red light, the emitted beam points to the left. By aiming the electron beam at other particles, the radiation profile can be manipulated. This is comparable to the targeted transmission of radio beams by antennae, which is realised by selecting which elements receive current.