“Our work focuses on how plasmonic nanoantennas polarize light,” explains team leader Otto Muskens, “and specifically how these nanostructures scatter light with a polarization perpendicular to an incident light wave falling on them.”

A nanoantenna is a device that that collects and focuses light at optical wavelengths – as opposed to a conventional television or radio antenna that works at radio frequencies. Optical antennas can control light at the nanometre scale and they localize, enhance and redirect light at this scale. As such, they will be crucial for developing nanophotonics devices in the future.

All types of antenna are based on the principle of oscillating charges along the length of the structure, which means that the size of the antenna must fit to a resonant mode for the wavelength of the radiation it supports. This is why an antenna must be scaled down to nanometre dimensions to make it work at optical frequencies.

However, to think that optical antennas are simply scaled-down versions of radio antennas would be oversimplifying things. At optical frequencies metals such as gold and silver exhibit plasmonic resonances that intensify the light field enhancement. Optical antennas demonstrate plasmonic “modes” that can be tuned to be resonant to electronic transitions in nearby molecules and it is these modes that make the coupling between the light emitted by the molecule and the antenna strong enough for the antenna to control the direction, or polarization, of light emission. In particular, the angle of emission depends on the dominant antenna mode, which in turn depends on the antenna design.

Spatial modulation spectroscopy analyses L-shaped antenna

Muskens’ team has now measured the spectrum of light scattered by individual L-shaped nanoantennas made of gold. The light was polarized either identical or orthogonal to the incident light and the researchers analysed it using a special technique called spatial modulation spectroscopy. “This technique give us direct information about how light is scattered at different cross sections in the nanoantenna, and the technique is based on a periodic modulation of the nanostructure in the focus of an optical microscope,” explained Muskens. “By detecting this periodic variation in the scattered light we can recover the fraction of light scattered by a single antenna structure.”

The team optimized its set up so that it covered a wide spectral range from the visible to the near-infrared parts of the electromagnetic spectrum – a range that is important for real-world telecoms applications.

To better understand their experimental observations, Arnaud Arbouet from Toulouse and colleagues also developed a theoretical model describing their nanoantenna. The model describes two springs oriented at right angles to each other. Each spring has a different resonant frequency that corresponds to a light polarization that is at 45° with respect to the antenna arms (see figure).

Simple model gives valuable information

“Combining these two oscillators results in a specific polarization state of the light that depends strongly on the frequency of the incoming light with respect to the two resonance frequencies,” Arbouet told nanotechweb.org. “As a result, and in the spectral range lying between the two resonances, an incident excitation light beam polarized along one direction can be scattered with a perpendicular polarization. This simple model gives us valuable information about the phase, intensity and polarization of the light scattered by our nanoantenna – and backs up our experimental results."

The findings could help us better design phased nanoantenna arrays for making flat lenses, he added. “As we move towards ever smaller all-optical systems for telecoms, low-loss on-chip polarization conversion elements like these will be increasingly required. With specific nanoantenna configurations, we can generate perfectly circularly polarized light that may even have many interesting applications in sensing and potential biosensing of chiral molecules in the living world.”

The Southampton team says that it is now busy trying to make real devices from its nanoantennas. “The knowledge we have gained from our single-antenna studies will be put to good use in designing metasurfaces,” said Muskens. “We will also be trying to further optimize the nanoantennas' response to light by combining many oscillators with different properties. Particularly interesting is the possibility of actively controlling the coupling between the elements to make tuneable components for flat optics.”

The current work is reported in ACS Nano DOI: 10.1021/nn501889s.