Building optical antennas at the nanoscale
NCCR Bio-Inspired Materials researchers are working on the frontiers of nanophotonics, developing quantum devices known as optical antennas, which could improve the performance of solar cells or boost nanoscale LED lighting.
Antennas as such are not particularly novel. Today, while less visible than they were a few decades ago, they are found in our mobile phones, and used for example for satellite communications and in a wide variety of devices relying on the micro and radio-wave part of the electromagnetic spectrum. They can either capture waves and convert their energy into electrical signals, or take electrical signals and convert them into waves. In theory this is also possible with light, which is part of the electromagnetic spectrum.
Typically, light is commonly manipulated with lenses and mirrors. However, using these elements light cannot be focused to dimensions much smaller than the optical wavelength (~ 1 micrometer), due to the diffraction of light waves. In order to overcome this hurdle, recent research in nano-optics and plasmonics has focused on so-called optical antennas, with physicists exploring ways of translating established radio wave and microwave antenna theories into the optical frequency regime. It is hoped that such antennas could increase the efficiency of light-matter interactions in important applications, such as light-emitting devices, photovoltaics, and spectroscopy.
The purpose of these optical antennas is therefore to convert the energy of free propagating light to localized energy, and vice versa. Although this is similar to what radio and microwave antennas do, optical antennas commonly exploit the unique properties of metal nanostructures, which exhibit strong collective oscillations of their free electrons at optical frequencies an effect known as localized surface plasmons.
NCCR Principal Investigator Prof. Guillermo Acuna and his colleagues at the University of Fribourg have focused on fabricating optical nano-antennas, and studying their interaction with single photon emitters, including the effects of near field excitation and emission directionality. Their work calls upon the so-called DNA origami technique, developed over a decade ago by US researchers. This is based on “folding” a long single-stranded DNA sequence with the aid of shorter single-strands into a predesigned shape typically with dimensions in the hundreds of nanometers. Acuna and his team use the DNA origami as a framework where metallic nanoparticles or quantum dots for example can be precisely positioned. “This allows us to form vast numbers of theses antennas in parallel, permitting us to study fundamental interactions,” explains Acuna.
The NCCR researchers have recently been investigating how DNA self-assembled optical antennas can direct the emission of single photon emitting fluorophores. The origami technique is applied to create optical antennas composed of two colloidal gold nanoparticles separated by a predefined gap. They then placed a single red fluorescent molecule in the middle of this space. By doing this, the nanoparticles act as an optical antenna that mediates the emission of the fluorophore thus manipulating its directionality.
“This work is intended to set out the basis for manipulating the emission pattern of single molecules with self-assembled optical antennas based on colloidal nanoparticles,” adds Acuna. “This provides us with solid ground for more sophisticated photon routing experiments.” In the longer term, this could lead to applications in fields such as sensing and super-resolution microscopy.
Reference: Hübner, K.; Pilo-Pais, M.; Selbach, F.; Liedl, T.; Tinnefeld, P.; Stefani, F. D.; Acuna, G. P. Directing single-molecule emission with DNA origami-assembled optical antennas, Nano Lett., 2019, 19, 6629–6634.