Nanogap focuses light tighter than a lens

A team of researchers from institutes in Germany, Spain and the US has used nanoscale structures to focus light on tiny areas, a move that could massively improve the sensitivity of optical sensors.

Based at CIC nanoGUNE, Donostia International Physics Center DIPC, Centro de Física de Materiales of CSIC/UPV-EHU in San Sebastián, Spain, Harvard University in the US and the Max Planck Institute of Biochemistry in Munich, Germany, the team adopted tuning concepts from radio-frequency technology.

With an optical antenna, light can potentially be focused to nanometre dimensions, several orders of magnitude smaller than what conventional lenses can achieve. Tiny objects such as molecules or semiconductors that are placed in hot spots of the antenna can efficiently interact with light. Therefore optical antennas boost single molecule spectroscopy or signal-to-noise in detector applications.

In their experiments, published in Nature Photonics, the researchers studied infrared antennas, that feature a very narrow gap at the centre. These gap-antennas generate a very intense hot spot inside the gap, allowing for highly efficient nano-focusing of light. To study how the presence of matter inside the gap affects antenna behaviour, the researchers constructed small metal bridges inside the gap.

They mapped the near-field oscillations of the different antennas with a modified version of the scattering-type near-field microscope that the Max Planck and nanoGUNE researchers had pioneered over the last decade. For this work, they chose dielectric tips and operated in transmission mode, allowing for imaging local antenna fields in details as small as 50nm without disturbing the antenna.

“By monitoring the near-field oscillations of the different antennas with our novel near-field microscope, we were able to directly visualise how matter inside the gap affects the antenna response. The effect could find interesting applications for tuning of optical antennas,” claimed Rainer Hillenbrand, leader of the nano-optics group at the newly established research institute CIC nanoGUNE Consolider.

The nano-optics group from DIPC and CSIC-UPV/EHU led by Javier Aizpurua in San Sebastián confirmed and helped to understand the experimental results using electrodynamic calculations. The calculated maps of the antenna fields are in good agreement with the experimentally observed images. The simulations provide insights into the dependence of the antenna modes on the bridging.

A simple circuit model showed remarkable agreement with the results of the numerical calculations of the optical resonances, the researchers claimed.

“By extending circuit theory to visible and infrared frequencies, the design of novel photonic devices and detectors will become more efficient. This bridges the gap between these two disciplines,” said researcher Javier Aizpurua.

With this work, the researches provide first experimental evidence that the local antenna fields can be controlled by gap-loading. This opens the door for designing near-field patterns in the nanoscale by load manipulation, without the need to change antenna length, which could be highly valuable for the development of compact and integrated nanophotonic devices.

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