Speaker: L (Kobus) Kuipers

Department: ImPhys

Subject: Light at the nanoscale

Location: Delft University of Technology

Date: 2016-01-06

Author: Romano van Genderen

The seminar was divided into two parts, one about light in nanostructures and one about optical rogue waves. Unfortunately, he did not have enough time to go into depth on these rogue waves.

He started his seminar by explaining why control of light at the nanoscale is important. Most of this information dealt with the use of light in computers, because light is much faster and more energy efficient than electricity. Especially the prospects of turning light beams on and off with other light beams would be a huge step in this process.

Afterwards he explained that in nanostructures there is a strong correlation between the geometrical shape and the optical effects. But it is hard to understand what precisely happens in the structures, because of the diffraction limit. The subwavelength information is called evanescent. This means at long distances it is impossible to measure. But he used a probe structure close (≈½λ) to the structure to measure it, which solves this problem. Using this probe he found both amplitude and phase components. This enables him to use the Fourier transform to find the frequency components of the cosine and so to say something about the relation between energy and frequency components. Also, using interference, the relative phase and polarisation of each frequency component can be measured.

But light of course is made of two oscillating field, the electric (E) and magnetic (H)-field. Usually, the magnetic field is much weaker, but because the probe has a metallo-dielectric structure, magnetic dipoles can play a major role. So there is no way to distinguish the two. But both waves evolve differently with height, the magnetic field falls off way quicker. So they used a system of equations, stating that Lx=aEx+bHy and Ly=aEy+bHx and fitted the data. They found that a and b, the factors showing the relative contribution of electric and magnetic field to the results were of the same order of magnitude. So both fields are measured simultaneously.

The final topic on nanostructures he mentioned were so-called isogyres. These are points where polarisation singularities are encountered. These are the points where the electric field can have more than one direction (asymptotes are encountered in the vector field). Quantum mechanical bits (Qubits) have special properties in such a singularity which enables them to interact with photons. This could have its applications in computing.

The second topic he barely glanced over were rogue waves. These are waves that significantly spike out of nowhere. He used a nanostructure to investigate the rogue waves and he showed that a nanostructure must have at least an entrance and an exit in order for rogue waves to occur. He wanted to tell more, but he was out of time.

This was a very interesting topic about optics. It also how often the ideal measurement just simply cannot exist, especially in the weird world of quantum mechanical scales.

Image 1: The optical ellipse and optical singularities. Right in the middle of these holes a singularity can be found. Source: de Hoogh, A.; Kuipers, L.; Visser, T.D.; Rotenberg, N. Creating and Controlling Polarization Singularities in Plasmonic Fields. *Photonics* **2015**, *2*, 553-567.