18. Nov. 2019
Michal Horák from the research group Fabrication and Characterization of Nanostructures has been involved in CEITEC BUT for the fourth year in electron energy loss spectroscopy and transmission electron microscopy. Recently, he and his colleagues completed a project of the Grant Agency of the Czech Republic in which they investigated and mapped plasmon resonances in plasmonic nanoanthenes using the Babinet´s principle. They were the first in the world to explore the limits of this principle for plasmonics.
Four years ago, when new microscopes were installed at CEITEC BUT, Michal Horák decided to explore their possibilities. “In the topic of measuring plasmonic resonance with the use of electron energy loss spectroscopy in transmission electron microscope I started pretty much from scratch,” recalled Horák who maps this area in his dissertation work. According to him the main advantage of this technique is combination of nanometer spatial resolution and good energy resolution.
In the recent project Michal Horák and his colleagues explored the Babinet´s principle applied to plasmonics. „It is a principle originally from light optics. It says that the diffraction pattern from an opaque disk is the same as the diffraction pattern from a hole in an impermeable screen having the same shape and size. In plasmonics we expected that particle antennas and aperture antennas, i.e. holes in a metal layer, of the same shape and size, should have similar properties. And by similar properties we mean similar resonant energy. Furthermore, the nearby electric field is replaced by magnetic, which means that the electric field in the particle corresponds to the magnetic field in the aperture and vice versa,” explained Michal Horák.
According to Horák the principle was confirmed in qualitative terms. “It actually works that way. The antennas and apertures have a similar resonant energy and the fields have a similar pattern. On the other hand, in the visible area where we moved the antennas, the principle does not correspond quantitatively,” pointed out Horák and added that the magnitude of the variables varies. “One of the possible reasons may be that the ideal validity of the principle requires an infinitely thin structure with infinite conductivity and an infinite number of electrons inside,” added Horák with a smile. But the Czech team was the first in the world to explore the limits of the Babinet´s principle in plasmonics. “It can still be used to approximate the magnetic field distribution. Electron energy loss spectroscopy is sensitive only to the electric field. If we want to measure the magnetic field, one possibility is to create a complementary structure and measure the electric field in this complementary structure. This will give us some idea of the magnetic field in the original structure,” said Horák, stating that the findings and limits are essential especially for applied research and use in production.
Also, an article on bow-tie and diabolo antennas is currently under review. “We used the Babinet´s principle to visualize the magnetic field. On these antennas it is easy to see how we can qualitatively display where the field is for a given mode. We make a direct structure, measure the distribution of the electric field for the given modes, produce an inverted structure, and measure the distribution of the magnetic field for the same modes. In the final, we know the distribution of both near electric and magnetic fields,” explained Michal Horák.