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Dipartimento di Fisica - Politecnico di Milano

Thorsten Feichtner

Thorsten Feichtner's picture
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Potential-dependent Second-Harmonic Generation in Optical Antennas measured Time-resolved


The efficient nonlinear optical conversion of photons at the nanoscale is one of the challenges faced by future integrated optics. In the project PoSHGOAT I will control the surface charges in metallic nanoparticles – optical antennas with plasmonic resonances in the infrared and visible - by applying voltages and pump-probe spectroscopy. By this and evolutionary optimization of numerical models the second-harmonic generation in optical antennas will be examined, controlled and optimized. This will increase the understanding of nonlinear processes in plasmonic resonators and ultimately impact on improved background-free spectroscopy and ultra-fast switching for optical circuitry.

This project is about optical antennas. This are really tiny metallic particles, which are designed to work like a radio antenna and convert electromagnetic waves into currents. As the antenna needs to be of the size of radiations wavelength, radio antenna dimensions are between meters and centimeters, where optical antennas are only hundreds of nanometers large.

This project is also about non-linear optics. This means effects, where the energy of the involved light is changing the result not in a one-to-one fashion. Here we especially look at second harmonic generation (SHG), where the energy of two incoming photons with frequency ω is added to generate a single outgoing photon with twice the energy and twice the frequency 2ω.

To make SHG work, the incoming light has to be concentrated to ensure two incoming photons being at the same spot at the same time. In addition one needs a material at the interaction volume, because there is no non-linear effect without light-matter interaction . Optical antennas provide both – as they can concentrate light to very small volumes much below the diffraction limit, because they show plasmonic resonances. Plasmons are collective oscillations of light and free electrons, and metals can form plasmons at their surface. By the shape of the surface and constricting it like in a nano particle, resonances can be tuned, just like with a radio wave antenna.

Project topics
There are several aspects of SHG generation with optical antennas which are more complex than described above or even have never been tackled before. They are central aspects of PoSHGOAT.

SHG needs light-matter interaction. However, centro-symmetric matter leads to destructive interference of the non-linear emission. As our optical antennas are made from gold, which is a centrosymmetric material with an fcc-lattice, only the surface geometry can break the symmetry and, thus, the shape of the nano-particles becomes crucial. In the project the particles are shaped by focused ion beam milling and one of the novel aspects is the use of Helium ions.

A more demanding way to break the symmetry of an already fabricated nano structure is to change the charges on their surface. Here we make use of the monocrystalline substrate which allows to realize electrical contacts with only a few ten nanometers thickness.

And finally PoSHGOAT aims to use femtosecond laser pulses to excite the metal electrons. When an external voltage is applied, the pulses can lead to the electrons being emitted, which gives a possibility to control the non-linearity of an electrically contacted optical antenna on the fs-time scale.

Until of today there are very promising first results of SHG modulation in electrically contacted optical antennas. However, the first result which is published is about the optimization of the fabrication process of optical antennas:

Deinhart, Victor, et al. "The patterning toolbox FIB-o-mat: Exploiting the full potential of focused helium ions for nanofabrication." Beilstein journal of nanotechnology 12.1 (2021): 304-318.