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Research

Nanophotonics Group - Research Topics

Nanostructured materials provide unique opportunities to control light at the subwavelength scale. We pursue an active research program in which we exploit nanophotonic systems to tailor light-matter interactions and to create devices with novel properties. Brief summaries of current research projects can be found below.


Small is beautiful

Our research group has extensive experience in the design and fabrication of high quality metallic nanostructures for optical applications. Electron beam lithography is our method of choice for production of planar nanostructures with feature sizes down to 50 nm. For the fabrication of 3D nanostructures, we operate a direct laser writing system. Additionally, we collaborate with the group of Dr. Stephan Irsen (caesar) to fabricate photonic nanodevices by focused ion beam milling.

 Fabrication

 


Quantum simulators based on plasmonic waveguide arrays

Discrete photonic systems such as coupled waveguide arrays can show interesting dynamics that resemble quantum mechanical condensed matter phenomena. The basis for this is the mathematical equivalence between the time dependent Schrödinger equation and the coupled mode equation used to describe the propagation of light in arrays of waveguides. This allows to map the time-dependent probability distribution of an electronic wave packet onto the spatial light intensity distribution in the waveguide array and hence to directly visualize the quantum mechanical evolution in a coherent, yet classical wave environment.

In our experiments we employ arrays of evanescently coupled dielectric-loaded surface plasmon polariton waveguides as simulators of an electronic wavefunction in one-dimensional crystals. The arrays are fabricated by negative-tone gray-scale electron beam lithography. The propagation of the surface plasmon polaritons in the arrays is monitored by real and Fourier-space leakage radiation microscopy. This powerful detection technique allows us to access the temporal evolution of the electronic probability density as well as the band structure of the simulated crystal. Using this approach, a wide range of condensed matter phenomena have been already demonstrated by our group, for instance, Bloch oscillations, Anderson localization, or topological edge states just to name a few. Of special interest for the current research are topological and time-periodic, i.e., Floquet systems as well as non-Hermitian systems.

Plasmonic Quantum Simulator

 

Optical antennas

An antenna is a device that converts the energy of a propagating wave to localized energy and vice versa. The technological progress of the last years in the field of nano-fabrication has paved the way to transfer this concept to the optical domain. Optical antennae fabricated for the near infrared and visible spectral range offer new and exciting possibilities to manipulate and control light, e.g., by focusing beyond the diffraction limit or by directing the emission of quantum emitters.

A key topic of our current research is the coupling of solid-state quantum emitters (semiconductor quantum dots or nanodiamonds with color centers) to metallic or dielectric optical antennas. The basic idea is here that we can use the optical antennas to tailor the electromagnetic environment of the emitters. For instance, we have recently demonstrated the directional emission of colloidal semiconductor quantum dots coupled to dielectric leaky-wave nanoantennas [M. Peter et al., Nano Lett. 17,  4178 (2017)] .

Dielectric optical antenna

 

 

Near-field Microscopy: Gaining a deeper insight

Nanostructures offer many new and exciting optical properties. The in-depth understanding of these properties requires the knowledge of the electromagnetic near-field distribution in the vicinity of the nanostructures. Due to its evanescent nature, the near-field distribution can not be directly observed by standard optical microscopy. However, it can be accessed by a scattering-type scanning near-field optical microscope (s-SNOM), where a sharp metallic tip is brought in close proximity to the sample surface. The tip acts as an antenna that converts the bound near-field to propagating radiation (conventional light) that can be measured in the far-field with a conventional detector. Utilizing an interferometric detection scheme, s-SNOM is capable of measuring the amplitude and phase of the near-field distribution directly above the sample. The figure below shows an example of a s-SNOM measurement on a single-crystalline gold flake. The edge-launched surface plasmon polaritons interfere in the middle of the flake.

SPP

 This project is carried out in close collaboration with the Electron Microscopy and Analysis (EMA) group at the research center caesar in Bonn.

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