Research
Nanostructured materials can provide unprecedented control over the light field at a subwavelength scale. In particular, they allow converting propagating optical waves into highly localized excitations and vice versa. This opens fascinating opportunities to study nanoscale light-matter interactions and to create materials with tailored optical properties.
Our group experimentally explores the optical properties of nanostructured materials. For this purpose, we combine state-of the art nanofabrication methods with different spectroscopic and microscopic techniques. Brief summaries of current research projects can be found below.
We have extensive experience in the design and fabrication of high quality metallic and dielectric nanostructures for optical applications. Electron beam lithography is our method of choice for the production of planar nanostructures. 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.
We operate a Zeiss Sigma field emission scanning electon microscope equipped with a Raith Elphy lithography attachment. This system is used for electron beam lithography as well as for the inspection of the fabricated samples.
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.
Our group operates a scattering type scanning near-field microscope that uses a femtosecond optical parametric oscillator as a tunable light source for near-infrared light. The system is equipped with an interferometer and is thus capable of delivering phase resolved near-field data.
We have studied plasmonic slot waveguides in single-crystalline and poly-crystalline gold films with slot widths down to 50 nm. The quality of the gold film has a profound impact for small gap widths. In this regime, Ohmic losses are the dominant loss channel. In contrast, for gap widths
exceeding 100 nm, the waveguides in single-crystalline and poly-crystalline gold films show a comparable performance since leakage radiation into the substrate becomes the dominant loss channel. Details can be found here.
Arrays of evanescently coupled dielectric-loaded surface plasmon polariton waveguides (DLSPPWs)
provide a powerful platform to experimentally study tight-binding systems in one dimension. 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.
Dielectric loaded surface plasmon polariton waveguides (DLSPPW) combine several favorable characteristics, which make them ideally suited for our experimental studies of tight-binding systems. DLSPPWs feature relatively low losses, are easy to fabricate by electron beam lithography and the SPP propagation can be conveniently monitored by leakage radiation microscopy.
We have studied the driven Su-Schrieer-Heeger model using arrays of evanescently coupled plasmonic waveguides.This system hosts for suitable driving frequencies a topologically protected edge state that has no counterpart in static systems, the so-called anomalous π-mode. By using real and Fourier-space leakage radiation microscopy in combination with edge- and bulk excitation, we have unequivocally identified this mode and studied its frequency dependence. Details can be found here.
We have studied the propagation of surface plasmon polaritons in arrays of DLSPPWs with a propagation constant gradient acting as an effective external potential. Using leakage radiation microscopy, we have observed in real-space an oscillatory motion of the excited wavepacket. The corresponding momentum resolved spectra are composed of sets of equally spaced modes. Details can be found here.
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.
We have developed a fabrication process for optical antennas with emitters that combines two lithography steps with the selective functionlization of the substrate surface. Using this approach, we can deposit colloidal semiconductor quantum dots or nanodiamonds from an aqueous solution to freely definable sites on a substrate. Details can befound here.
Dielectric materials can serve as an attractive platform for travelling wave optical antennas for directional light emission. Because of their non-resonant
nature, these optical antennas are characterized by a large bandwidth and robustness against fabrication imperfections. Details can be found here.
We study the interaction between excitons in 2D transitionmetal dichalcogenide (TMDC) monolayers and plasmonic modes supported by metallic nanostructures. These hybrid nanostructures are an attractive platform to investigate lightmatter interactions in open nanosystems as they combine the large oscillator strength of
TMDC monolayer exciton resonances with the tailored electromagnetic environment provided by the plasmon modes. The hybrid nanostructures are prepared by dry transfer of mechanically exfoliated TMDC monolayers onto suitable plasmonic nanostructures defined by electron beam lithography or focused ion beam milling.
We just have finished setting up a pump-probe experiment for the ultrafast spectroscopy on hybrid nanostructures. As light sources, we employ an ultrafast Ti:Sapphire laser and an optical parametric oscillator (OPO). This laser system delivers femtosecond pulses with a typical pulse length of 150 - 250 fs and covers a broad spectral range in the visible and near-infrared (500 nm to 1600 nm wavelength). A helium-flow cryostat can be used to cool the sample down to 4K.