The talk is focused on optical excitations in mono- and bilayer transition metal dichalcogenides (TMDs). In the first part, I will discuss the results of spectroscopic experiments on molybdenum ditelluride (MoTe₂), a TMD material with its excitonic transition wavelength around 1000 nm. As this is a highly relevant spectral window, the hitherto little investigated MoTe₂ may have considerable application potential, in particular as I show evidence that for this material also the bilayer is still a direct semiconductor.

The second part of the talk will be devoted to time- and polarization resolved studies of tungsten diselenide (WSe₂). I will show the potential of resonant and near-resonant pump-probe spectroscopy in identifying states and scattering channels of TMDs. In particular, the technique allows us to identify a biexciton state with a binding energy less than the trion state, in full agreement with the latest theoretical findings. On the trion side, using the polarization resolution, we can selectively excite two distinct trion states, which display very different polarization dynamics.

Einladende: D. Reiter

Electron energy loss spectroscopy (EELS) and microscopy allow probing of evanescent fields of particle plasmons with nanometer resolution. In EELS swift electrons pass by or through a metallic nanoparticle and lose a tiny fraction of their kinetic energy by exciting particle plasmons. By spectrally analyzing the energy loss and raster-scanning the electron beam over the specimen one can map the plasmon polariton nearfields with sub-eV and nanometer resolution. By a similar token, EELS with extremely high energy resolution of about 10 meV allows investigating nanoscale phonon polariton properties [1], which have recently received tremendous attention in the context of phononics and nearfield heat transport at the nanoscale.

In this talk I will start by discussing our recent efforts to correlate experimental and simulated EELS maps of coupled nanostructures [2,3]. The comparison can be brought to a quantitative level when using the precise 3D geometry of the nanoparticles, reconstructed through electron tomography, as an input for simulation. This work paves the way for detailed investigations of the enhanced fields of realistic and complex plasmonic nanostructures. When additionally using a series of rotated EELS maps, it becomes possible to reconstruct the full 3D photonic environment of the plasmonic nanoparticles [2].

I will also report about our recent EELS studies using a strongly improved energy resolution of about 10 meV, which allowed observation of bulk and surface phonon polariton modes (corner, face, edge) in a single MgO cube with sub-nanometer spatial resolution [1]. This opens the route towards true atomic-resolution phonon spectroscopy in nanostructures and for a detailed understanding of nanoscale energy transport.

[1] M. Lagos et al., Nature 543, 533 (2017).

[2] A. Hörl et al., Nature Commun. 8, 37 (2017).

[3] G. Haberfehlner et al., Nano Lett. 15, 7726 (2015).

Einladende: T. Kuhn, R. Bratschitsch

Plasmonics research has ushered in a new era of precise control over light in sub-diffraction volumes. In recent years there has been strong interest in exploring the ultimate small size limit of plasmonics with the ambition that, by the manipulation of individual atoms/molecules, the properties of light, such as the local field enhancement, gradient and polarisation, can be controlled on the nano/angstrom scale. This is known as quantum plasmonics [1,2] and could lead to developments in quantum optics, as well as nano-localised photochemistry and the design of efficient molecules for light–matter interaction. There is also interesting fundamental questions still to be addressed with the apparently innocent question 'what is a plasmon in the quantum limit?' still able to provoke heated debate.

In my talk I will discuss some of our recent work on the plasmonic response of single-atom thick atomic chains within the framework of time dependent density functional theory [3]. In particular I discuss how the collectivity of an excitation can be used as a potential measure of 'plasmonicity' and how, despite the small size of the systems, large field enhancements can be achieved analogously to classical plasmonics.

[1] Fitzgerald, Jamie M., et al. "Quantum plasmonics." Proceedings of the IEEE 104.12 (2016): 2307-2322.

[2] Fitzgerald, Jamie M., and Vincenzo Giannini. "Perspective on molecular quantum plasmonic nanoantennas.", J. Opt.19 (2017) 060401.

[3] Fitzgerald, Jamie M., Sam Azadi, and Vincenzo Giannini. "Quantum plasmonic nanoantennas." Physical Review B 95.23 (2017): 235414

Einladende: Doris Reiter

As a graphene analogue, atomically thin hexagonal BN (h-BN) has recently attracted growing attention because of its wide bandgap and excellent thermal and chemical stability. Indeed, large-area h-BN related quasi-two-dimensional nanomaterials have been fabricated successfully thanks to the progress of modern nanotechnology. According to the reduced charge screening and enhanced electron-electron correlation in these low-dimensional systems, the single and few-layer h-BN nanosheets [1], BN co-doped graphene [2] and BN-graphene heterostructures [3] present novel phenomena that have not been found in bulk h-BN. Such prominent many-body effects, including electron-electron and electron-hole interactions, give rise to unusual electronic structures and significant optical exciton behaviour that pave the way for the promising applications of 2D nanosystems in next-generation optoelectronic and photonic devices. In this talk, we perform first-principles many-body perturbation calculations on quasi-2D BN related nanosystems and BN-graphene heterostructures to explore the origin of these unique electronic and optical properties.