This project aims at the study of ultrafast electronic dynamics in the class of layered materials, ranging from two-dimensional systems with atomic thickness to topological insulators and nonconventional superconductors where dimensionality and the microscopic physics of two-dimensional electron gases play a key role. This research topic will require significant advances in ultrafast spectroscopy techniques and the development of a new setup for the study of femtosecond dynamics driven by impulsive photoexcitation. The particular interest in the physics of layered materials is related to their peculiar characteristics that can be technologically exploited in mono-atomically thick devices for optoelectronics and electronics, thus overcoming the miniaturization limitations of current semiconductor technologies. A full study of the fundamental processes, such as electron-electron and electron-phonon scattering mechanisms occurring at this dimensional scale, calls for the implementation of an ultrafast spectroscopy system capable of extreme temporal resolution combined to broad spectral coverage and exceptional sensitivity.In particular, we will focus on the generation of ultrashort optical pulses in various spectral ranges to be employed in two-color pump-probe experiments with the goal to obtain sub-10-fs temporal resolution. High repetition rate will ensure sensitive detection of the small signal arising from few/mono-layered materials. With the novel spectroscopic tools developed in the project, it will be possible to unveil the ultimate charge dynamics that are at the basis for the optical and electronic properties in 2-dimensional systems.

The GEMINI project aims at laying the foundations of a novel paradigm in optical sensing by introducing molecule-specific strong light-matter interaction at mid-infrared wavelengths through the engineering of plasmonic effects in group-IV semiconductors. Our contribution to the collaborative European project is the time-resolved nonlinear spectral characterization of the new heavily doped materials and nanoantenna devices. We focus on their fundamental properties to assess their potential for sensing. Furthermore, we explore the possibility of active optical activation of Ge resonances via control of the plasma frequency by near-IR excitation.Single resonant antennas will be excited by using a dispersion-free reflective confocal microscope setup coupled to advanced systems for the generation of ultrashort THz and mid-IR radiation.The key enabling technology is the novel germanium-on-silicon material platform: heavily-doped Ge films display plasma frequencies in the mid-infrared range. This allows for the complete substitution of metals with CMOS-compatible semiconductors in plasmonic infrared sensors, with enormous advantages in terms of fabrication quality and costs. Moreover, the mid-infrared range offers the unique opportunity of molecule specificity to target gases in the atmosphere, analytes in a solution or biomolecules in a diagnostic assay. Impacts of the proposed research go far beyond transforming optical sensing technology. Lab-on-chip disposable biosensors with integrated readout for medical diagnostics would for example radically cut healthcare costs.

Over the past few years, plasmonics has been established as an important field of physics. This fact is largely due to its advantages in the controlling and manipulating of light on sub-wavelength scales.A plasmon is a quantized density fluctuation of the electron gas in a solid. These oscillations can propagate inside the bulk or just along the surface, which is pointed out by referring to them as surface plasmons. If propagation is additionally restricted in the other spatial dimensions, we are dealing with localized particle plasmons. They can be induced in metallic nanoparticles, since there, the propagation is confined due to the small particle size. In our ultrafast plasmonics laboratory at the University of Konstanz we generate and investigate surface plasmons as well as particle plasmons in micro- and nanostructures. Applying different techniques with a resolution on the nanometer scale, we can customize the structures for special applications. The equipment in our plasmonics laboratory enables us to optically excite plasmons in single nanostructures with intense ultrashort light pulses. The pulses can be tuned over a broad spectral range and are used to characterize the structures. It turns out that the possibility to produce nanostructures with high precision not only paves the way to manipulate light on the shortest spatial scales: also the temporal answer of plasmonic devices can be tailored. This is even feasible down to the attosecond regime, that is shorter than one single oscillation of the light field. Just as their counterparts for radiofrequencies, optical antennas are ideally suited for the conversion of freely propagating electromagnetic waves into localized energy and vice versa. They enable us to control light on sub-wavelength dimensions. Thus, the diffraction-limited possibilities of light manipulation by conventional free-space optics like lenses or mirrors are vastly expanded. Furthermore, the near-field amplification that can be achieved at optical antennas is also of great importance for advanced light-matter coupling. For instance, this goal can be achieved with hybrid systems consisting of semiconductor quantum dots coupled to antennas. Additionally, the highly nonlinear optical response of nanoantennas promises to hold a huge application potential.

The study of ultrafast phenomena requires us to develop novel laser sources. For the reason we have a dedicated research line for the design of new laser concepts that allow us to scale pulse energy and repetition rate while preserving the possibility to deliver ultrashort pulses. Our systems are based on Er:fiber laser technology that show excellent robustness and flexibility. We employ this sources to seed high power Yb: and Tm:fiber amplifiers able to produce µJ level pulses at 10 MHz repetition rate. In alternative, thanks to thin disk technology.