Best photos coming from Department of Dielectrics in 2021:

Graphene is a single sheet of carbon atoms forming a two-dimensional infinite honeycomb lattice. For example, a macroscopic ordered stack of such layers forms the graphite; however, the behavior of the graphene single atomic layer is dramatically different from that of graphite.

The electronic properties of graphene are determined by the behavior of charge carriers (electrons and holes); their energy band structure is quite different from typical structures of classical semiconductors or metals and resembles much more the energy scheme of photons.

Depending on the position of the so-called Fermi level (which may be shifted, e.g., due to the nature of the surrounding material and dynamically controlled by illumination or by electric current) graphene can be an excellent conductor or a good insulator. Applications of graphene in electronics and optoelectronics rely on various approaches to the Fermi level tuning by electronic, chemical or optical stimuli.

Free-standing graphene layer is very fragile and is not very useful for practical applications. Therefore, in this study, the group of Petr Kužel focused on graphene layers epitaxially grown on silicon carbide (SiC) substrates. Properties of films prepared under certain conditions approach those of an ideal free-standing graphene layer. By varying the technological conditions, the properties of graphene can be tuned. However, the substrate surface is not perfectly flat even if the greatest care is devoted to its preparation. Instead, it always consists of a set of nanoscopic terraces.

The researchers studied graphene layers using ultrashort laser pulses and terahertz optoelectronic probing (see Fig. 1): an optical pulse excited charge carriers and a delayed ultrashort terahertz pulse tested the state of these carriers. Measured changes in the terahertz pulse shape provided the conductivity spectra, which reflect the distribution of charges within the energy band structure.

The obtained spectra exhibited the so-called localized plasmon resonance, which expresses the collective motion of charge carriers, and which is related to the existence of terraces on the substrate surface. In the investigated graphene samples the build-up and decay of these plasmons on picosecond time scale (1 ps = 10^–12 s) was observed.

In brief, the observed behavior can be described as follows. Before the arrival of the optical laser pulse a significant concentration of equilibrium charge carriers exists in the sample, the graphene film is conducting. Immediately after the optical excitation, the newly generated carriers gain a very high temperature, they exchange energy with equilibrium carriers through elastic scattering, undergo a fast recombination process, and efficiently transfer their energy to the graphene crystal lattice.

During the first picosecond the optically generated carriers practically vanish, and only significantly heated equilibrium carriers remain; the Fermi level exhibits a pronounced decrease with respect to the equilibrium state. The subsequent nonlinear dynamics of plasmons are entirely controlled by the Fermi level of excited carriers through their temperature T_c (see Figure 1). The nonlinear behavior of graphene in this state is a consequence of the unique band structure of graphene which enables very high rate of elastic collisions of carriers. The decay of the nonlinear regime depends on the degree of disorder in the graphene layer. The technological control of the disorder in graphene layers thus allows one to tune the THz response of the material which is important for its optoelectronic applications.

text: Petr Kužel

Reference
[1] V. C. Paingad, J. Kunc, M. Rejhon, I. Rychetský, I. Mohelský, M. Orlita, and P. Kužel, Ultrafast plasmon thermalization in epitaxial graphene probed by time-resolved THz spectroscopy, Adv. Funct. Mater. 31, 2105763 (2021).

The Falling Walls Lab is a world-class pitch competition that brings together a diverse and interdisciplinary pool of students and researchers, and pioneering innovators. One of the preliminaries of the final Falling Walls Lab in Berlin is yearly organized by the University of Wrocław and is devoted to participants from the Central and Eastern Europe. Three minutes is all it takes to succeed in this competition. What matters is an original idea and the ability to present it in a clear way. In Wroclaw, Karel Tesař has captured attention of Polish scientists by his project Post sternotomy chest pain.

The front cover (see Fig. 1) shows collective dynamics of spontaneously formed vortices of electric polarization in ferroelectric PbTiO₃ layers of PbTiO₃/SrTiO₃ superlattices which have been investigated by Marek Paściak, Jirka Hlinka, and Christalle Kadlec in a world-wide collaboration (the Argonne National Laboratory, the Pennsylvania State University, the University of California, Berkeley and other American institutions) [1].

The characteristic ultrafast collective polarization dynamics of vortices was discovered by pump-probe experiments, concretely using a terahertz-field excitation and femtosecond X-ray diffraction measurements. Lattice dynamics calculations with first-principle-based interatomic potentials done in Department of Dielectrics unveiled the atomistic picture of the observed excitations (see Fig. 2). In particular, the most coherent and electric-field susceptible mode at the sub-THz frequencies called a vortexon has been recognized as the transverse oscillation of whole polar vortices. The frequency of the vortexon mode has been experimentally and theoretically found to be tunable by temperature or the substrate strain. Moreover, the mode is behind a phase transition from the state in which the vortices are symmetrical to the one where they are staggered. The experiment and calculations prove that THz pulses can excite the polarization state on the nanometer scale which opens opportunities for electric-field-driven data processing in topological structures with ultrahigh speed and density.