Nonequilibrium carrier dynamics in single exfoliated graphene layers on muscovite substrates are studied by ultrafast optical pump-probe spectroscopy and compared with microscopic theory. The very high 10-fs-time resolution allows for mapping the ultrafast carrier equilibration into a quasi-Fermi distribution and the subsequent slower relaxation stages. Coulomb-mediated carrier-carrier and carrier-optical phonon scattering are essential for forming hot separate Fermi distributions of electrons and holes which cool by intraband optical phonon emission. Carrier cooling and recombination are influenced by hot phonon effects.

Single photon laser enabled Auger decay (spLEAD) has been redicted theoretically [Phys. Rev. Lett. 111, 083004 (2013)] and here we report its first experimental observation in neon. Using coherent, bichromatic free-electron laser pulses, we have detected the process and coherently controlled the angular distribution of the emitted electrons by varying the phase difference between the two laser fields. Since spLEAD is highly sensitive to electron correlation, this is a promising method for probing both correlation and ultrafast hole migration in more complex systems.

The structure and morphology on different length scales dictate both the electrical and optical properties of organic semiconductor thin fi lms. Using a combination of spectroscopic methods, including scanning near-fi eld optical microscopy, we study the domain structure and packing quality of highly crystalline thin fi lms of oligomeric PQT-12 with 100 nanometer spatial resolution. The pronounced optical anisotropy of these layers measured by polarized light microscopy facilitates the identifi cation of regions with uniform molecular orientation. We fi nd that a hierarchical order on three different length scales exists in these layers, made up of distinct well-ordered dichroic areas at the ten-micrometer-scale, which are sub-divided into domains with different molecular in-plane orientation. These serve as a template for the formation of smaller needle-like crystallites at the layer surface. A high degree of crystalline order is believed to be the cause of the rather high fi eld-effect mobility of these layers of 10 − 3 cm 2 V − 1 s − 1 , whereas it is limited by the presence of domain boundaries at macroscopic distances.

Elementary excitations at the polar (000-1) surface of a 20 nm pseudomorphically grown ZnO thin film are examined by steady state and time-resolved photoluminescence spectroscopy at low temperature. We control the density of emission centers through the deposition of prototypical organic molecules with a carboxylic acid anchor group by the Langmuir-Blodgett technique. Knowledge of the precise film thickness, defect concentrations and number density of deposited molecules leads us to associate the surface exciton emission to defect-related localization centers that are generated through a photochemical process.

Excitons play a key role for the optoelectronic properties of hybrid systems. We apply near-field scanning optical microscopy (NSOM) with a 100-nm spatial resolution to study the photoluminescence of surface excitons (SX) in a 20 nm thick ZnO film capped with a monolayer of stearic acid molecules. Emission from SX, donor-bound (DX), and -at sample temperatures T>20 K -free (FX) excitons is separated in steady-state and time-resolved photoluminescence spectra. The 4 meV broad smooth envelope of SX emission at T<10 K points to an inhomogeneous distribution of SX transition energies and spectral diffusion caused by diffusive SX transport on a 50 nm scale with a diffusion coefficient of D SX (T<10K)=0.30 cm²/s. PACS number(s): 68.37. Uv, 73.20.At, 78.66.Hf