Excitons dominate the light absorption and re-emission spectra of monolayer transition-metal dichalcogenides (TMD). Microscopic investigations of the excitonic response in TMD almost invariably extract information from the radiative recombination step, which only constitutes one part of the picture. Here, by exploiting imaging spectroscopic ellipsometry (ISE), we investigate the spatial dependence of the dielectric function of chemical vapor deposition (CVD)-grown WS2 flakes with a microscopic lateral resolution, thus providing information about the spatially varying, exciton-induced light absorption in the monolayer WS2. Comparing the ISE results with imaging photoluminescence spectroscopy data, the presence of several correlated features was observed, along with the unexpected existence of a few uncorrelated characteristics. The latter demonstrates that the exciton-induced absorption and emission features are not always proportional at the microscopic scale. Microstructural modulations across the flakes, having a different influence on the absorption and re-emission of light, are deemed responsible for the effect.

The interaction of ultrashort pulsed laser radiation with intensities of 1013 W cm−2 and above with materials often results in an unexpected high X-ray photon flux. It has been shown so far, on the one hand, that X-ray photon emissions increase proportionally with higher laser power and the accumulated X-ray dose rates can cause serious health risks for the laser operators. On the other hand, there is clear evidence that little variations of the operational conditions can considerably affect the spectral X-ray photon flux and X-ray emissions dose. In order to enhance the knowledge in this field, four ultrashort pulse laser systems for providing different complementary beam characteristics were employed in this study on laser-induced X-ray emissions, including peak intensities between 8 × 1012 W∙cm−2 < I0 < 5.2 × 1016 W∙cm−2, up to 72.2 W average laser power as well as burst/bi-burst processing mode. By the example of AISI 304 stainless steel, it was verified that X-ray emission dose rates as high as H˙′ (0.07) > 45 mSv h−1 can be produced when low-intensity ultrashort pulses irradiate at a small 1 µm intra-line pulse distance during laser beam scanning and megahertz pulse repetition frequencies. For burst and bi-burst pulses, the second intra-burst pulse was found to significantly enhance the X-ray emission potentially induced by laser pulse and plasma interaction.

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Optimizing the processing of organic photovoltaic devices by laser radiation requires a fundamental understanding of the excitation processes during laser radiation−matter interaction. Spatially and temporally resolved pump−probe ellipsometry on poly(methyl methacrylate) (PMMA), excited by single-pulsed ultrafast mid-IR laser radiation, reveals electronic and vibrational excitation as competing excitation processes. Mid-IR laser radiation in the femtosecond regime induces mainly wavelength independent nonlinear excitation of electrons, as predicted by a theoretical model of the induced free charge carrier density. In contrast, mid-IR laser radiation in the picosecond regime enables linear vibrational excitation, provided the laser radiation frequency corresponds to a resonance frequency of PMMA. Thereby, a wavelength selective processing of organic materials may enable faster processing of multilayer systems depending on the polymer specific vibrational modes.

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In contrast to conventional methods for increasing the conductivity of transparent polymer electrodes based on additives, solvent treatment, or hot plate thermal processing, laser heating enables the selective annealing of defined areas of a material on the micrometer scale. This study investigates the spatial conductivity distribution of thin PEDOT:PSS films after selective heating by continuous wave (cw) laser radiation without damaging the material. The sheet resistances of the samples decreased abruptly above a certain threshold intensity and a changed relative permittivity of the irradiated material was detected by imaging ellipsometry. A theoretical model has been derived to estimate the resulting spatial conductivity distribution induced by the inhomogeneous heating with the Gaussian spatial intensity distribution of the applied laser radiation. In combination with the experimental measurements, the model revealed that the conductivity increased proportionally to the intensity above the threshold intensity, but remained unchanged below, resulting also in a Gaussian spatial conductivity distribution. Thus, in contrast to conventional oven annealing where only large-area averaged annealing is possible, laser annealing allows to tailor the spatial distribution of the conductivity on a micrometer scale opening applications for future micro systems.

We demonstrate micrometer-resolved imaging of the transient dielectric function of a c-ZnO thin film with femtosecond resolution in the visible to near-IR spectral range measured by pump-probe ellipsometry at five different probe photon-energies. The spatial profile of the real part of the dielectric function broadens drastically with increasing time delay, which we associate with the combined effect of carrier cooling and fast carrier transport with an effective diffusion coefficient of (1.1±0.1)×104 cm2/s. A ring structure is detected in the image after a few picoseconds, which can be explained by a random-walk model including ballistic transport due to the thermal gradient induced by the hot-phonon effect.

The ablation efficiency during laser processing strongly depends on the initial and transient reflectance of the irradiated material surface. This article reports on the transient relative change of the reflectance ΔR/R of stainless steel during and after ultrashort pulsed laser excitation (800 nm, 40 fs) by spatially resolved pump–probe reflectometry. The spatial resolution of the setup in combination with the spatial Gaussian intensity distribution of the pump radiation enables a fluence-resolved detection of ΔR/R. Within the first picosecond after irradiation with a peak fluence of 2 J/cm2, the spatially resolved ΔR/R of stainless steel evolves into an annular shape, in which the center almost remains at its initial reflectance, whereas the outer region features a decreased reflectance. The decreasing trend of ΔR/R is qualitatively supported by applying a two-temperature model, considering the transient optical properties of stainless steel from the literature. At larger fluences and thus higher electron temperatures, the experimental data deviate from the transient reflectance given in the literature. A drastically decreased occupation of the states below the Fermi energy and the subsequent excitation of electrons into these new vacant states by the probe radiation are considered being the most probable origin for this behavior at high fluences.