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There have been controversial reports about the type of magnetic coupling and hence ferromagnetism in ZnMnO. We have investigated the magnetic properties of ZnxMn1−xO layers grown by Molecular Beam Epitaxy (MBE) on sapphire substrates. At first sight, the investigated ferromagnetism in our ZnMnO layers deposited on Al2O3 substrates seemed trivial, but the results of extensive analyses of the layers and also data from the Al2O3 substrates used strongly indicate that the influence of the substrate on magnetic property measurements needs to be considered when confirming ferromagnetism in Mn-doped ZnO layers. Our ZnMnO layers showed a paramagnetic behaviour.

Induced transmittance is observed with polycarbonate laser filters when they are irradiated by laser pulses with a width of 200 fs. These results are compared with measurements in the nanosecond time domain. Calculations by a simple absorption model show, that single photon absorption is dominating the light-matter interaction at the pulse widths used in this study, at least at radiant exposure levels at which the filter material is not damaged. The use of eyewear showing induced transmittance can lead to a risk for the user, if the filter is irradiated by a strong laser beam in an accident. This situation can be even worse when an additional temperature effect is taken into account, which shifts the transmittance characteristics to lower optical densities when the temperature of the laser filter is maintained at a lower level. An example for this effect is given for nanosecond-pulse laser interactions.

We demonstrate an optical limiter for ultra-short (∼100-fs) laser pulses. The device has a dynamic range (= damage energy/onset-of-limiting energy) of more than 10000 and an onset-of-limiting energy of only ∼10 nJ. The output-pulse energy is kept below 1.3 μJ. The limiting mechanism is based on two-photon absorption and refractive nonlinearities in a 20-mm piece of ZnSe. We discuss the importance of the different nonlinearities, damage issues, and guidelines for the construction of the device.

We investigate the nonlinear optical properties of low-temperature-grown (LT) GaAs. It is shown that the strength of the absorptive nonlinearity is weak in as-grown undoped LT-GaAs with a fast recovery time of the nonlinear absorption. The reason for this behavior is revealed by a quantitative study of the nonlinear optical properties of the most important defect transition from the neutral As antisite to the conduction band. Based on this analysis, we demonstrate two methods for the improvement of the ultrafast nonlinearity of LT-grown semiconductors: post-growth annealing or Be doping of LT-GaAs yields a material with a strong ultrafast nonlinear optical response.

We present a combined atomic force microscopy (AFM) and magnetic force microscopy (MFM) study of bacterial magnetosomes, consisting of membrane-covered magnetite particles. AFM imaging unambiguously demonstrates the existence of single isolated magnetosomes with a size of about 40 nm, in addition to various clusters of magnetosomes. With MFM we have particularly addressed the magnetic properties of single magnetosomes. These nanoparticles behave like single mono-domain nanomagnets, as shown by the comparison of the MFM data to simple simulations. These simulations also provide information on the size of the magnetic kernel of the magnetosomes. Moreover, the invasiveness of the MFM tip is addressed.

The dynamics of optical excitations in a PPP-type ladder polymer have been investigated by means of femtosecond pump-probe experiments. Stimulated emission (SE) is observed around the S 1 → S 0 transition while photoinduced absorption (PA) dominates the differential transmission at lower energies. The stimulated emission is due to singlet exciton recombination. Comparative studies of the temporal behaviour of the SE and the PA demonstrate that the SE and the PA signals originate from different species. Our results provide evidence that the PA is due to spatially separated electron-hole pairs (indirect excitons).

We study the dynamics of optical excitations in solid films of ladder-type poly(para-phenylene) (LPPP). A low-energy emission band is observed in the photoluminescence spectrum of solid films of this π-conjugated polymer, which is absent in LPPP solutions. We show that the low-energy luminescence can be attributed to aggregates between subunits of the polymer chains. Energy transfer into aggregate states strongly affects the dynamics of excitations in solid LPPP. We further discuss the influence of non-radiative defects on the dynamics.

We have quantitatively studied the optical nonlinearity in annealed and in beryllium doped low-temperature grown (LT) GaAs. This study has been complemented by measurements of the temporal decay of the absorption modulation on the 20-fs time scale. Our results show that both be doping and annealing of LT GaAs yield a material with superior properties for ultrafast all-optical switching applications.

We present a combined experimental and theoretical study of the spatio-temporal carrier dynamics in photoconductive switches. Experimental photoluminescence patterns demonstrate substantially different electron-hole dynamics across the excitation laser spot. The density dependence of screening combines with the lateral variation of the carrier density to produce the observed effects. These conclusions are also reached from self-consistent calculations of the drift-diffusion equation and Poisson's equation. Moreover, the theoretical studies show that the current density transients substantially vary over the excitation laser spot. Hence, the inhomogeneous current distribution should be taken into account for future switch optimization.

We have set up a laser system which simultaneously provides synchronized picosecond pulses in the visible and at a wavelength of 1.06 μm with a repetition rate of 76 MHz. The set-up consists of a dye laser synchronously pumped by the second harmonic of a cw mode-locked Nd:YAG laser and a fiber-grating compressor for the fundamental wavelength of the Nd;YAG laser. Crosscorrelation measurments reveal the time jitter between the two pulse trains to be less than 10 ps. As a first application we have performed non-degenerate transient grating experiments in semiconductors. The non-degenerate technique allows to use excitation energies well above the bandgap energy and to separate non-linear refractive index effects from photoinduced absorption or transmission changes.