We have measured the low-temperature (4.2 K) exciton lifetimes of zinc-blende CdTe nanocrystal quantum dots (NQDs), 2.6-3.8 nm in diameter, in magnetic fields up to 30 T. The exciton photoluminescence decay time decreases with both dot size and magnetic field. We explain the decrease in decay time in magnetic fields by the mixing of bright and dark exciton states due to a small shape asymmetry in the zinc-blende CdTe NQDs. We show that this behavior resembles that of wurtzite CdSe NQDs, and we demonstrate that an asymmetry of NQDs caused by either shape or crystal structure leads to similar exciton decay dynamics.

We have prepared 5 nm diameter, micrometer long tetra(p-phenylenevinylene) (OPV)-based nanofibers on a graphite surface. The fluorescence emission of an individual fiber shows a profound polarization over its entire length that directly corresponds to its orientation on the substrate. Quantitative analysis of the fluorescence polarization, including the depolarizing effect of the underlying graphite, evidences the high degree of organization within chiral fibers with the OPV molecules perpendicular to the fiber axis. The control of the internal order within self-assembled fibers, and the ability to measure it, is a crucial step to obtain uniform organic fibers that can be applied in nanosized electronics at room temperature.

We determine the low-temperature optical properties of dark-exciton states in CdSe/CdS nanocrystal quantum dots ͑NQDs͒. By using resonant laser excitation we distinguish zero-phonon from phonon-assisted photoluminescence. The NQDs show a decreasing zero-phonon intensity with decreasing temperature, resulting in a redshift of the nonresonant photoluminescence. This redshift is undone by application of a magnetic field. Our results show that dark-exciton luminescence originates from the intricate competition of phonon-assisted and zero-phonon transitions, the latter of which are enhanced by dark-bright-exciton mixing due to unpassivated surface states or a magnetic field.

Self-assembled rings (with diameters of 0.1−2.0 μm) consisting of porphyrin dodecamer molecules are formed by evaporating chloroform solutions on glass substrates. Fluorescence microscopy experiments on individual rings reveal a strongly polarized optical absorption and emission. Quantitative analysis of the fluorescence images evidences the high degree of order within the rings, consisting of radially oriented columnar stacks of porphyrin dodecamers, and the absence of energy transport along the rings on length scales resolved by fluorescence microscopy.

We measure the energy levels and charging spectra of holes in self-assembled InAs quantum dots using capacitance-voltage and polarized photoluminescence spectroscopy in high m agnetic fields. The pronounced circular polarization of the optical emission, together with the optical selection rules for orbital and spin quantum numbers, allows us to separate the individual electron and hole levels. The magnetic field dependence of the single-particle hole energy levels can be understood by considering a spin-orbit coupled valence band and agrees well with the observed behavior of the charging peaks in the capacitance-voltage spectra.

We have quantified the sensitivity of a simple method to measure the frequency spectrum of pulsed terahertz (THz) radiation. The THz pulses are upconverted to the optical regime by sideband generation in a zinc telluride (ZnTe) crystal using a continuous wave (cw) narrow-bandwidth near-infrared laser. A single-shot spectral measurement of sideband pulses with a high resolution spectrometer directly provides the spectral information of the THz pulses without the need of adjustable elements in the detection setup. This method has been applied at the free electron laser FELIX, where, for a wavelength of 150 μm (2 THz), pulse trains of 5 μs duration with an integrated energy of 800 nJ, as well as single pulses with an energy as low as 13 nJ could be characterized on a single-shot basis.

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