Using low temperature single molecule spectroscopic methods as applied to rigid-rod conjugated polymers, U. Lemmer (Light Technology Institute) and his coworkers were able to identify homogeneously broadened, strongly polarized emission from individual chromophore units of a single semiconducting polymer chain. Gated fluorescence spectroscopy allowed real time imaging of intramolecular energy transfer for the first time. It was found that the polymer chain behaves as a series of weakly interacting chromophores. Energy transfer is controlled by the chromophoric spectral linewidth, which depends on temperature. Linewidths exceeding intramolecular disorder lead to incoherent chromophore coupling and collective fluorescence phenomena. This constitutes the first observation of individual states of a semiconducting polymer using low-temperature single molecule optical spectroscopy (Phys. Rev. Lett. 91, 267403 (2003) and Appl. Phys. Lett. 84, 1183 (2004).).
The group of M. Kappes (Institute of Physical Chemistry) has developed setups for confocal microscopy of single-walled carbon nanotubes (SWNT) in the near-infrared spectral range. These allow for Raman and photoluminescence measurements under a wide range of different sample conditions including pressures of up to 10 GPa and temperatures down to 5 K. Such measurements have provided insight into the chiral vector dependent electronic properties of semiconducting SWNTs – including their dependence on uniaxial strain and hydrostatic compression (Nano Lett. 4, 2349 (2004)). Raman imaging of individual, electrically contacted SWNTs has led to identification of the dominant mechanism for electron-phonon coupling in metallic SWNTs (Nano Lett. 5, 1761 (2005)). Near-infrared photoluminescence and Raman probes have also played a key role in the development of a dielectrophoretic (DEP) trapping method, which allows spatially selective deposition of individual SWNTs from aqueous surfactant suspensions. Studies of the frequency dependence of DEP trapping led to the development of a DEP based method for the separation of semiconducting from metallic SWNTs (Science 301, 344 (2003)).
U. Spicher (Institute for Reciprocating Engines) has pioneered novel spectroscopy and imaging techniques for the diagnosis of combustion processes in automotive engines. Ongoing cooperative research projects with leading car manufacturers aim at the development of energy efficient cars. Recently, a new technique for the measurement of three-dimensional flame propagation in internal combustion engines was presented (7. International Symposium on Internal Combustion Diagnostics, Baden-Baden, Germany (2006)). The radiation emitted from combustion flames in a spark-ignited engine with spray-guided direct-injection was detected simultaneously from different perspectives by three endoscopic fiber optical visualization systems. The measurement system consists of an assembly of 3 high-speed visualization systems based on detection with photomultipliers (High-Speed Visualization of Combustion in Modern Gasoline Engines, 2. Int. Conference on Optical and Laser Diagnostics, London (2005)), which detects light signals in the UV range from the combustion chamber via fiber-optical endoscopes. The combustion process can be measured with a temporal resolution of 200 kHz. Consequently, cycle-resolved measurements are possible at an engine speed up to 3000 rpm.
The group of H. Kalt (Institute for Applied Physics) has established a setup for time-resolved confocal microscopy which combines picosecond laser excitation and streak camera detection (temporal resolution 5 ps) with focusing and imaging via microscope objectives. The spatial resolution is further enhanced by the use of hemispherical solid immersion lenses to a value of some 200 nm. The semiconductor samples are placed in compact He-flow cryostats for experiments at lattice temperatures between 5 and 300 K. This setup allows for example the spectroscopy of excitonic relaxation processes in single quantum dots, of spontaneous and stimulated emission from single ZnO nanorods or of excitonic transport processes on a length scale below the wavelength of light and on a time scale faster than the interaction time with the phonon bath. Recently, this has led to novel insights into the coherent spatiotemporal dynamics of excitons in semiconductor quantum well structures (Phys. Rev. Lett. 89, 097401 (2002) and Phys. Rev. Lett. 94, 137402 (2005)). A breathing-like spatial oscillation of the exciton density was found and allowed the evaluation of both the coherence length and coherence time of quasi-ballistic excitons.
