Selection of dominant multi-exciton transitions in disordered linear J-aggregates

We show that the third-order optical response of disordered linear J-aggregates can be calculated by considering only a limited number of transitions between (multi-) exciton states. We calculate the pump-probe absorption spectrum resulting from the truncated set of transitions and show that, apart from the blue wing of the induced absorption peak, it agrees well with the exact spectrum.Comment: 8 pages, 2 figures, accepted to Journal of Luminescenc

Collective Optical Response of Molecular Aggregates

Relative fluorescence quantum yield, fluorescence lifetime, optical dephasing, and absorption line shift data are presented for both excitonic transitions of the pseudoisocyanine (PIC) J-aggregate. The data indicate activation of the optical dephasing by exciton-acoustic phonon scattering at low temperatures. Above approximately 80 K the optical dephasing, line shift as well as the fluorescence lifetime lengthening call for activation by high frequency phonons. Moreover, resonance Raman data indicate the delocalization of low frequency vibrational modes in the aggregate

Structure-property relationships in laser assisted and mechanically deformed advanced materials

This research concentrates on the structural crystallographic properties in laser assisted and mechanically deformed shapes within hexagonal closed packed (HCP) material. This thesis explores a new ‘challenge and opportunity’ for lasers in the field of manufacturing processing, i.e. using a laser as an innovative tool for materials forming purposes. Indeed the laser have been used before as a sophisticated heat source for manufacturing products but the processing-structure-property relationships have been largely neglected. Here, we present a detailed and in-depth study of the structure-performance relationships in laser assisted and mechanically deformed advanced materials

Decoherence of Excitons in Multichromophore Systems: Thermal Line Broadening and Destruction of Superradiant Emission

We study the temperature-dependent dephasing rate of excitons in chains of chromophores, accounting for scattering on static disorder as well as acoustic phonons in the host matrix. From this we find a powerlaw temperature dependence of the absorption line width, in excellent quantitative agreement with experiments on dye aggregates. We also propose a relation between the line width and the exciton coherence length imposed by the phonons. The results indicate that the much debated steep rise of the fluorescence lifetime of pseudo-isocyanine aggregates above 40 K results from the fact that this coherence length drops below the localization length imposed by static disorder.Comment: 4 pages, 2 figure

Exciton Dephasing and Thermal Line Broadening in Molecular Aggregates

Using a model of Frenkel excitons coupled to a bath of acoustic phonons in the host medium, we study the temperature dependence of the dephasing rates and homogeneous line width in linear molecular aggregates. The model includes localization by disorder and predicts a power-law thermal scaling of the effective homogeneous line width. The theory gives excellent agreement with temperature dependent absorption and hole-burning experiments on aggregates of the dye pseudoisocyanine.Comment: 11 pages, 3 PostScript figure

Fatigue of intermittently stimulated quadriceps during imposed cyclical lower leg movements

During prolonged experiments the influence of knee angular velocity, and stimulation parameters (interpulse interval (IPI), duty cycle (DC), number of pulses per cycle (NP)) on fatigue-induced torque\ud decline of paralyzed human quadriceps was studied. Identification of torque-angle and -angular velocity was also performed. The overall loss of maximum torque (MT) and torque-time integral ('lTI) per cycle during sustained intermittent stimulation during isokinetic movement had a typical exponential decay reaching asymptotic values. Larger knee velocities resulted in a significantly faster and relative larger decay of MT and TTI. The rate and relative magnitude of fatigue during concentric contractions are in direct relation\ud to NP. The results may be valuable in the design of optimal control systems for FES which pursue minimization of muscle fatigue

Engineering molecular aggregate spectra

We show that optical properties of linear molecular aggregates undergo drastic changes when aggregates are deposited on a metal surface. The dipole-dipole interactions of monomers with their images can result in strong {re-structuring of both the exciton band and the absorption spectrum, depending on the arrangement of the monomer transition dipoles with respect to the surface.Comment: 4 two-column pages, Proceedings of the 8th International Conference on Excitonic Processes in Condensed Matter, June 22-27, Kyoto, Japa