1st talk:

Decoherence of Rabi oscillations in diluted paramagnets

Speaker:

Prof. Dr. Kristel Michielsen, JSC

Contents:

Electron paramagnetic resonance experiments on diluted spin systems show that the decay of the Rabi oscillations depends on the Rabi frequency and on the concentration of the spins. In this talk, we scrutinize the various mechanisms that may lead to the decoherence of these Rabi oscillations. By direct numerical solution of the timedependent Schrodinger equation of the many-body system, we show that a system of dipolar-coupled spins subject to an inhomogenous microwave field suffices to explain the salient features of the experimental observations. Some experimental examples are given to illustrate the potential of the numerical simulation approach. The simulations are performed using a parallel algorithm implementation based on a massively parallel quantum computer simulator.

2nd talk:

Optical nano-structures for thin-film solar cells

Speakers:

Prof. Dr. Reinhard Carius, Dr. Karsten Bittkau, IEK-5

Contents:

Since a rapidly expanding photovoltaic market demands cheap and efficient solar cells and modules, one important focus of research is the improvement of total light absorption even in very thin films. Statistically textured surfaces are presently used to scatter the impinging light in a way that it is trapped inside the absorber layer by total internal reflection. These textures consist of structures with sizes in the order of the wavelength or even smaller for the relevant spectral range of sunlight and their optimization is based on trial and error rather than on understanding. Due to the resonant sizes, simple optical approximations are insufficient to fully describe the light trapping. Therefore, a rigorous solution of Maxwells equations is necessary taking into account the measured surface textures, the thicknesses of each layers and the dielectric function of the materials. The Finite-Difference Time-Domain method is proven to be very suitable for this purpose. By taking the software package MEEP, developed at the MIT, the local light scattering properties of the rough interfaces as well as the local absorption profiles in the solar cell are investigated. The local light intensity distribution in air is experimentally determined by near-field scanning optical microscopy and a comparison with the FDTD simulations shows a good agreement. It is therefore concluded that detailed information about light distribution inside the solar cell can be gained and used to support the optimization process for light trapping structures. Additionally, the effect of artificial texture modifications and photonic structures to the layer stack are computed in order to develop novel light trapping concepts that outperforms existing surface textures.