Coherent versus incoherent excitation dynamics in dissipative many-body Rydberg systems

We study the impact of dephasing on the excitation dynamics of a cloud of ultracold two-level Rydberg atoms for both resonant and off-resonant laser excitation, using the wave function Monte Carlo (MCWF) technique. We find that while for resonant laser driving, dephasing mainly leads to an increase of the Rydberg population and a decrease of the Mandel Q parameter, at off-resonant driving strong dephasing toggles between direct excitation of pairs of atoms and subsequent excitation of single atoms, respectively. These two excitation mechanisms can be directly quantified via the pair correlation function, which shows strong suppression of the two-photon resonance peak for strong dephasing. Consequently, qualitatively different dynamics arise in the excitation statistics for weak and strong dephasing in off-resonant excitation. Our findings show that time-resolved excitation number measurements can serve as a powerful tool to identify the dominating process in the system's excitation dynamics.Comment: 10 pages, 10 figure

Collective excitation of Rydberg-atom ensembles beyond the N\sqrt{N} enhancement

In an ensemble of laser-driven atoms involving strongly interacting Rydberg states, the excitation probability is usually strongly suppressed. In contrast, here we identify a regime in which the steady-state Rydberg excited fraction is enhanced by the interaction. This effect is associated with the build-up of many-body coherences, induced by coherent multi-photon excitations between collective states. The excitation enhancement should be observable under currently-existing experimental conditions, and may serve as a direct probe for the presence of coherent multi-photon dynamics involving collective quantum states.Comment: 7 pages, 4 figure

Spatial light modulator used for the Fourier transformation of video images

Coherent-optical Fourier transformation of video images can be very powerful; a set-up will be described. The modulation of the coherent beam occurs in a spatial light modulator where the liquid crystal display image of the object is projected onto an optically addressed spatial light modulator that uses bismuth silicium oxide for photoconduction and a twisted nematic liquid crystal as a birefringent layer. Edge enhancing of the image can occur; in addition, it appears that the Fourier spectrum of an edge is asymmetric due to local phase variations. The set-up was applied to recognize objects at different positions. A charge-coupled device matrix camera detects the Fourier plane, in the personal computer the pixels are summed up to ring-wedge-segments and processed for pattern recognition

Low coherence interferometry in selective laser melting

© 2014 The Authors. Published by Elsevier B.V. Selective Laser Melting (SLM) is an additive layer manufacturing technology that offers several advantages compared to conventional methods of production such as an increased freedom of design and a toolless production suited for variable lot sizes. Despite these attractive aspects today's state of the art SLM machines lack a holistic process monitoring system that detects and records typical defects during production. A novel sensor concept based on the low coherence interferometry (LCI) was integrated into an SLM production setup. The sensor is mounted coaxially to the processing laser beam and is capable of sampling distances along the optical axis. Measurements during and between the processing of powder layers can reveal crucial topology information which is closely related to the final part quality. The overall potential of the sensor in terms of quality assurance and process control is being discussed. Furthermore fundamental experiments were performed to derive the performance of the system

Toward Understanding High-Z′ Organic Molecular Crystals through the Superspace Method: The Example of Glycyl-L-valine

The high-Z′ (Z′ = 7) structure of glycyl-l-valine has been redetermined at 298 K, using synchrotron radiation and exploiting the superspace approach. The analysis of the diffraction data reveals that the structure can be described as a commensurately modulated crystal structure with superspace group P2$_1$2$_1$2$_1$(0σ$_2$0)000 and modulation wave vector q = (0, 2/7, 0). A new Z′ = 1 phase has been discovered for this compound at 323 K that is related to the known Z′ = 7 phase. The origin of the modulated phase has been explored by analyzing intermolecular interactions and molecular conformations in terms of t-plots and through a comparison to (1) the newly discovered high-temperature phase, (2) a hypothetical Z′ = 1 structure at 298 K, and (3) a similar Z′ = 1 structure of the related compound glycyl-l-leucine. The results show that the conformational flexibility of the glycyl-l-valine molecule ensures optimization of the hydrogen-bond network which causes the modulation and thus a high Z′ value of the supercell. This study highlights the elegance and convenience of the superspace approach to explain the occurrence of at least part of the rarely occurring high-Z′ structures of molecular compounds