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Waller E.H.,University of Kaiserslautern | Freymann G.V.,University of Kaiserslautern | Freymann G.V.,Fraunhofer Institute for Physical Measurement Techniques
Optics Express

Independent control of the spatial intensity, phase and polarization distribution has numerous applications in direct laser writing, microscopy and optical trapping. Especially, it is well known that the inversion of the Debye-Wolf diffraction integral usually leads to spatially varying intensity, phase and polarization maps. Here, we present a prism and grating free setup built around a single phase-only spatial-light-modulator for full control of spatial intensity, phase and polarization distributions. These distributions are not limited to non-diffractive beams and do not require any change of setup. We verify the versatility of the proposed method with wavefront and intensity measurements. © 2013 Optical Society of America. Source

Waller E.H.,University of Kaiserslautern | Von Freymann G.,University of Kaiserslautern | Von Freymann G.,Fraunhofer Institute for Physical Measurement Techniques
Optics Express

The generation of multi foci is an established method for high-speed parallel direct laser writing, scanning microscopy and for optical tweezer arrays. However, the quality of multi foci reduces with increasing resolution due to interference effects. Here, we report on a spatial-lightmodulator- based method that allows for highly uniform, close to Gaussian spots with diffraction limited resolution using a wavelength of 780 nm. We introduce modifications of a standard algorithm that calculates a field distribution on the entrance pupil of a high numerical aperture objective splitting the focal volume into a multitude of spots. Our modified algorithm compares favourably to a commonly used algorithm in full vectorial calculations as well as in point-spread- function measurements. The lateral and axial resolution limits of spots generated by the new algorithm are found to be close to the diffraction limit. ©2013 Optical Society of America. Source

Nielsch K.,University of Hamburg | Bachmann J.,University of Hamburg | Kimling J.,University of Hamburg | Bottner H.,Fraunhofer Institute for Physical Measurement Techniques
Advanced Energy Materials

Thermoelectric materials could play an increasing role for the efficient use of energy resources and waste heat recovery in the future. The thermoelectric efficiency of materials is described by the figure of merit ZT = (S 2 σT )/κ (S Seebeck coefficient, σ electrical conductivity, κ thermal conductivity, and T absolute temperature). In recent years, several groups worldwide have been able to experimentally prove the enhancement of the thermoelectric effi ciency by reduction of the thermal conductivity due to phonon blocking at nanostructured interfaces. This review addresses recent developments from thermoelectric model systems, e.g. nanowires, nanoscale meshes, and thermionic superlattices, up to nanograined bulk-materials. In particular, the progress of nanostructured silicon and related alloys as an emerging material in thermoelectrics is emphasized. Scalable synthesis approaches of high-performance thermoelectrics for high-temperature applications is discussed at the end. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Rahm M.,University of Kaiserslautern | Rahm M.,Fraunhofer Institute for Physical Measurement Techniques | Li J.-S.,China Jiliang University | Padilla W.J.,Boston College
Journal of Infrared, Millimeter, and Terahertz Waves

We review different techniques for modulation of the electromagnetic properties of terahertz (THz) waves. We discuss various approaches for electronic, optical, thermal and nonlinear modulation in distinct material systems such as semiconductors, graphene, photonic crystals and metamaterials. The modulators are classified and compared with respect to modulation speed, modulation depth and categorized by the physical quantity they control as e.g. amplitude, phase, spectrum, spatial and temporal properties of the THz wave. Based on the review paper, the reader should obtain guidelines for the proper choice of a specific modulation technique in view of the targeted application. © 2012 Springer Science+Business Media New York. Source

Luedtke F.,University of Bonn | Buse K.,Albert Ludwigs University of Freiburg | Buse K.,Fraunhofer Institute for Physical Measurement Techniques | Sturman B.,Russian Academy of Sciences
Physical Review Letters

We show that a continuous-wave (cw) pump beam at a wavelength of 532 nm produces substantial light-induced (LI) absorption in the visible range in initially transparent undoped LiNbO 3 crystals. The LI absorption coefficient stays linear in the pump intensity I p up to Ipmax=48kW/cm2. Together with other features including long-term stretched-exponential relaxation of the LI absorption, it indicates that the present concept of LI electron processes in this important optical material must be revised: the amount of photoactive electrons increases already within the cw intensity range. A quantitative model is proposed that explains the experimental data and employs two-step excitations from filled localized states near the valence band via intermediate deep centers into the conduction band. The introduced localized states serve as a hidden reservoir of electrons. © 2012 American Physical Society. Source

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