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Ludeke S.,Albert Ludwigs University of Freiburg | Pfeifer M.,Fraunhofer Institute for Physical Measurement Techniques | Fischer P.,Fraunhofer Institute for Physical Measurement Techniques
Journal of the American Chemical Society | Year: 2011

Vibrational circular dichroism (VCD) spectra were recorded with a tunable external-cavity quantum-cascade laser (QCL). In comparison with standard thermal light sources in the IR, QCLs provide orders of magnitude more power and are therefore promising for VCD studies in strongly absorbing solvents. The brightness of this novel light source is demonstrated with VCD and IR absorption measurements of a number of compounds, including proline in water. © 2011 American Chemical Society.

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 | Year: 2012

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.

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 | Year: 2011

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.

« New one-pot high-yield “high-gravity” process for cellulosic ethanol; potential for drop-in fuels | Main | Automotive sales in Russia in 2015 down 35.7% » Alphabet Energy is commercializing low-cost, efficient thermoelectric materials for power generation leveraging technology initially developed at the Lawrence Berkeley National Laboratory. (Earlier post.) The company has now announced characterization from the Fraunhofer Institute for Physical Measurement Techniques IPM of heat flow and thermal resistance (in air) of the Alphabet Energy PowerCard, the company’s core thermoelectric device for power generation. The PowerCard has shipped to customers in a variety of industries, including automotive; has been tested extensively; and is now entering high-volume production. The PowerCard generates power from exhaust source temperatures ranging from 400-600 °C using Alphabet Energy’s proprietary thermoelectric materials: tetrahedrite and magnesium silicide stannide. Competing materials, such as skutterudites and half-Heuslers, rely on rare and critical elements subject to scarcity and price volatility, making them unreliable for commercial scale, Alphabet Energy said. The tetrahedrite and magnesium silicide stannide combination consists of the most abundant and scalable elements available for high-temperature materials, enabling the PowerCard to meet commercial requirements for a wide range of applications from remote power generation in industrial settings to waste heat recovery in the automotive industry. In testing conducted at Alphabet Energy’s labs, the PowerCard generates over 9 watts of electricity at 5% efficiency with a hot-side temperature of 400 °C and a cold-side temperature of 100 °C, outperforming competing technologies that are able to produce around six watts (skutterudites, half-Heuslers) and two watts (bismuth telluride) when tested under the same conditions. Furthermore, the PowerCard has displayed high reliability through large numbers of thermal cycles and time at temperature in air. In addition to the significant advancements with thermoelectric materials science, the PowerCard represents a culmination of technological advances in manufacturing, metallization, package design, coatings, and assembly materials and processes. Alphabet Energy has shipped PowerCard prototypes to a variety of customers, both stand-alone and as part of the larger PowerModule (which contains many PowerCards), in energy-intensive markets including, automotive, commercial trucking, oil & gas, industrial manufacturing, defense, and consumer appliances. Automotive. According to a McKinsey Quarterly report, the connected cars of the future will “become less like metal boxes and more like integrators of multiple technologies, productive data centers.” The automotive industry is working to meet the increased electrical power requirements of the future car (e.g., connected, semiautonomous) while also achieving fuel efficiency standards (e.g., US EPA CAFE Standards). The Alphabet Energy PowerModule is being used by an automotive OEM and tier-one supplier to address this challenge, and is expected to improve fuel efficiency by 5%, reducing the load on the alternator and generating the necessary electrical power to keep up with the future car’s electronics.

Waller E.H.,University of Kaiserslautern | Freymann G.V.,University of Kaiserslautern | Freymann G.V.,Fraunhofer Institute for Physical Measurement Techniques
Optics Express | Year: 2013

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.

Waller E.H.,University of Kaiserslautern | Von Freymann G.,University of Kaiserslautern | Von Freymann G.,Fraunhofer Institute for Physical Measurement Techniques
Optics Express | Year: 2013

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.

Pfeifer M.,Fraunhofer Institute for Physical Measurement Techniques | Fischer P.,Fraunhofer Institute for Physical Measurement Techniques
Optics Express | Year: 2011

We present a new form of optical activity measurement based on a modified weak value amplification scheme. It has recently been shown experimentally that the left- and right-circular polarization components refract with slightly different angles of refraction at a chiral interface causing a linearly polarized light beam to split into two. By introducing a polarization modulation that does not give rise to a change in the optical rotation it is possible to differentiate between the two circular polarization components even after post-selection with a linear polarizer. We show that such a modified weak value amplification measurement permits the sign of the splitting and thus the handedness of the optically active medium to be determined. Angular beam separations of Δθ ∼ 1 nanoradian, which corresponds to a circular birefringence of Δn∼1×10-9, could be measured with a relative error of less than 1%. © 2011 Optical Society of America.

Petermann C.,Fraunhofer Institute for Physical Measurement Techniques | Fischer P.,Fraunhofer Institute for Physical Measurement Techniques
Optics Express | Year: 2011

We demonstrate a coupling scheme for cavity enhanced absorption spectroscopy that makes use of an intracavity acousto-optical modulator to actively switch light into (and out of) a resonator. This allows cavity ringdown spectroscopy (CRDS) to be implemented with broadband nonlaser light sources with spectral power densities of less than 30μW/nm. Although the acousto-optical element reduces the ultimate detection limit by introducing additional losses, it permits absorptivities to be measured with a high dynamic range, especially in lossy environments. Absorption measurements for the forbidden transition of gaseous oxygen in air at ∼760nm are presented using a low-coherence cw-superluminescent diode. The same setup was electronically configured to cover absorption losses from 1.8×10-8 cm -1 to 7.5% per roundtrip. This could be of interest in process analytical applications. © 2011 Optical Society of America.

Fischer P.,Fraunhofer Institute for Physical Measurement Techniques | Ghosh A.,Indian Institute of Science
Nanoscale | Year: 2011

Significant progress has been made in the fabrication of micron and sub-micron structures whose motion can be controlled in liquids under ambient conditions. The aim of many of these engineering endeavors is to be able to build and propel an artificial micro-structure that rivals the versatility of biological swimmers of similar size, e.g. motile bacterial cells. Applications for such artificial "micro-bots" are envisioned to range from microrheology to targeted drug delivery and microsurgery, and require full motion-control under ambient conditions. In this Mini-Review we discuss the construction, actuation, and operation of several devices that have recently been reported, especially systems that can be controlled by and propelled with homogenous magnetic fields. We describe the fabrication and associated experimental challenges and discuss potential applications. © 2011 The Royal Society of Chemistry.

Vogel M.,University of Kaiserslautern | Chumak A.V.,University of Kaiserslautern | Waller E.H.,University of Kaiserslautern | Langner T.,University of Kaiserslautern | And 4 more authors.
Nature Physics | Year: 2015

Structuring of materials is the most general approach for controlling waves in solids. As spin waves - eigen-excitations of the electrons' spin system - are free from Joule heating, they are of interest for a range of applications, such as processing, filtering and short-time data storage. Whereas all these applications rely on predefined constant structures, a dynamic variation of the structures would provide additional, novel applications. Here, we present an approach for producing fully tunable, two-dimensionally structured magnetic materials. Using a laser, we create thermal landscapes in a magnetic medium that result in modulations of the saturation magnetization and in the control of spin-wave characteristics. This method is demonstrated by the realization of fully reconfigurable one- and two-dimensional magnonic crystals - artificial periodic magnetic lattices. © 2015 Macmillan Publishers Limited. All rights reserved.

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