Fraunhofer Institute for Physical Measurement Techniques

This work describes the first thermoelectric devices based on the V-VI-compounds Bi/sub 2/Te/sub 3/ and (Bi,Sb)/sub 2/Te/sub 3/ which can be manufactured by means of regular thin film technology in combination with microsystem technology. Fabrication concept, material deposition for some 10-μm-thick layers and the properties of the deposited thermoelectric materials will be reported. First device properties for Peltier-coolers and thermogenerators will be shown as well as investigations on long term and cycling stability. Data on metal/semiconductor contact resistance were extracted form device data. Device characteristics like response time for a Peltier-cooler and power output for a thermogenerator will be compared to commercial devices.

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.

New applications in the realms of terahertz (THz) technology require versatile adaptive optics and powerful modulation techniques. Semiconductors have proven to provide fast all-optical terahertz wave modulation over a wide frequency band. We show that the attenuation and modulation depth in optically driven silicon modulators can be significantly enhanced by deposition of graphene on silicon (GOS). We observed a wide-band tunability of the THz transmission in a frequency range from 0.2 to 2 THz and a maximum modulation depth of 99%. The maximum difference between the transmission through silicon and GOS is Δt = 0.18 at a low photodoping power of 40 mW. At higher modulation power, the enhancement decreased due to charge carrier saturation. We developed a semianalytical band structure model of the graphene-silicon interface to describe the observed attenuation and modulation depth in GOS.

Abstract 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 efficiency 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.

In this contribution, we present a highly accurate approach for thickness measurements of multi-layered automotive paints using terahertz time domain spectroscopy in reflection geometry. The proposed method combines the benefits of a model-based material parameters extraction method to calibrate the paint coatings, a generalized Rouard's method to simulate the terahertz radiation behavior within arbitrary thin films, and the robustness of a powerful evolutionary optimization algorithm to increase the sensitivity of the minimum thickness measurement limit. Within the framework of this work, a self-calibration model is introduced, which takes into consideration the real industrial challenges such as the effect of wet-on-wet spray in the painting process.

We incorporate dielectric indium tin oxide nanocrystals into the hot-spot of gold nanogap-antennas and perform third harmonic spectroscopy on these hybrid nanostructure arrays. The combined system shows a 2-fold increase of the radiated third harmonic intensity when compared to bare gold antennas. In order to identify the origin of the enhanced nonlinear response we perform finite element simulations of the nanostructures, which are in excellent agreement with our measurements. We find that the third harmonic signal enhancement is mainly related to changes in the linear optical properties of the plasmonic antenna resonances when the ITO nanocrystals are incorporated. Furthermore, the dominant source of the third harmonic is found to be located in the gold volume of the plasmonic antennas.

In this progress report the development of three‐dimensional μ‐printing and its impact as an enabling technology onto different scientific fields is reviewed. Driven by direct laser writing via two‐photon absorption, the technology has reached a level of maturity and ease of application such that 3D printing on the micrometer scale can now be considered. While the underlying technology is still developing towards higher resolution and increasing speed of fabrication, the last five years have seen new fields rising that were obviously enabled by 3D μ‐printing. Among the recent technological developments discussed in this progress report are the fields of super‐resolution lithography and spatial light modulator‐based lithography. Novel fields relying on these technological advances like aperiodic photonic structures, mechanical metamaterials, and structures for biological studies will be reviewed. A glimpse into developing research directions will also be provided.

Whispering-gallery resonators (WGR's), based on total internal reflection, possess high quality factors in a broad spectral range. Thus, nonlinear-optical processes in such cavities are ideally suited for the generation of broadband or tunable electromagnetic radiation. Experimentally and theoretically, we investigate the tunability of optical parametric oscillation in a radially structured WGR made of lithium niobate. With a 1.04 μm pump wave, the signal and idler waves are tuned from 1.78 to 2.5 μm--including the point of degeneracy--by varying the temperature between 20 and 62 °C. A weak off centering of the radial domain structure extends considerably the tuning capabilities. The oscillation threshold lies in the mW-power range.

An algorithm for the calculation of the propagation constant of integrated-optics waveguides is developed. The waveguide may consist of any number of layers with complex refractive indexes due to gain and loss. It allows the additional determination of the field distribution. Thus, all parameters necessary for heterostructure laser development can be calculated. The resulting program was run on a personal computer; numerical results are presented.

A novel innovative approach towards a marketable lab-on-chip system for point-of-care in vitro diagnostics is reported. In a consortium of seven Fraunhofer Institutes a lab-on-chip system called "Fraunhofer ivD-platform" has been established which opens up the possibility for an on-site analysis at low costs. The system features a high degree of modularity and integration. Modularity allows the adaption of common and established assay types of various formats. Integration lets the system move from the laboratory to the point-of-need. By making use of the microarray format the lab-on-chip system also addresses new trends in biomedicine. Research topics such as personalized medicine or companion diagnostics show that multiparameter analyses are an added value for diagnostics, therapy as well as therapy control. These goals are addressed with a low-cost and self-contained cartridge, since reagents, microfluidic actuators and various sensors are integrated within the cartridge. In combination with a fully automated instrumentation (read-out and processing unit) a diagnostic assay can be performed in about 15 min. Via a user-friendly interface the read-out unit itself performs the assay protocol, data acquisition and data analysis. So far, example assays for nucleic acids (detection of different pathogens) and protein markers (such as CRP and PSA) have been established using an electrochemical read-out based on redoxcycling or an optical read-out based on total internal reflectance fluorescence (TIRF). It could be shown that the assay performance within the cartridge is similar to that found for the same assay in a microtiter plate. Furthermore, recent developments are the integration of sample preparation and polymerase chain reaction (PCR) on-chip. Hence, the instrument is capable of providing heating-and-cooling cycles necessary for DNA-amplification. In addition to scientific aspects also the production of such a lab-on-chip system was part of the development since this heavily affects the success of a later market launch. In summary, the Fraunhofer ivD-platform covers the whole value chain ranging from microfluidics, material and polymer sciences, assay and sensor development to the production and assembly design. In this consortium the gap between diagnostic needs and available technologies can be closed.

The lattice dynamics in Bi${}_{2}$Te${}_{3}$ and Sb${}_{2}$Te${}_{3}$ were investigated both microscopically and macroscopically using ${}^{121}$Sb and ${}^{125}$Te nuclear inelastic scattering, x-ray diffraction, and heat capacity measurements. In combination with earlier inelastic neutron scattering data, the element-specific density of phonon states was obtained for both compounds and phonon polarization analysis was carried out for Bi${}_{2}$Te${}_{3}$. A prominent peak in the Te specific density of phonon states at $13\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$, that involves mainly in-plane vibrations, is mostly unaffected upon substitution of Sb with Bi revealing vibrations with essentially Te character. A significant softening is observed for the density of vibrational states of Bi with respect to Sb, consistently with the mass homology relation in the long-wavelength limit. In order to explain the energy mismatch in the optical phonon region, a $\ensuremath{\sim}$$20%$ force constant softening of the Sb-Te bond with respect to the Bi-Te bond is required. The reduced average speed of sound at $20\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ in Bi${}_{2}$Te${}_{3}$, $1.75(1)\phantom{\rule{0.28em}{0ex}}\mathrm{km}/\mathrm{s}$, compared to Sb${}_{2}$Te${}_{3}$, $1.85(4)\phantom{\rule{0.28em}{0ex}}\mathrm{km}/\mathrm{s}$, is not only related to the larger mass density but also to a larger Debye level. The observed low lattice thermal conductivity at $295\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, $2.4\phantom{\rule{4.pt}{0ex}}{\text{Wm}}^{\ensuremath{-}1}{\text{K}}^{\ensuremath{-}1}$ for Sb${}_{2}$Te${}_{3}$ and $1.6\phantom{\rule{4.pt}{0ex}}{\text{Wm}}^{\ensuremath{-}1}{\text{K}}^{\ensuremath{-}1}$ for Bi${}_{2}$Te${}_{3}$, cannot be explained by anharmonicity alone given the rather modest Gr\"uneisen parameters, $1.7(1)$ for Sb${}_{2}$Te${}_{3}$ and $1.5(1)$ for Bi${}_{2}$Te${}_{3}$, without accounting for the reduced speed of sound and more importantly the low acoustic cutoff energy.

We demonstrate a plasmonic Mach-Zehnder (MZ) modulator with a flat frequency response exceeding 170 GHz. The modulator comprises two phase modulators exploiting the Pockels effect of an organic electro-optic material in plasmonic slot waveguides. We further show modulation at 100 GBd NRZ and 60 GBd PAM-4. The electrical drive signals were generated using a 100 GSa/s digital to analog converter (DAC). The high-speed and small-scale devices are relevant for next-generation optical interconnects.

Multi-quantum-well structures of Bi2Te3 are predicted to have a high thermoelectric figure of merit ZT. Bi2Te3 thin films and Bi2Te3∕Bi2(Te0.88Se0.12)3 superlattices (SLs) were grown epitaxially by molecular beam epitaxy on BaF2 substrates with periods of 12 and 6nm, respectively. Reflection high-energy electron diffraction confirmed a layer-by-layer growth, x-ray diffraction yielded the lattice parameters and SL periods and proved epitaxial growth. The in-plane transport coefficients were measured and the thin films and SL had power factors between 28 and 35μW∕cmK2. The lattice thermal conductivity varied between 1.60W∕mK for Bi2Te3 thin films and 1.01W∕mK for a 10nm SL. The best figures of merit ZT were achieved for the SL; however, the values are slightly smaller than those in bulk materials. Thin films and superlattices were investigated in plan view and cross section by transmission electron microscopy. In the Bi2Te3 thin film and SL the dislocation density was found to be 2×1010cm−2. Bending of the SL with amplitudes of 30nm (12nm SL) and 15nm (6nm SL) and a wavelength of 400nm was determined. Threading dislocations were found with a density greater than 2×109cm−2. The superlattice interfaces are strongly bent in the region of the threading dislocations, undisturbed regions have a maximum lateral sie of 500nm. Thin films and SL showed a structural modulation [natural nanostructure (nns)] with a wavelength of 10nm and a wave vector parallel to (1,0,10). This nns was also observed in Bi2Te3 bulk materials and turned out to be of general character for Bi2Te3. The effect of the microstructure on the thermoelectric properties is discussed. The microstructure is governed by the superlattice, the nns, and the dislocations that are present in the films. Our results indicate that the microstructure directly affects the lattice thermal conductivity. Thermopower and electrical conductivity were found to be negatively correlated and no clear dependence of the two quantities on the microstructure could be found.

We present a metamaterial-based terahertz (THz) sensor for thickness measurements of subwavelength-thin materials and refractometry of liquids and liquid mixtures. The sensor operates in reflection geometry and exploits the frequency shift of a sharp Fano resonance minimum in the presence of dielectric materials. We obtained a minimum thickness resolution of 12.5 nm (1/16 000 times the wavelength of the THz radiation) and a refractive index sensitivity of 0.43 THz per refractive index unit. We support the experimental results by an analytical model that describes the dependence of the resonance frequency on the sample material thickness and the refractive index.

We employ three highly sensitive spectrometers: a photoacoustic spectrometer, a photothermal common-path interferometer and a whispering-gallery-resonator-based absorption spectrometer, for a comparative study of measuring the absorption coefficient of nominally transparent undoped, congruently grown lithium niobate for ordinarily and extraordinarily polarized light in the wavelength range from 390 to 3800 nm. The absorption coefficient ranges from below 10(-4) cm(-1) up to 2 cm(-1). Furthermore, we measure the absorption at the Urbach tail as well as the multiphonon edge of the material by a standard grating spectrometer and a Fourier-transform infrared spectrometer, providing for the first time an absorption spectrum of the whole transparency window of lithium niobate. The absorption coefficients obtained by the three highly sensitive and independent methods show good agreement.

Presently, the only commercially available power generating thermoelectric (TE) modules are based on bismuth telluride (Bi2Te3) alloys and are limited to a hot side temperature of 250 °C due to the melting point of the solder interconnects and/or generally poor power generation performance above this point. For the purposes of demonstrating a TE generator or TEG with higher temperature capability, we selected skutterudite based materials to carry forward with module fabrication because these materials have adequate TE performance and are mechanically robust. We have previously reported the electrical power output for a 32 couple skutterudite TE module, a module that is type identical to ones used in a high temperature capable TEG prototype. The purpose of this previous work was to establish the expected power output of the modules as a function of varying hot and cold side temperatures. Recent upgrades to the TE module measurement system built at the Fraunhofer Institute for Physical Measurement Techniques allow for the assessment of not only the power output, as previously described, but also the thermal to electrical energy conversion efficiency. Here we report the power output and conversion efficiency of a 32 couple, high temperature skutterudite module at varying applied loading pressures and with different interface materials between the module and the heat source and sink of the test system. We demonstrate a 7% conversion efficiency at the module level when a temperature difference of 460 °C is established. Extrapolated values indicate that 7.5% is achievable when proper thermal interfaces and loading pressures are used.

The usability of pulsed broadband terahertz radiation for the inspection of composite materials from the aeronautics industry is investigated, with the goal of developing a mobile time-domain spectroscopy system that operates in reflection geometry. A wide range of samples based on glass and carbon fiber reinforced plastics with various types of defects is examined using an imaging system; the results are evaluated both in time and frequency domain. The conductivity of carbon fibers prevents penetration of the respective samples but also allows analysis of coatings from the reflected THz pulses. Glass fiber composites are, in principle, transparent for THz radiation, but commonly with significant absorption for wavelengths >1 THz. Depending on depth, matrix material, and size, defects like foreign material inserts, delaminations, or moisture contamination can be visualized. If a defect is not too deep in the sample, its location can be correctly identified from the delay between partial reflections at the surface and the defect itself.

We report on an enhanced thermoelectric figure of merit ZT=σS2T/λ (where σ is electrical conductivity, S is thermopower, T is absolute temperature, and λ is thermal conductivity) for PbTe/PbSe0.20Te0.80 superlattices (SLs) and PbTe doping SLs due to a reduction of the thermal conductivity λ parallel to the layer planes. Despite a small decrease of the power factors σS2 due to a reduction of σ in these superlattices, the figure of merit is higher as compared to the corresponding bulk materials and reaches maximum values in the temperature range between 400 and 570 K.

Data from the first year of operation of the Pioneer Venus orbiter retarding potential analyzer are presented, showing the variation with solar zenith angle of median values of ion and electron temperatures and density. Ion density is found to decrease by about an order of magnitude from dayside to nightside. Median ion temperatures above 300‐km altitude are essentially constant with solar zenith angle for solar zenith angles less than 150° and have values near 2300°K at the ionopause. Near the antisolar point, the ion temperature rises to 5000°K. At altitudes below 300 km the median ion temperature is nearly constant with solar zenith angle throughout the dayside but increases with solar zenith angle on the nightside. Ion temperatures on the dayside are nearly constant and are larger than the neutral temperature in the altitude range between 175 and 225 km. A strong heat source near 160 km is required to model the observations. Joule heating is suggested as the probable source. The median electron temperature at a given height is independent of both solar zenith angle and ionopause height. The electron temperatures suggest a constant heat flux into the electron gas at the ionopause. The flux may be provided by dissipation of energy by whistler mode plasma waves at the ionopause and/or conduction of heat from the ionosheath through the mantle.

Ferroelectric domain walls are interfaces between areas of a material that exhibits different directions of spontaneous polarization. The properties of domain walls can be very different from those of the undisturbed material. Metallic-like conductivity of charged domain walls (CDWs) in nominally insulating ferroelectrics was predicted in 1973 and detected recently. This important effect is still in its infancy: The electric currents are still smaller than expected, the access to the conductivity at CDWs is hampered by contact barriers, and stability is low because of sophisticated domain structures or proximity of the Curie point. Here, we report on large, accessible, and stable conductivity at CDWs in lithium niobate (LN) crystals - a vital material for photonics. Our results mark a breakthrough: Increase of conductivity at CDWs by more than 13 orders of magnitude compared to that of the bulk, access to the effect via ohmic and diode-like contacts, and high stability for temperatures T ≤ 70 °C are demonstrated. A promising and now realistic prospect is to combine CDW functionalities with linear and nonlinear optical phenomena. Our findings allow new generations of adaptive-optical elements, of electrically controlled integrated-optical chips for quantum photonics, and of advanced LN-semiconductor hybrid optoelectronic devices.