Energy Relaxation of Electrons in GaAs/AlGaAs Quantum Wells and Superlattices
Intersubband Transitions in Quantum Wells: Physics and Devices, 1998
We discuss two experiments aimed at the investigation of the electronic energy relaxation rate in... more We discuss two experiments aimed at the investigation of the electronic energy relaxation rate in semiconductor quantum structures. In a doped GaAs/AlGaAs superlattice the steady-state energy-loss rate and the relaxation time are determined using the temperature dependence of the interminiband absorption. The intersubband relaxation in wide GaAs/AlGaAs quantum wells is measured with a picosecond free-electron laser and anlyzed using a energy balance model. Both experiments show the crossover from acoustic-phonon to optical-phonon dominated energy relaxation when the electron temperature rises.
Ternary Composite for Controlled Shape Memory Transformations
2019 IEEE SENSORS
In this contribution we present an approach towards programmable shape memory transformations uti... more In this contribution we present an approach towards programmable shape memory transformations utilizing the intrinsic physical properties of a ternary polymer-based composite. The underlying idea is to use an open porous elastic skeleton as the base, where the pores are filled with a component that can alter its elasticity abruptly upon an external stimulus. In this study we use PDMS as the base network, and Paraffin as the pore filler, which causes a huge change in elasticity of the composite when undergoing the solid to liquid phase change at elevated temperature. This binary composite, already featuring shape memory characteristics, is then additionally made controllable by adding nano-particles with specific stimulus responsive properties. We demonstrate the programmable shape transformation capability of the devised ternary composite by the example of carbon black and ferromagnetic iron oxide nano-particles.
We review the basic features of interminiband absorption in superlattices, focussing on the joint... more We review the basic features of interminiband absorption in superlattices, focussing on the joint density of states, the oscillator strength and the associated sum rule. Then we will discuss infrared spectroscopic studies under application of an electric eld. With a eld in the plane of the layers, a hot-electron distribution can be generated. Using the temperature dependence of the interminiband absorption, energy loss and relaxation rates can be determined. A vertical electric eld leads to negative di erential resistance, domain formation and the break-up of the minibands into Wannier-Stark ladders. We will present experiments showing the formation of interpenetrating Wannier-Stark ladders in the continuum, which are strongly coupled by Zener resonances. This is the rst time to correlate transport and infrared absorption in a biased superlattice.
Single and multilayer metamaterials fabricated by nanoimprint lithography This article has been d... more Single and multilayer metamaterials fabricated by nanoimprint lithography This article has been downloaded from IOPscience. Please scroll down to see the full text article.
Nested, meander shaped strain gauges for temperature compensated strain measurement
2017 IEEE SENSORS, 2017
In recent years, the implementation of printed strain gauges as cost-effective alternatives to co... more In recent years, the implementation of printed strain gauges as cost-effective alternatives to conventional glued strain gauges was investigated. Printed strain gauges are, in general, not temperature compensated due to the lacking availability of appropriate materials. Their accuracy (and applicability) therefore is strongly affected by temperature drifts. In this contribution, we propose a concept for temperature compensated strain measurement with printed strain gauges based on two nested, meander shaped strain gauges consisting of different materials. Due to the proximity of the sensors, they are both affected by the same strain and the same local temperature. It is shown that, by measurement of the resistance of both strain gauges, the strain applied to the strain gauges can accurately be determined even if the sensors are subject to temperature fluctuations.
Electromagnetically Actuated Membrane-Based Micropumps with Integrated Magnetic Yoke
Journal of Microelectronics and Electronic Packaging, 2009
Micropumps are an essential part of laboratory-on-a-chip applications, which are used for cell ma... more Micropumps are an essential part of laboratory-on-a-chip applications, which are used for cell manipulations, DNA sequencing, and clinical diagnostics, just to name a few. Electromagnetic actuators are used for fluid transportation, because of relatively high forces and high displacements. Most published electromagnetic actuators for micro pumps are designed without a magnetic flux guiding yoke. In our paper, we present electromagnetic micro pumps with two alternative actuation principles, that is, Lorentz forces and reluctance forces, both with an integrated magnetic yoke. The high magnetic permeability and the higher magnetic flux enhance the mechanical force at the same electric energy input or primarily enable the functionality of the reluctance pump. The electromagnetic forces are used to drive a fluid flow via membrane deflection and passive valves without moving parts. The geometry of these passive valves is optimized to via finite element simulations (Comsol Multiphysics). D...
Convective flows in 3-dimensional microfluidic networks induced by localized microwave heating
2010 IEEE Sensors, 2010
ABSTRACT We present a natural convection microfluidic loop, based on localized microwave heating ... more ABSTRACT We present a natural convection microfluidic loop, based on localized microwave heating of fluids, as a concept for driving flows in 3-dimensional lab-on-a-chip architectures. As a proof of principle we fabricated test devices by using standard power RF-components, originally developed for mobile wireless communication applications, which are combined with Polydimethylsiloxane (PDMS) imprint technology for the realization of fluidic driving unit and channel, respectively. It is demonstrated that microwave fields coupled into the channel by the open end of a miniature coax cable lead to localized dielectric and, to a minor degree, also resistive heating of the fluid which induces a unidirectional, convective flow in the microfluidic network.
We demonstrate for the first time a fast and easy nanoimprint lithography (NIL) based stacking pr... more We demonstrate for the first time a fast and easy nanoimprint lithography (NIL) based stacking process of negative index structures like fishnet and Swiss-cross metamaterials. The process takes a few seconds, is cheap and produces three-dimensional (3D) negative index materials (NIMs) on a large area which is suitable for mass production. It can be performed on all common substrates even on flexible plastic foils. This work is therefore an important step toward novel and breakthrough applications of NIMs such as cloaking devices, perfect lenses and magnification of objects using NIM prisms. The optical properties of the fabricated samples were measured by means of transmission and reflection spectroscopy. From the measured data we retrieved the effective refractive index which is shown to be negative for a wavelength around 1.8 μm for the fishnet metamaterial while the Swiss-cross metamaterial samples show a distinct resonance at wavelength around 1.4 μm.
Integration of impedance spectroscopy sensors in a digital microfluidic platform
Microsystem Technologies, 2012
ABSTRACT Digital microfluidics combines the advantages of a low consumption of reagents with a hi... more ABSTRACT Digital microfluidics combines the advantages of a low consumption of reagents with a high flexibility of processing fluid samples. For applications in life sciences not only the processing but also the characterization of fluids is crucial. In this contribution, a microfluidic platform, combining the actuation principle of electrowetting on dielectrics for droplet manipulations and the sensor principle of impedance spectroscopy for the characterization of the fluid composition and condition, is presented. The fabrication process of the microfluidic platform comprises physical vapor deposition and structuring of the metal electrodes onto a substrate, the deposition of a dielectric isolator and a hydrophobic top coating. The key advantage of this microfluidic chip is the common electric nature of the sensor and the actuation principle. This allows for fabricating digital microfluidic devices with a minimal number of process steps. Multiple measurements on fluids of different composition (including rigid particles) and of different conditions (temperature, sedimentation) were performed and process parameters were monitored online. These sample applications demonstrate the versatile applications of this combined technology.
A Magnetic Membrane Actuator in Composite Technology Utilizing Diamagnetic Levitation
IEEE Sensors Journal, 2013
ABSTRACT We present the application of diamagnetic levitation of a small floater magnet for the r... more ABSTRACT We present the application of diamagnetic levitation of a small floater magnet for the realization of a magnetically actuated membrane actuator in polymer composite technology. Following this approach the neutral position of the actuating membrane can be adjusted by diamagnetically stabilized levitation and the actuation of the membrane is induced by an integrated coil which is driven by an ac-signal. This setup holds the advantage that the neutral position of the flexible membrane can be stabilized with no external energy input, and the separation of the off-position from the driving mechanism allows for many possible applications of the device in, for instance, microfluidic systems.
Stacked Fishnet and Swiss cross samples fabricated by NIL
Intersubband Absorption in Strongly Coupled Superlattices: Miniband Dispersion, Critical Points, and Oscillator Strengths
Quantum Well Intersubband Transition Physics and Devices, 1994
ABSTRACT
Sensor Decals: a Minimum Effort Approach to Sensors on Unconventional Substrates and Surfaces
2019 IEEE SENSORS
In this contribution we present a minimum effort approach to sensors for physical quantities like... more In this contribution we present a minimum effort approach to sensors for physical quantities like, e.g., force, pressure, strain, and temperature without the need for cost-intensive lab equipment. Only adhesive tape, carbon black, some kind of cutting tool, and a heat source are needed to fabricate sensors that can easily be affixed also to unusual and curved surfaces. We demonstrate the ease of fabrication and usage at the example of a rather generic sensor structure that can be used to measure strain, force/pressure, and temperature at virtually any chosen point of interest.
A levitating sphere viscometer operating in a rotational mode
2016 IEEE SENSORS
We present a special operation mode of a levitating sphere viscometer which allows steady rotatio... more We present a special operation mode of a levitating sphere viscometer which allows steady rotations of a levitated magnet around its vertical axis. In contrast to miniaturized oscillatory viscosity sensors, this mode of operation allows to obtain viscosity measurements more comparable to standard laboratory viscometers. Moreover, the measurement range is increased to higher viscosities, where no resonance peak is detectable in with oscillatory sensors. Using electromagnetical actuation and the readout, all fluids except ferrofluids are measurable. Due to the levitation and the contactless readout, only the measurement body has to be in contact with the fluid, all readout and actuation parts can be placed outside the measurement cell, which makes the setup particularly useful for, e.g., sterile, toxic or poisonous fluids.
Flexible and Transparent Pyroelectric Polymer Sensor for Light Spot Position Detection
Sheet type position sensors might trigger a wealth of applications in flexible electronics. Progr... more Sheet type position sensors might trigger a wealth of applications in flexible electronics. Progress in the field is rapidly embracing the demonstration of simple, transparent and flexible large area sensor concepts reacting sensitively to temperature changes caused by incident light. A promising concept for an efficient yet simple and thus low cost, flexible and transparent light spot position sensor is based on ferroelectric and conductive polymers. In this contribution we present a concept for a one dimensional light point localization model system using the position dependent signals generated by pyroelectric films when excited by a laser diode. The sensor consists of a PVDF stripe with PEDOT:PSS (DMSO) electrodes on both sides. For the theoretical description the detector can be treated mathematically analogous to a transmission line. The solutions of the equations and the experimental results are in agreement.
Embedded Temperature and Anti-Icing Monitoring Systems Directly Printed on 3D Shaped Substrates
IEEE Sensors Journal
The field of machine health and system monitoring has gained interest in different application fi... more The field of machine health and system monitoring has gained interest in different application fields, e.g., monitoring of lubricants, vibrations, surroundings, environmental conditions or the temperature of critical machine parts. This work presents two different condition monitoring devices, which are fabricated in a low cost printing process directly on the non-planar surface of the object to be monitored. The first system is used for thermal condition monitoring, where temperature monitoring is performed by 24 thermocouples resulting in a 24 pixel temperature image of the sample. The thermocouples are printed in an array arrangement, which enables the reduction of the required electrical connections from 48 to 25. The second device is intended for the use in aerodynamic systems, e.g., airplane wings, rotor blades of wind turbines, and helicopters where icing is a critical issue. The aerodynamic lift of aircraft is critically influenced by deposits on the wing or rotor blade surface. Also in other rotor blade systems, e.g., wind turbines the efficiency is drastically dropped and in worst case the icing can cause the failure of the system. The aim is the fabrication of an active monitoring system which is able to detect and also remove the ice from the monitored surface. To this end the system is directly fabricated on the 3D shaped substrate like a wing or rotor blade and the icing is detected using a capacitive sensor and a thermocouple sensor array. In case of icing a heater removes the critical ice deposit.
The energy relaxation of hot electrons in a strongly coupled, n-type GaAs/AlGaAs superlattice is ... more The energy relaxation of hot electrons in a strongly coupled, n-type GaAs/AlGaAs superlattice is measured by analysing the temperature and electric-field dependence of the interminiband absorption. Electrons are heated up by an electric-field pulse and the resulting change of the electron temperature is measured by monitoring the spectral change of the infrared absorption. The measured energy-loss rates are higher than predicted by a simple 3D model including acoustic and LO phonon emission. Other possible relaxation channels, such as folded acoustic phonons or coupled plasmon-phonon modes are discussed. An energy relaxation time can be extracted from the power balance and results to vary between 300 ps at 15 K and 20 ps at 48 K.
Ethanol Fermentation as the Basis for Autonomous, Long-term and High-pressure Fluid Transport in Microfluidics
Procedia Engineering
Abstract We present a concept for autonomous, long-term and high-pressure fluid transport on disp... more Abstract We present a concept for autonomous, long-term and high-pressure fluid transport on disposable microfluidic chips, which are fully bio-compatible and compostable. The actuation principle is based on ethanol fermentation, a well-known biological process in which yeast cells convert molecules like sucrose into cellular energy and thereby produce ethanol and carbon dioxide (CO 2 ) as metabolic waste products. A two-chamber fluidic system separated by a flexible membrane is suggested for active fluid transport utilizing a bio-reactor. One chamber, connected to the outside via a pressure-sensitive valve, contains the fluid to be actuated, the other one the culture medium for the yeast cells. Once the yeast cells are injected into the culture medium, ethanol fermentation and thus the production of CO 2 starts, which builds up pressure on the membrane and hence also on the fluid chamber. As soon as the switching point of the pressure sensitive valve is reached, fluid transport at a predefined and constant flow rate starts.
We present the application of a composite material, consisting of a ferromagnetic neodymium powde... more We present the application of a composite material, consisting of a ferromagnetic neodymium powder and a silicone elastomer, alone to realize a microfluidic network with pressure sensitive passive valves. The mixing ratio of the composite and the degree of magnetization can be utilized to adjust the switching point of each single valve. By combining differently adjusted valves within a microfluidic network it is possible to realize a sort of liquid logic and to control the flow in the network just by changing the inward flow respectively pressure. This concept holds the advantage that it is purely passive, the switching points can be adjusted over a wide pressure range and the valves are not sensitive to contamination and/or chemical degradation. In our contribution we report on details of the fabrication technology and on the performance of the valves in a test device.
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