Daniel Kopp

Optofluidic laser scanner based on a rotating liquid prism

We demonstrate an electrowetting-actuated optofluidic system based on a rotatable liquid prism implemented as a two-dimensional laser scanner. The system is fabricated through a novel technology using a patterned flexible polymeric foil on which a high density of electrodes is structured and which is subsequently inserted into a cylindrical housing. The resulting radial electrode array is used for electrowetting actuation of two fluids filled into the cylinder, which allows a controllable tilt and orientation of the planar liquid interface and thus represents a tunable rotating prism. Finite element simulations and subsequent experimental verification show that this highly planar and precisely positionable liquid/liquid interface may be actuated to a deflection angle of ±6.4°, with a standard deviation of ±0.18°, and rotated 360° about the vertical axis. Power consumption is limited to several microwatts, and switching times of several hundred milliseconds were determined.

Tubular astigmatism-tunable fluidic lens

We demonstrate a new means to fabricate three-dimensional liquid lenses which may be tuned in focal length and astigmatism. Using actuation by electrowetting-on-dielectrics, astigmatism in arbitrary directions may be tuned independently, with almost no cross talk between orthogonal orientations. The lens is based on electrodes structured on planar polyimide foils and subsequently rolled, enabling high-resolution patterning of complex electrodes along the azimuthal and radial directions of the lens. Based on a design established through fluidic and optical simulations, the astigmatism tuning is experimentally verified by a change of the corresponding Zernike coefficients measured using a Shack-Hartmann wavefront sensor. It was seen that the back focal length can be tuned by 5 mm and 0° and 45° astigmatism by 3 μm through application of voltages in the range of 50  Vrms. It was observed that the cross talk with other aberrations is very low, suggesting a novel means for astigmatism control in imaging systems.

Tubular Focus-Tunable Fluidic Lens Based on Structured Polyimide Foils

We demonstrate an entirely new means for fabrication of focus-tunable fluidic lenses using a tubular geometry with great design flexibility. The technology is based on patterned, flexible polymeric foils inserted into a cylindrical tube, which is filled with two liquids of different refractive indices. Electrowetting is used for actuation of the liquid/liquid interface, thus realizing focus tunability. To show the utility of this approach, a single liquid lens with a 4.5-mm aperture and of a back focal length tuning by a factor of 7, requiring actuation voltages below 100 Vrms, is demonstrated. The tubular geometry optical resulted in a high optical surface quality over the entire actuation range, with rms values for wavefront error below λ/2, is dominated by spherical aberration.

Determination of the thermoelectric figure of merit of doped polysilicon thin films by micromachined test structures

This paper reports on the determination of the thermoelectric figure of merit ZT of doped polysilicon (poly-Si) thin films. For the extraction of accurate thermal properties, micromachined membrane test structures for the Seebeck coefficient S and the thermal conductivity κ of poly-Si were developed. The electrical resistivity ρ is measured using van-der-Pauw structures. From the results the temperature dependent value of ZT = S²Tk^-l ρ^-1 is extracted. The material parameters were measured in the temperature range from 90 to 370 K. A ZT of (10.08±0.14)×10^-3 and (10.29±0.12)×10^-3 was obtained at 290 K for n- and p-doped poly-Si thin films. The corresponding material parameters for n-doped material are k = 25.0 Wm^-1K^-1, S = -81 μVK^-1 and ρ = 7.58 μΩm. For p-doped poly-Si the values are found to be κ = 40.6 Wm^-1K^-1, S = 240 μVK^-1 and ρ = 40.1 μΩm.

Piezo-hall effect in CMOS-based vertical hall devices

This paper reports on the piezo-Hall effect in CMOS-based five-contact vertical Hall sensors (VHS) able to measure in-plane magnetic field components. The geometry of such devices and the characteristic current flow in the deep n-wells strongly differ from those of structures used so far to characterize the piezo-Hall response. In contrast to standard planar Hall plates, homogeneous mechanical in-plane stress was found to cause a weak change in the magnetic sensitivity of the VHS. The paper presents the custom-made measurement setup and its detailed characterization as well as experimental results acquired using single VHS and coupled sensor systems comprising four VHS connected in parallel. The experimental results are supported by finite element simulations. It is concluded, that the low sensitivity change is due to the vertical current density changes induced by the applied mechanical stress.

Piezoresistive Response of Vertical Hall Devices

This paper reports on the piezoresistive response of CMOS-based five-contact vertical Hall sensors (VHS) under mechanical stress. Single sensor elements and coupled sensor systems comprising four individual VHS were exposed to normal in-plane stress σxx along the sensor axis using a four-point bending bridge setup. The resulting stress sensitivity of the offset signals at an input voltage of Vin = 3 V was below 2 μV/MPa. In addition, an inhomogeneous stress distribution was generated by applying vertical forces close to the sensor systems using an SU-8 pillar. A resulting maximum offset increase by 109 μV/N at Vin = 3 V was measured. In conclusion, the influence of mechanical stress on the offset behavior of VHS is small compared to other offset sources such as the junction field effect and variations in sensor geometry.

Piezoresistive response of five-contact vertical Hall devices

This paper reports on the piezoresistive effect and resulting offset behavior of CMOS-based five-contact vertical Hall sensors (VHS) under mechanical stress. Single sensor elements and coupled sensor systems comprising four individual VHS were exposed to normal in-plane stress σxx along the sensor axis using a four-point bending bridge setup. The resulting stress sensitivity of the offset signal at a bias voltage of bias = 3 V was below 2 µV/MPa. In addition, an inhomogeneous stress distribution was generated applying vertical forces close to a four-sensor system using a polymer cylinder. A resulting maximum offset increase by 20 µV/N at Vbias = 3 V was achieved. In conclusion, the influence of mechanical stress on the offset behavior of VHS is small compared to other offset sources such as the junction field effect and variations in sensor geometry.

Tubular optofluidics as a versatile optical toolbox

We present a new technology which enables the versatile realization of a wide spectrum of optofluidic elements, including focus- and aberration-tunable lenses, prisms with adjustable tip and tilt, and variable apertures, as well as the combination of these into multi-element optical systems. These systems, based completely on liquid elements and with no moving parts, are axially configured in a tubular housing and actuated using electrowetting through complex axially- and azithumally configured electrode patterns. We discuss the basis of the technology, which uses microfabricated flexible polymeric foils with the electrodes for actuation as well as dielectric layers and surfaces with varying wetting properties, and the assembly techniques. Optical performance of these elements shows that the tubular optofluidics approach is a significant step to realizing optofluidic systems with good imaging performance.

Modular optofluidic systems: A toolbox for fast and simple assembly of a photonic lab on a chip

A powerful modular optofuidic systems (MOPS) toolbox, which allows in-situ implementation of reconfigurable Photonic lab on a chip (PhLoC) has been developed. Each module has specific optic, fluidic or optofluidic elements that enable defining arbitrarily complex PhLoC systems. After having measured the optical total losses of the MOPS concept, it has been used for defining either absorbance or fluorescence measurements. In the first case, LoD below the μM level of Crystal Violet was obtained, whereas in fluorescence, the use of a specifically designed MOPS building block allows blocking excitation wavelength, resulting in a 2-fold increase of the sensitivity.

Modular Optofluidic Systems (MOPS)

Elementary PDMS-based building blocks of fluidic, optical and optofluidic components for Lab on a chip (LOC) platforms has here been developed. All individual modules are compatible and can be anchored and released with the help of puzzle-type connectors This approach is a powerful toolbox to create modular optofluidic systems (MOPS), which can be modified/upgraded to user needs and in-situ reconfigurable. In addition, the PDMS can locally be functionalized, defining a modular biosensor. Measurements in absorbance and fluorescence have been pursued as demonstrator.