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Passive graduated filters with fixed absorption profile are currently used in image recording to avoid overexposure. However, a whole set of filters with prescribed gradients is required to cope with changing illumination conditions. Furthermore, they demand mechanical adjustment during operation. To overcome these deficiencies we present a microfabricated active electrochromic graduated filter which combines multiple functionalities: The overall absorbance, the position of medium transmission as well as the magnitude of its gradient can be tuned continuously by electrical means. Live image control is possible using low operation voltages in the range of ±2 V to reach a high change in optical density ΔOD of 1.01 (400 nm to 780 nm) with a coloration and bleaching time 1.3 s and 0.2 s, respectively. Owing to their low volume and power consumption they are suitable for widespread applications like in smartphones, surveillance cameras or microscopes.
Spatial optical Fourier filtering is a widespread technique for in situ image or light
field processing. However, conventional fixed absorbing patterns or mechanical irises only allow
an inflexible, very restricted control. Thus, we present two electrochromic spatial filters with
ring-shaped or directional segments, which can be individually addressed and continuously tuned
in transmission resulting in up to 512 different filtering states. For realization of the electrochromic
devices, we overcome technical obstacles to realize seamless, gap-free electrochromic segments.
We describe this novel fabrication process and demonstrate the successful application in an
optical Fourier transform set-up.
Exploiting Direct Laser Writing for Hydrogel Integration into Fragile Microelectromechanical Systems
(2019)
The integration of chemo-responsive hydrogels into fragile microelectromechanical systems (MEMS) with reflective surfaces in the micron to submicron range is presented. Direct laser writing (DLW) for 3D microstructuring of chemoresponsive “smart” hydrogels on sensitive microstructures is demonstrated and discussed in detail, by production of thin hydrogel layers and discs with a controllable lateral size of 2 to 5 µm and a thickness of some hundred nm. Screening results of polymerizing laser settings for precision microstructuring were determined by controlling crosslinking and limiting active chain diffusion during polymerization with macromers. Macromers are linear polymers with a tunable amount of multifunctional crosslinker moieties, giving access to a broad range of different responsive hydrogels. To demonstrate integration into fragile MEMS, the gel was deposited by DLW onto a resonator with a 200 nm thick sensing plate with high precision. To demonstrate the applicability for sensors, proof of concept measurements were performed. The polymer composition was optimized to produce thin reproducible layers and the feasibility of 3D structures with the same approach is demonstrated.
Graduated optical filters are commonly used for spatial image control as they are capable of darkening the overexposed parts of the image specifically. However, they lack flexibility because each filter has a fixed transmission distribution. We herein present a fully controllable graduated filter based on the electrochromic device. Its graduated transmission distribution can be spatially controlled by the application of multiple electric potentials. In this way, the control of the gradient’s position and its width, transmission and angular orientation is possible. Simulation of both the spatial potential distribution and the resultant optical absorption distribution are conducted to optimize the electrode configuration and furthermore to derive a control dataset that facilitates the adjustment and thus the application of the graduated filter. Based on three objective and quantitative criteria, we identify the electrode configuration with the highest flexibility in all four controls, manufacture the device using a gravure printing process for the nanoparticle electrodes and show its successful application.
We report the design, fabrication and experimental investigation of a spectrally wide-band terahertz spatial light modulator (THz-SLM) based on an array of 768 actuatable mirrors with each having a length of 220 μm and a width of 100 μm. A mirror length of several hundred micrometers is required to reduce diffraction from individual mirrors at terahertz frequencies and to increase the pixel-to-pixel modulation contrast of the THz-SLM. By means of spatially selective actuation, we used the mirror array as reconfigurable grating to spatially modulate terahertz waves in a frequency range from 0.97 THz to 2.28 THz. Over the entire frequency band, the modulation contrast was higher than 50% with a peak modulation contrast of 87% at 1.38 THz. For spatial light modulation, almost arbitrary spatial pixel sizes can be realized by grouping of mirrors that are collectively switched as a pixel. For fabrication of the actuatable mirrors, we exploited the intrinsic residual stress in chrome-copper-chrome multi-layers that forces the mirrors into an upstanding position at an inclination angle of 35°. By applying a bias voltage of 37 V, the mirrors were pulled down to the substrate. By hysteretic switching, we were able to spatially modulate terahertz radiation at arbitrary pixel modulation patterns.
In this work, we investigate and compare the condensation behavior of hydrophilic, hydrophobic, and biphilic microgrooved silicon samples etched by reactive ion etching. The microgrooves were 25 mm long and 17−19 μm deep with different
topologies depending on the etching process. Anisotropically etched samples had 30 μm wide rectangular microgrooves and silicon ridges between them. They were either left hydrophilic or covered with a hydrophobic fluorocarbon or photoresist layer.
Anisotropically etched samples consisted of 48 μm wide semicircular shaped microgrooves, 12 μm wide silicon ridges between them, and a 30 μm wide photoresist stripe centered on the ridges. The lateral dimensions were chosen to be much smaller than the capillary length of water to support drainage of droplets by coalescence rather than droplet sliding. Furthermore, to achieve a low thermal resistance of the periodic surface structure consisting of water-filled grooves and silicon ridges, the trench depth was also kept small. The dripped-off total amount of condensate (AoC) was measured for each sample for 12 h under the same boundary
conditions (chamber temperature 30 °C, cooling temperature 6 °C, and relative humidity 60%). The maximum increase in AoC of 15.9% (9.6%) against the hydrophilic (hydrophobic) reference sample was obtained for the biphilic samples. In order to elucidate their unique condensation behavior, in situ optical imaging was performed at normal incidence. It shows that the drainage of droplets from the stripe’s surface into the microgrooves as well as occasional droplet sliding events are the dominant processes to clear the surface. To rationalize this behavior, the Hough Circle Transform algorithm was implemented for image processing to receive
additional information about the transient droplet size and number distribution. Postprocessing of these data allows calculation