Wolfgang Mönch

Electrowetting for Tunable Microoptics

The design, fabrication, and optical properties of tunable liquid microoptical components based on electrowetting are presented. Two optical microsystems are discussed: a single liquid microlens with tunable focal length, centered and stabilized in a micromachined positioning structure, and laterally repositionable tunable liquid microlens arrays. The latter employs electrowetting for lateral movement as well as focal length tuning of the optical elements. Through the use of novel microstructuring techniques, a high positioning accuracy may be achieved with low electrode count.

Tunable Bragg filters based on polymer swelling

We report on the optical properties of Bragg mirrors and filters fabricated from photo-cross-linked standard optical polymers. The transmittance spectra of these devices in the visible to near-infrared spectral range were measured. We demonstrate efficient tuning of the filter peak of the polymer Bragg filters over several hundred nanometers by adding organic solvents to the surrounding atmosphere of the filter. This represents what we believe to be a novel tuning principle for Bragg filters relying on the use of polymeric materials.

Flory-Huggins swelling of polymer Bragg mirrors

Bragg mirrors have been fabricated from cross-linked standard optical polymers (polystyrene and polymethylmethacrylate). In an ambient containing organic solvents, these polymer multilayers swell, causing the transmission peak wavelength to shift towards higher wavelengths. The transmission spectra of these Bragg mirrors as a function of the solvent partial pressure in the surrounding atmosphere have been measured for different organic solvents. From the spectral signatures, we show that the mirror swelling is well described by a Flory-Huggins-type sorption behavior. This result suggests that polymer swelling may be used as a robust tuning principle for optical Bragg structures.

A fully integrated optofluidic attenuator

A fast and reliable, fully integrated optofluidic optical attenuator is demonstrated. The concept employs only liquid and thus has no mechanically moving parts. Transparent and opaque aqueous liquid droplets are displaced using an on-chip electrowetting actuator and, due to the flexibility in the choice of liquids, various transmission spectra can be defined. The microfluidic attenuator system is fabricated using wafer-level bonding and dry film resists resulting in an ultra-compact (11×23×1.6 mm3) device requiring no external components for operation. The measured dynamic range of optical transmission is up to 47 dB, while the response times are below 100 ms for a 2 mm input beam. Using a novel double-actuator configuration, actuation speeds of the liquids of up to 39 mm s−1 were measured.

Tunable solid-body elastomer lenses with electromagnetic actuation

We present novel biconvex solid-body elastomer (polydimethylsiloxane) lenses, which can be tuned in focal length by using magnetic or mechanical actuation. The focal length change is induced by applying radial elastic strain and is investigated for different initial radii of curvature of the lenses and different actuation designs. In all cases, a linear correlation between induced strain and focal length tuning, in the range of about 10% (approximately 3 mm), is found. These results compare favorably with finite element simulations.

Biochip reader with dynamic holographic excitation and hyperspectral fluorescence detection

A highly parallel microarray scanner for functional genomic research using a dynamically reconfigurable holographic excitation and hyperspectral fluorescence detection is described. The light from two laser sources (405 and 532 nm) is split into an arbitrary number of focused spots on the biochip using a pair of spatial light modulators and a novel imaging system. The parallel optical scanner includes a hyperspectral detection unit with a high-sensitivity CCD camera to detect and analyze the emitted fluorescence spectra (from 430 to 800 nm) of all illuminated spots simultaneously. By using an xy scanner, the spectra of all spots on an entire array can be read out line by line.

Tunable photonic crystals on a freestanding polymer membrane

Two- and three-dimensional polymer photonic crystal structures are fabricated by multi-beam interference lithography. The cross-linked photonic crystal structures are obtained on a freestanding P(MMA-5% MaBP) membrane by combining with the standard silicon process. The resulting photonic crystal structures swell in response to the sorption of acetone solvent in the surrounding atmosphere. The optical effect of the swelling process is characterized by dynamic diffraction experiments. The results indicate that controlled polymer swelling may be used to tune the period (as well as the thickness, and thus efficiency) of the polymer-based photonic crystal structures on freestanding membranes.

Fabrication and Characterization of a Repositionable Liquid Micro Lens System

A novel method to improve both the lateral positioning and focal length tuning accuracy of a liquid lens in a micro lens system, based on the electrowetting on dielectrics (EWOD) principle, is described. This system consists of a device, encapsulated with a packaging technique, fabricated on a transparent substrate featured with an array of electrodes and a structured surface whose periodicity is smaller than the electrodes dimension. With variable applied voltage signals, the liquid lens is actuated precisely in the lateral position and also tuned dynamically irrespective of the lens position on the substrate. The accuracy of lateral positioning is mainly dependent on the structure of the dielectric surface. Measurements of the focal length tuning range and the positioning accuracy are presented. A first design of reconfigurable micro lens system, which has diverse application as adaptive wave-front sensing, optical tweezers and etc, is demonstrated.

Bistable optofluidic attenuator with integrated electrowetting actuation and high dynamic range

We present a novel miniaturized transmission-type optical attenuator based on optofluidic technology. A microfluidic chip has been developed which is fabricated on wafer scale, based on full-wafer-bonding with a dry film resist. The optical transmission of the chip can be changed simply by displacing a highly absorbent liquid with a transparent one. Due to the integration of an electrowetting actuator for driving the liquids, the attenuator is ultra-compact (11×23×1:6mm3) and does not need any external components. When using oil soluble dyes as the absorbent liquid, various transmission spectra can be defined. Depending on the input wavelength range, attenuations as high as 47 dB can be reached, while the switching times for a 2mm aperture can be as low as 100 ms.

Reconfigurable liquid micro-lens system for variable fiber coupling

A system capable of both two-dimensional lateral positioning and focusing of a liquid micro-lens for variable fiber coupling is presented. Lateral positioning and focus adjustment is accomplished by electrowetting-on-dielectrics. The chip is fabricated on a transparent Pyrex substrate and is equipped with a two dimensional array of electrodes featuring a well-defined microstructure which allows accurate positioning of the liquid micro-lens. The system is used as a variable optical fiber coupling element, and measurements of optical power coupled to a fiber as a function of the applied voltage are presented. Applications such as adaptive wave front sensing, optical tweezers, or three- dimensional focus control for microscopy are envisaged.