Jens Brunne

Complex three-dimensional high aspect ratio microfluidic network manufactured in combined PerMX dry-resist and SU-8 technology

In this paper we present a new fabrication method that combines for the first time popular SU-8 technology and PerMX dry-photoresist lamination for the manufacturing of high aspect ratio three-dimensional multi-level microfluidic networks. The potential of this approach, which further benefits from wafer-level manufacturing and accurate alignment of fluidic levels, is demonstrated by a highly integrated three-level microfluidic chip. The hereby achieved network complexity, including 24 fluidic vias and 16 crossing points of three individual microchannels on less than 13 mm(2) chip area, is unique for SU-8 based fluidic networks. We further report on excellent process compatibility between SU-8 and PerMX dry-photoresist which results in high interlayer adhesion strength. The tight pressure sealing of a fluidic channel (0.5 MPa for 1 h) is demonstrated for 150 μm narrow SU-8/PerMX bonding interfaces.

Tunable MEMS axicon mirror arrays

Highly aspheric reflective micro-optics for the generation of quasi-nondiffracting beams are of great interest for a wide variety of applications. However, up to now it was impossible to fabricate tunable arrays of these elements. In this Letter, we demonstrate the first array of purely reflective tunable microelectromechanical systems (MEMS) microaxicon mirrors with a conical shape and a continuous surface. The actuation is achieved by thermal expansion in a solid state design and the tuning range allows for large conical angles and is able to form concave as well as convex axicons. The deflection of the mirror surface and the propagation of the resulting quasi-Bessel beams have been characterized to prove the functionality of the device.

Sub-3-cycle vortex pulses of tunable topological charge

Novel types of reflective spiral micro-electro-mechanical systems were used to generate few-cycle vortex pulses of variable topological charge from a Ti:sapphire laser oscillator. The phase profile of these components was controlled by varying the temperature. The temporal properties of the pulses were characterized with spatially resolved nonlinear autocorrelation. The beam structure resembles a slightly distorted Laguerre-Gaussian distribution. The different topological charges were indicated by detecting Poynting-vector maps with a programmable Shack-Hartmann sensor of enhanced angular sensitivity.

Focusing mirror with tunable eccentricity

We present a new kind of varifocal mirror with independently adjustable curvatures in the major directions. For actuation we use two stacked piezo bending actuators with crossed in-plane polarization. This mirror can be used for example as an off-axis focusing device with tunable focal length and compensation for a variable angle of incidence or for coma correction. We demonstrate the prototype of such a mirror and characterize the mechanical deflection, as well as the focusing capabilities.

MEMS axicons for nondiffracting line shaping of ultrashort pulses

For a growing number of applications in nonlinear spectroscopy, micro- and nano-machining, optical data processing, metrology or medicine, an adaptive shaping of ultrashort pulsed, ultrabroadband laser beams into propagation-invariant linear focal zones (light blades) is required. One example is the femtosecond laser high-speed large area nanostructuring with moving substrates and cylindrical optics we reported about recently. Classical microoptical systems, however, distort the temporal pulse structure of few cycle pulses by diffraction and dispersion. The temporal pulse transfer can be improved with innovative types of reflective MEMS axicons based on two integrated rectangular mirrors, tilted by a piezoelectric bending actuator. In contrast to pixelated liquid-crystal-on-silicon (LCoS) based devices, cutoff frequencies in multi-kilohertz range, a purely reflective setup and continuous profiles with larger phase shift are realized which enable for shaping extended propagation-invariant zones at a faster and more robust operation. Additionally, a fixed phase offset can be part of the structure. Here, the performance of a prototype of linear mechanically tunable MEMS axicon is demonstrated by generating a pseudo-nondiffracting line focus of variable diameter and depth extension from a femtosecond laser pulse. The temporal transfer of 6-fs pulses of a Ti:sapphire laser oscillator is characterized with spectral phase interferometry for direct electric-field reconstruction (SPIDER) and spatially resolved nonlinear autocorrelation. Spatial and temporal self-reconstruction properties were studied. The application of the flexible focus to the excitation of plasmon-polaritons and the self-organized formation of coherently linked deep sub-wavelength laser-induced periodic surface structures (LIPSS) in semiconductors and dielectrics is reported.

In-plane DEAP stack actuators for optical MEMS applications

Recently, stacked dielectric polymer actuators have gained a lot of attention as MEMS actuators. In this paper we present a new kind of in-plane stack actuator. In contrast to its multilayer counterparts, it consists of only one active layer with inter-digitated microstructured soft electrodes which allow for a linear, radial or even asymmetric pulling motion in the working plane. The single layer design makes it in principle compatible with standard MEMS processes like deep reactive ion etching as well as silicone casting for optical components. Nevertheless, the wafer level fabrication process does not require any photolithography or clean room processes. The actuator consists of a microstructured layer of carbon black or nanotube filled PDMS which is suspended over a KOH etched trench on a (111) silicon wafer. The conductive PDMS electrodes are structured by laser ablation and subsequently embedded in a dielectric. The use of a (111) silicon wafer enables a mask less definition of the trench as the (111) layer is almost not attacked by the KOH etchant. The trench is defined by laser induced damage of the silicon wafer, so only exposed areas are etched. This allows for a true rapid prototyping of actuators with a fabrication time of less than one day.

A new dimension for piezo actuators: free-form out-of-plane displacement of single piezo layers

We present a controlled mode of ‘topological’ displacement of homogeneous piezo films that arises solely from an inhomogeneous in-plane strain due to an inhomogeneous polarization. For the rotationally symmetric case, we develop a theoretical model that analytically relates the shape of the displacement to the polarization for the cases of in-plane and out-of-plane polarization. This is verified experimentally for several examples, and we further demonstrate controlled asymmetric deformations.

A reflective tunable blazed-grating for high energy femtosecond laser pulses

We present a tunable reflective blazed-grating for the use with high energy femtosecond laser pulses. The technology is based on the surface deformation of a viscoelastic polymer induced by the volume shrinkage of the polymer during crosslinking. The blaze-angle of the grating is tunable by thermal actuation. Thermal actuation and light propagation have been characterized.

Adaptive piezoelectric axicon mirrors for high power femtosecond laser applications

In this work, we present the first mechanically tunable axicon mirror. The intended applications for such a device are nanomachining with high energy femtosecond lasers, microscopy and optical tweezers. The device consists of a piezodisc with structured electrodes, a reflective layer and a soft PDMS supporting ring that allows a rotational movement of the device around its outer perimeter. The performance of the device is optimized by an automated FEM-Simulation. A rapid prototyping fabrication process is presented as well as the mechanical and optical characterization of the system.

Adaptive generation of Bessel-like beams by reflective multi-electrode piezo-axicons

For novel spatio-temporal pulse characterization techniques, adaptive materials processing [1], advanced microscopy, tweezers and other emerging applications, adaptive axicons capable to shape highly robust nondiffracting beams of variable parameters are of increasing interest. Conical phase profiles can be approximated by pixellated liquid crystal based spatial light modulators [2] which reach a high spatial resolution even at ultrashort pulses but suffer from limitations with respect to obtainable phase steps, switching speed, diffraction losses and damage resistance. One alternative approach is to combine the specific advantages of highly reflective analogue micro-electromechanical systems (MEMS) with the optical functionality of continuous conical axicon profiles [3]. This promises to work at minimum dispersion and minimum diffraction and, simultaneously, with extended undistorted propagation zones. Here we report on first experimental results obtained with a new type of tunable mechanical axicon mirror. Its design is based on a piezo-disk with 5 concentric circular electrodes (Ag) and a broadband reflecting aluminium layer on top (Figure 2). The local bending radii of the disk correspond to the voltages at the individual electrodes (≤ 60 V). The envelope phase function was approximated by automated FEM simulations (about 50 iterations). Beam propagation of shaped ultrashort pulses was simulated with VirtualLab (Light Trans). Supporting structures and the substrate for the mirror were fabricated from Si wafers coated with polydimethylsiloxane (PDMS) and aluminium, respectively. The surface quality was characterized interferometrically (Figure 1). A maximum initial bending of < 3 µm over 15 mm diameter was found. Voltage dependent shape variations were detected (a) for the stationary case by phase shift interferometry and with a laser profilometer, (b) by analyzing the far field intensity patterns of a cw source at 532 nm (Figures 2 c–e), and dynamically with a stroboscopic setup. It was shown that the device can be operated up to a frequency of 3 kHz. Variable Bessel-like zones (here indicated by narrow rings in the far field) were created by addressing variable combinations of electrodes (Figures 2 c–e). Femtosecond pulse transfer was studied with a Ti:sapphire oscillator (Venteon, pulse duration < 6 fs, spectral bandwidth > 200 nm, repetition rate 80 MHz) and an LX-SPIDER (APE) for detecting spectral phase changes. Scattering and slightly asymmetric angular profiles indicate small deviations from a perfect geometry which still have to be overcome. To conclude, adaptive MEMS axicon mirrors based on piezo-disks with multiple individually addressable electrodes were studied in proof-of-principle experiments. The capability to flexibly shape Bessel-like beams at ultrashort pulse durations was demonstrated. Improvements of quality and design, a further development of characterization techniques and applications in ultrashort-pulse physics are a subject of continuing experiments.