Matthias C. Wapler

Magnetic properties of materials for MR engineering, micro-MR and beyond.

We present the results of a systematic measurement of the magnetic susceptibility of small material samples in a 9.4 T MRI scanner. We measured many of the most widely used materials in MR engineering and MR micro technology, including various polymers, optical and substrate glasses, resins, glues, photoresists, PCB substrates and some fluids. Based on our data, we identify particularly suitable materials with susceptibilities close to water. For polyurethane resins and elastomers, we also show the MR spectra, as they may be a good substitute for silicone elastomers and good casting resins.

A Compact, Large-Aperture Tunable Lens with Adaptive Spherical Correction

In this paper, we present the proof of concept a very fast adaptive glass membrane lens with a large aperture/diameter ratio, spherical aberration correction and integrated actuation. The membrane is directly deformed using two piezo actuators that can tune the focal length and the conical parameter. This operating principle allows for a usable aperture of the whole membrane diameter. Together with the efficient actuation mechanism, the aperture is around 2/3 of the total system diameter -- at a thickness of less than 2mm. The response time is a few milliseconds at 12mm aperture, which is fast compared to similar systems.

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.

Quasi-Bessel beams from asymmetric and astigmatic illumination sources

We study the spatial intensity distribution and the self-reconstruction of quasi-Bessel beams produced from refractive axicon lenses with edge emitting laser diodes as asymmetric and astigmatic illumination sources. Comparing these to a symmetric mono-mode fiber source, we find that the asymmetry results in a transition of a quasi-Bessel beam into a bow-tie shaped pattern and eventually to a line shaped profile at a larger distance along the optical axis. Furthermore, we analytically estimate and discuss the effects of astigmatism, substrate modes and non-perfect axicons. We find a good agreement between experiment, simulation and analytic considerations. Results include the derivation of a maximal axicon angle related to astigmatism of the illuminating beam, impact of laser diode beam profile imperfections like substrate modes and a longitudinal oscillation of the core intensity and radius caused by a rounded axicon tip.

Ultra-compact, large-aperture solid state adaptive lens with aspherical correction

We report on a solid state adaptive lens based on an elastomer lens body and an active glass-piezo composite membrane. In contrast to existing lenses with integrated actuation, this allows for the control of the aspherical behavior and has a large aperture of 6.25 mm at 7.9 mm outer diameter (9 mm including packaging). Using a polymer instead of a fluid as a refractive medium gives a good robustness and allows for a simple layout and fabrication. Depending on the choice of polymer, we find a short response time or a large focal range of 21 dpt.

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.

Segmented Bessel beams

We investigate segmented Bessel beams that are created by placing different ring apertures behind an axicon that is illuminated with a plane wave. We find an analytical estimate to determine the shortest possible beam segment by deriving a scale-invariant analytical model using appropriate dimensionless parameters such as the wavelength and the axicon angle. This is verified using simulations and measurements, which are in good agreement. The size of the ring apertures was varied from small aperture sizes in the Frauhofer diffraction limit to larger aperture sizes in the classical limit.

Optimal electric load prediction from the KLM model for ultrasound energy receivers

In this paper, we compare the zero reflection and the power maximization conditions to calculate the optimal electric load for maximum US energy absorption by receivers with the KLM model. In experiments with a 1-3 composite transducer, the predictions agree with the measurements at the resonance frequency. Results at the anti-resonance deviate slightly from predictions due to a parasitic load and the limited agreement of the KLM model with the measured electric impedance in air.

Adaptive Fresnel mirror for ultrashort-pulse laser beam shaping

In this contribution we present a fast and robust adaptive Fresnel mirror for advanced nanomachining applications with femtosecond lasers. The Fresnel mirror consists of two mirror segments that are actuated with a piezoelectric bimorph. All components are fabricated by low-cost rapid prototyping processes without the need for a clean-room. Light propagation and pulse transfer have been characterized as well as the static and dynamic actuation properties.

Axial scanning and spherical aberration correction in confocal microscopy employing an adaptive lens

We present a fluid-membrane lens with two piezoelectric actuators that offer versatile, circular symmetric lens surface shaping. A wavefront-measurement-based control system ensures robustness against creeping and hysteresis effects of the piezoelectric actuators. We apply the adaptive lens to correct synthetic aberrations induced by a deformable mirror. The results suggest that the lens is able to correct spherical aberrations with standard Zernike coefficients between 0 μm and 1 μm, while operating at refractive powers up to about 4m-1. We apply the adaptive lens in a custom-built confocal microscope to allow simultaneous axial scanning and spherical aberration tuning. The confocal microscope is extended by an additional phase measurement system to include the control algorithm. To verify our approach, we use the maximum intensity and the axial FWHM of the overall confocal point spread function as figures of merit. We further discuss the ability of the adaptive lens to correct specimen-induced aberrations in a confocal microscope.