arXiv subjects

Ulrike Wallrabe

Publications and source records attributed to Ulrike Wallrabe.

18 recordsLinked to original sources

arXiv2025

Enhanced spectral range of strain-induced tuning of quantum dots in circular Bragg grating cavities

Tunable sources of entangled and single photons are essential for implementing entanglement-based quantum information protocols, as quantum teleportation and entanglement swapping depend on photon indistinguishability. Tunable devices are fabricated from indium arsenide (InAs) quantum dots (QDs) embedded in gallium arsenide (GaAs) nanomembranes placed on monolithic piezoelectric substrates. Circular Bragg grating (CBG) resonators enhance emission brightness and exploit the Purcell effect; however, the inclusion of CBGs reduces strain-mediated tunability compared to planar nanomembranes. A simple and effective solution is introduced: filling the CBG trenches with a stiff dielectric (aluminum oxide) via atomic layer deposition (ALD) restores up to 95% of the tunability of planar structures. Finite element analysis (FEA) confirms that the tunability loss originates from bending in the device layers due to strain relief in the CBG geometry. Lowering the stiffness of intermediate layers between the QDs and the piezoelectric actuator, such as in bonding or reflector layers, further increases strain losses in uncoated CBGs. Coated devices maintain 98-99% strain-tuning efficiency across all simulated underlayer stiffnesses. The results demonstrate that advantageous optical cavity properties can be effectively combined with piezoelectric strain tuning, enabling scalable, bright, and tunable quantum light sources.

cond-mat.mtrl-sci↗

arXiv2020

Micro Fresnel Mirror Array with Individual Mirror Control

We present the design and fabrication of a miniaturized array of piezoelectrically actuated high speed Fresnel mirrors with individual mirror control. These Fresnel mirrors can be used to generate propagation invariant and self-healing interference patterns. The mirrors are actuated using piezobimorph actuators, and the consequent change of the tilting angle of the mirrors changes the fringe spacing of the interference pattern generated. The array consists of four Fresnel mirrors each having an area of 2x2 mm2 arranged in a 2x2 configuration. The device, optimized using FEM simulations, is able to achieve maximum mirror deflections of 15 mrad, and has a resonance frequency of 28 kHz.

physics.app-ph↗

arXiv2019

Integrated Sensor System to Control the Temperature Effects and the Hysteresis on Adaptive Fluid-Membrane Piezoelectric Lenses

We present in this paper an integrated sensor system for closed-loop control of the temperature effects and the hysteresis on the refractive power of an adaptive fluid-membrane piezoelectric lens. The piezoelectric hysteresis and the fluid thermal expansion contribute to a nonlinear response of the lens refractive power defined as a function of the actuation electric field. Hence, a pressure sensor and a temperature sensor are integrated inside the lens to monitor and define the lens refractive power as a function of both the internal fluid pressure and the temperature, thus, allowing for the closed-loop control of the refractive power. The adaptive lens has a refractive power range varying from -16 m-1 to +17 m-1 at 25°C and from -15 m-1 to +28 m 1 at 75°C.

physics.app-ph↗

arXiv2019

Multiphysics simulation of the aspherical deformation of piezo-glass membrane lenses including hysteresis, fabrication and non-linear effects

In this paper we present and verify the non-linear simulation of an aspherical adaptive lens based on a piezo-glass sandwich membrane with combined bending and buckling actuation. To predict the full non-linear piezoelectric behavior, we measured the non-linear charge coefficient, hysteresis and creep effects of the piezo material and inserted them into the FEM model using a virtual electric field. We further included and discussed the fabrication parameters -- glue layers and thermal stress -- and their variations. To verify our simulations, we fabricated and measured a set of lenses with different geometries, where we found good agreement and show that their qualitative behavior is also well described by a simple analytical model. We finally discuss the effects of the geometry on the electric response and find, e.g., an increased focal power range from $\pm 4.5$ to $\pm 9\, \mathrm{m}^{-1}$ when changing the aperture from $14$ to $10\, \mathrm{mm}$.

physics.app-ph↗

arXiv2019

Properties of piezoceramic materials in high electric field actuator applications

In this paper, we compare the performance of 8 PZT ceramics and one PMN-PT material for typical bending actuator applications. This includes the measurement of non-linear transverse charge coefficient at high electric field strength and related quantities such as the Young's modulus, relative permittivity, coercive field and their temperature dependencies, and the Curie temperature. Most materials show much higher strains than what is expected from the datasheet values. We further study the operating region for fields against the polarization direction in different operating cycles and demonstrate a long-term stable quick re-poling method which increases the operating range of negative-only cycles from 50% of Ec to 75% of the coercive field.

physics.app-ph↗

arXiv2018

Diffraction-limited axial scanning in thick biological tissue employing an aberration correcting adaptive lens

Diffraction-limited focusing deep into biological tissue is challenging due to spherical aberrations that lead to a broadening of the focal spot particularly in axial direction. While the diffraction-limit can be restored employing aberration correction with a deformable mirror or spatial light modulator, a bulky optical setup results due to the required beam-folding. We propose a bi actor adaptive lens, that enables axial scanning and at the same time correction of specimen induced spherical aberrations while offering a compact setup. Using the bi-actor lens in a confocal microscope, we show diffraction limited axial scanning up to \SI{340}{\micro m} deep inside a phantom specimen. Applying this technique for in-vivo measurements of zebrafish embryos with reporter gene-driven fluorescence in the thyroid gland reveals substructures of thyroid follicles.

physics.optics↗

arXiv2017

Inverse design method for metalens with optical vortices towards light focusing through localized phase retardation

Metalenses can achieve diffraction-limited focusing through localized phase manipulation of the incoming light beam. Because these structures are ultrathin, less than a wavelength, this has the potential of achieving ultrathin optical elements, with a thickness limited mainly by the mechanical strength of the transparent substrate. Recently proposed metalenses are based on either dielectric nanofin arrays, or nanoparticles of large number, which leads to severe manufacturing challenges. To overcome these challenges, this paper predicts a new type of metalens with concentric-nanoring topology, where the number and size of the nanorings are determined using an inverse design method. By focusing the electrical field energy at a specified position, the convex-like metalens is inversely predicted with desired numerical aperture and a diffraction-limited focal spot. The Poynting vector distribution found demonstrates the mechanism of the lensing function, in which optical vortices are generated in the nanorings to achieve a matching of the phase and impedance between the substrate and free space, and further, to form a spherical wavefront and enhance the transmission of the optical energy. The inverse design method can also be extended to predict an axicon-like metalens with focal beam. The improved manufacturability is concluded from the geometry of the concentric-nanoring configurations.

physics.optics↗

arXiv2017

Stable dynamics of micro-machined inductive contactless suspensions

In this article, we present a qualitative approach to study the dynamics and stability of micro-machined inductive contactless suspensions (MIS). In the framework of this approach, the induced eddy current into a levitated micro-object is considered as a collection of m-eddy current circuits. Assuming small displacements and the quasi- static behaviour of the levitated micro-object, a generalized model of MIS is obtained and represented as a set of six linear differential equations corresponding to six degrees of freedom in a rigid body by using Lagrange-Maxwell formalism. The linear model allows us to investigate the general stability properties of the MIS as a dynamic system, and these properties are synthesized in three major theorems. In particular, we prove that the stable levitation in the MIS without damping is impossible. Based on the approach presented herewith, we give general guidelines for MIS designing. In addition to we demonstrate the successful application of this technique to study the dynamics and stability of the symmetric and axially symmetric MIS designs both based on 3D micro-coil technology.

physics.ins-det↗

arXiv2016

Depth-controlled Bessel beams

We present a ring aperture with independently switchable segments for the three-dimensional control of quasi propagation invariant beams. We demonstrate that our liquid crystal design concept preserves coherence and generates the Bessel beam structure.

physics.optics↗

arXiv2016

Magnetic Lenz lenses increase the limit-of-detection in nuclear magnetic resonance

A high NMR detection sensitivity is indispensable when dealing with mass and volume-limited samples, or whenever a high spatial resolution is required. The use of miniaturised RF coils is a proven way to increase sensitivity, but may be impractical and is not applicable to every experimental situation. We present the use of magnetic lenses, denoted as Lenz lenses due to their working principle, to focus the magnetic flux of a macroscopic RF coil into a smaller volume and thereby locally enhance the sensitivity of the NMR experiment - at the expense of the total sensitive volume. Besides focusing, such lenses facilitate re-guiding or re-shaping of magnetic fields much like optical lenses do with light beams. For the first time we experimentally demonstrate the use of Lenz lenses in magnetic resonance and provide a compact mathematical description of the working principle. Through simulations we show that optimal arrangements can be found.

physics.med-ph↗

arXiv2016

Topological in-plane polarized piezo actuation for compact adaptive lenses with aspherical correction

In this contribution, we investigate the effects of using in-plane polarized piezo actuators with topological buckling displacement to drive glass-piezo composite membranes for adaptive lenses with aspherical control. We find that the effects on the focal power and aspherical tuning range are relatively small, whereas the tuning speed is improved significantly with a first resonance of 1 kHz for a 13 mm aperture lens.

physics.optics↗

arXiv2016

Single-Sided Contacting of Out-of-Plane Polarized Piezo Films for Fluid Membrane Lenses

We present an adaptive lens which tunes its focal length with a glass membrane that is deformed by an integrated piezo bending actuator. A particular challenge in the fabrication of this type of actuator is the need to contact the electrodes on both sides of the piezo element, i.e. also on the glass surface. Therefore, we show a novel contacting method where the top electrode on the piezo is segmented and the backside electrode is left at a floating potential. With this setup we achieve tuning capabilities of more than 13 dpt and response times in the range of less than 4 ms at a usable aperture of 4.5 mm.

physics.optics↗

arXiv2014

A compact, large-aperture tunable lens with adaptive spherical correction

In this paper, we present the proof of concept of 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.

physics.optics↗

arXiv2014

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.

cond-mat.mtrl-sci↗

arXiv2014

Piezo films with adjustable anisotropic strain for bending actuators with tunable bending profiles

We present a method to produce in-plane polarized piezo films with a freely adjustable ratio of the strains in orthogonal in-plane directions. They can be used in piezo bending actuators with a tunable curvature profile. The strains are obtained as mean strains from a periodic polarization pattern produced by a suitable doubly interdigitated electrode structure. This mechanism is demonstrated for several examples using PZT sheets. We further discuss how this tuning and the parameters of the electrode layout affect the overall magnitude of the displacement.

cond-mat.mtrl-sci↗

arXiv2013

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 for several examples, and we further demonstrate controlled asymmetric deformations.

cond-mat.mtrl-sci↗

arXiv2013

Freeform high-speed large-amplitude deformable Piezo Mirrors

We present a new type of tunable mirror with sharply-featured freeform displacement profiles, large displacements of several 100μm and high operating frequencies close to the kHz range at 15mm diameter. The actuation principle is based on a recently explored "topological" displacement mode of piezo sheets. The prototypes presented here include a rotationally symmetric axicon, a hyperbolic sech-icon and a non-symmetric pyram-icon and are scalable to smaller dimensions. The fabrication process is economic and cleanroom-free, and the optical quality is sufficient to demonstrate the diffraction patterns of the optical elements.

physics.optics↗

arXiv2013

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.

physics.optics↗