Journal Articles, Book Chapters

We present a systematic investigation of an SiO2 etching process using a standard fluorocarbon chemistry ICP-RIE etch tool with a cryogenically cooled electrode. Our goal is to enable the control of the SiO2 feature morphology, i.e., the sidewall angle, in order to add a degree of freedom for the design of resonant micro-/nanooptical elements. For such elements as e.g., whispering gallery mode resonators with specific mode profiles, it is essential to maintain low surface roughness. To this end, we investigate a variety of gas compositions. For statistical evaluation, we use a surface response methodology for several parameters and investigate the influence of the substrate temperature on the sidewall angle. Different hypotheses from the literature for the cause of non-anisotropic etch behavior are discussed for our specific case. Various investigations based on the prior hypothesis are presented, which provide more information about the pseudo-isotropic etch profile. Finally, we present two use cases: firstly, a classical anisotropic etch with a high aspect ratio and very low roughness (<1 nm), and secondly, an etch process for the fabrication of whispering gallery mode resonators that confine the light at the bottom of the resonator.

Focus-tunable lenses, e.g., liquid filled membrane lenses (MLs), have found increasingly widespread application in optical systems. If a large refractive power range is to be used, the correction of chromatic aberrations is particularly challenging: a group containing a single ML cannot be corrected over the whole refractive power range. In analogy to hybrid achromats for lenses with constant focal lengths, we present the combination of an ML and a diffractive Alvarez-Lohmann-lens (ALL) for the compensation of axial color over a large refractive power range. In contrast to the combination of multiple MLs, this does not increase the axial length of the system significantly. At the same time, the flexible adaption of the phase function of the diffractive ALL can reduce spherical aberration over the whole focal range. Design examples with ray-tracing and wave-optical simulations demonstrate the performance of the resulting hybrid tunable element. Experimental data from fabricated sample lenses provide a proof of principle.

It is well known that tunable lenses, with refractive power that can be varied, e.g., by changing the curvature of a membrane, can replace the motion of lens groups in zoom systems. Similar to classical zoom systems, the performance of these systems is heavily influenced by the fundamental first-order layout. Moreover, the first-order layout sets the most important requirements for the employed tunable lenses. In this contribution, we present a method for the analysis of a large number of possible first-order solutions for typical requirements and for the selection of the most promising layouts. The first-order solution space is mapped, allowing the layouts to be automatically filtered and plotted depending on pre-defined characteristics. Ray tracing of the marginal and chief rays combined with the traditional thin lens aberration theory provide efficient estimations of the expected installation space requirements and performance for each first-order layout. Using an example, we demonstrate good agreement between these estimations and the corresponding real lens layout, optimized by commercial raytracing software. The presented design method for zoom systems based on tunable lenses is compared with similar approaches for classical zoom lenses.

In this contribution we introduce new multilayer (bilayer and trilayer) resist systems for the generation of nanostructures with high aspect ratios of up to 14:1 on 4-in. full wafer scale. The bilayer stack consists of a bottom resist layer (lift off polymer LOR1A) and an UV-curable top resist layer (UV-NIL resist mr-NIL210 200 nm). The top resist is structured by UV-nanoimprint lithography with a soft polydimethysiloxane (PDMS) stamp (soft UV-NIL). After removal of the residual layer a wet chemical development is performed to achieve an isotropic undercut underneath the nanostructures in the top layer. This undercut is mandatory in order to perform a reliable and precise lift-off. The bilayer system is applicable on both silicon and fused silica. For a higher variety and combination of different resists, a trilayer system is investigated. A layer stack with new materials for bottom and top layer is presented. An intermediate layer made of silicon oxide by low temperature ICP-PECVD is added between a tailor-made top resist (mr-NIL213FC 200 nm) and an organic transfer layer (UL1). The intermediate layer serves as hard mask in order to etch the bottom layer isotropically utilizing a plasma etch process and thus replacing the wet-chemical development step. Subsequently, a thin metal layer is deposited onto the structured resist stack by electron beam evaporation. After lift-off, a nanostructured metal mask remains on the substrate providing a high selectivity during the following plasma etch step. A cryogenic ICPRIE etch process creates high aspect ratio nanostructures within the substrate. An aspect ratio of 14:1 was achieved.

Searching for the relationship between the nanostructure and optical properties has always been exciting the researchers in the field of optics (linear optics as well as non-linear optics), energy harvesting (anti-reflective Si solar cells, perovskite solar cells, ..., etc.), and industry (anti-reflection coating on car windows, sunglasses, etc.). In this work, we present an approach for nanostructuring the silicon substrate to silicon photonic crystals. By precisely controlling the etching time and etching path after using nanoimprint lithography, ordered arrays of inverted Si nanopyramids and Si nanopillars with good homogeneity, uniform surface roughness, high reproducibility of pattern transfer, and a controllable aspect ratio are prepared. Experimental investigation of the optical properties indicates that the reflections of these Si nanostructures are mainly determined by the aspect ratio as well as the period of nanostructures. Furthermore, we have experimentally observed visible-light scattering (V-LS) patterns on inverted Si nanopyramids and Si nanopillars, and their corresponding patterns can be precisely controlled by the patterned nanostructures. The V-LS pattern, background, and "ghost peaks" on the angle-resolved scattering results are caused by constructive interference, destructive interference, and the interference situation between both. This controllable nanopatterning on crystalline Si substrates with precisely tunable optical properties shows great potential for applications in many fields, for example, optics, electronics, and energy.

The field of optical lithography is subject to intense research and has gained enormous improvement. However, the effort necessary for creating structures at the size of 20 nm and below is considerable using conventional technologies. This effort and the resulting financial requirements can only be tackled by few global companies and thus a paradigm change for the semiconductor industry is conceivable: custom design and solutions for specific applications will dominate future development (Fritze in: Panning EM, Liddle JA (eds) Novel patterning technologies. International society for optics and photonics. SPIE, Bellingham, 2021. https://doi.org/10.1117/12.2593229). For this reason, new aspects arise for future lithography, which is why enormous effort has been directed to the development of alternative fabrication technologies. Yet, the technologies emerging from this process, which are promising for coping with the current resolution and accuracy challenges, are only demonstrated as a proof-of-concept on a lab scale of several square micrometers. Such scale is not adequate for the requirements of modern lithography; therefore, there is the need for new and alternative cross-scale solutions to further advance the possibilities of unconventional nanotechnologies. Similar challenges arise because of the technical progress in various other fields, realizing new and unique functionalities based on nanoscale effects, e.g., in nanophotonics, quantum computing, energy harvesting, and life sciences. Experimental platforms for basic research in the field of scale-spanning nanomeasuring and nanofabrication are necessary for these tasks, which are available at the Technische Universität Ilmenau in the form of nanopositioning and nanomeasuring (NPM) machines. With this equipment, the limits of technical structurability are explored for high-performance tip-based and laser-based processes for enabling real 3D nanofabrication with the highest precision in an adequate working range of several thousand cubic millimeters.

Deep etching of glass and glass ceramics is far more challenging than silicon etching. For thermally insensitive microelectromechanical and microoptical systems, zero-expansion materials such as Zerodur or ultralow expansion (ULE) glass are intriguing. In contrast to Zerodur that exhibits a complex glass network composition, ULE glass consists of only two components, namely, TiO2 and SiO2. This fact is highly beneficial for plasma etching. Herein, a deep fluorine-based etching process for ULE 7972 glass is shown for the first time that yields an etch rate of up to 425 nm min^-1 while still achieving vertical sidewall angles of 87&ring;. The process offers a selectivity of almost 20 with respect to a nickel hard mask and is overall comparable with fused silica. The chemical surface composition is additionally investigated to elucidate the etching process and the impact of the tool configuration in comparison with previously published etching results achieved in Zerodur. Therefore, deep and narrow trenches can be etched in ULE glass with high anisotropy, which supports a prospective implementation of ULE glass microstructures, for instance, in metrology and miniaturized precision applications.

Phase-space analysis has been widely used in the past for the study of optical resonant systems. While it is usually employed to analyze the far-field behavior of resonant systems, we focus here on its applicability to coupling problems. By looking at the phase-space description of both the resonant mode and the exciting source, it is possible to understand the coupling mechanisms as well as to gain insights and approximate the coupling behavior with reduced computational effort. In this work, we develop the framework for this idea and apply it to a system of an asymmetric dielectric resonator coupled to a waveguide.

We demonstrate the excitation and characterization of whispering gallery modes in a deformed optical microcavity. To fabricate deformed microdisk microresonators we established a fabrication process relying on dry plasma etching tools for many degrees of freedom and a shape-accurate morphology. This approach allowed us to fabricate resonators of different sizes with a controlled sidewall angle and underetching in large quantities with reproducible properties such as a surface roughness RQ ≤ 2nm. The excitation and characterization of these modes were achieved by using a state-of-the-art tapered fiber coupling setup with a narrow linewidth tunable laser source. The conducted measurements in shortegg resonators showed at least two modes within a spectral range of about 237 pm. The highest Q-factors measured were in the range of 105. Wave optical eigenmode and frequency domain simulations were conducted that could partially reproduce the observed behavior and therefore allow us to compare the experimental results.

The display of phase-only holograms with a spatial light modulator (SLM) has many applications due to its potential for dynamic three-dimensional projection of arbitrary patterns. We describe an innovative modification of the quadratic phase method for hologram calculation that uses error diffusion for initialization of an iterative phase retrieval algorithm. We compare the performance of our algorithm to other variations of hologram calculation approaches that use the quadratic phase method in the Fourier regime. Parameter variation is conducted for finding the differences and limits of the methods. Experiments with an SLM show the validity of the simulations.

We demonstrate a method for creating a three-dimensional (3D) array of focal spots by combination of a multi-focal diffractive lens and a two-dimensional multi-value phase grating. The multi-focal Fresnel-based lens is created by means of encoding special nonlinearities into the phase structure of a Fresnel zone plate and is represented as a mathematical superposition of this phase function with a refractive lens. The imposed nonlinearity type enables the creation of multiple focal spots with uniform intensity along the optical axis. We demonstrate the example of a 3D multi-value phase grating, which creates five focal planes with a 5×5 transverse array of focal spots with equal energy distribution in each plane. Experimental results are included to verify the theoretical outcomes, where the phase pattern of a 3D multi-value phase grating is encoded onto a spatial light modulator.

The applications of freeform optical surfaces in modern optical systems are providing unique solutions over rotationally symmetric surfaces. These surfaces offer higher degrees of freedom to the designer to enhance the high-end performance of the optical system. The precise metrology of freeform optics is one of the major bottlenecks for its use in imaging applications. Modern optical fabrication methods (i.e. fast or slow tool servo configuration) are, in principle, capable to meet the challenges to generate complex freeform surfaces if supported by precise metrology feedback for error compensation. In the present work, we have developed a Shack-Hartmann sensor-based metrology technique that can be used for quantitative in situ measurement of freeform optics. The sensor head is used to measure freeform optics in the reflection mode by following the CNC tool path in the offline mode. The measurements are used as feedback for corrective machining. Quantitative analysis is also performed to estimate the error budget of the metrology system. Further, the proposed in situ metrology scheme is validated by measuring freeform surface using a coherence correlation interferometric optical profiler.

In the present work, a spline-based integration technique for the reconstruction of a freeform wavefront from the slope data has been implemented. The slope data of a freeform surface contain noise due to their machining process and that introduces reconstruction error. We have proposed a weighted cubic spline based least square integration method (WCSLI) for the faithful reconstruction of a wavefront from noisy slope data. In the proposed method, the measured slope data are fitted into a piecewise polynomial. The fitted coefficients are determined by using a smoothing cubic spline fitting method. The smoothing parameter locally assigns relative weight to the fitted slope data. The fitted slope data are then integrated using the standard least squares technique to reconstruct the freeform wavefront. Simulation studies show the improved result using the proposed technique as compared to the existing cubic spline-based integration (CSLI) and the Southwell methods. The proposed reconstruction method has been experimentally implemented to a subaperture stitching-based measurement of a freeform wavefront using a scanning Shack-Hartmann sensor. The boundary artifacts are minimal in WCSLI which improves the subaperture stitching accuracy and demonstrates an improved Shack-Hartmann sensor for freeform metrology application.

The metrology of freeform wavefront can be performed by the use of a noninterferometric method, such as a Shack-Hartmann sensor (SHS). Detailed experimental investigations employing an SHS as metrology head are presented. The scheme is of nonnull nature where small subapertures are measured using an SHS and stitched to give the full wavefront. For the assessment of complex misalignment errors during the spiral scanning, a library of residual slope errors has been created, which makes the alignment process fast converging for minimizing the scanning errors. A detailed analysis of the effects of slope and positioning error on reproducibility is presented. It is validated by null test where a null diffractive optical element has been used in a Mach-Zehnder configuration for compensating the freeform shape. A freeform optics is measured by both measurement schemes, and the results are in good agreement. Further, the nonnull-based scanning subaperture stitching scheme is also validated by performing measurements on an aspheric surface and compared with the measurements from the interferometric method (Zygo Verifire).

The performance of optical tweezers in the presence of monochromatic aberrations is investigated theoretically and experimentally. For the calculation of optical forces we use a ray-optics-based force simulation, which is embedded in a ray tracing routine. By considering optical path differences caused by monochromatic aberrations, we are able to simulate their effect on the trapping forces. Optical trapping experiments considering the influence of aberrations were performed with an optical tweezers system, specially adapted to trap particles in gaseous surrounding. The conformity of these measurements with the theoretical predictions verifies the correct performance of the optical force simulation routine. Based on the reliable simulation tool, further theoretical studies are performed on how optical aberrations affect the function and performance of optical tweezers.

In this work, we apply the Alvarez-Lohmann principle for varifocal lenses to diffractive off-axis elements tuned by rotation. Two methods to combine multiple elements into arrays are presented. Further, we show that inverse phase sections result from a 2π ambiguity of the rotation. Quantization techniques are applied to eliminate these unwanted sections. As proof of concept, a retrofocus lens using a tunable diffractive lens array is presented.

This letter introduces a novel principle to the confocal profilometry for simultaneous measurement of surface position and tilt. The principle relies on an angle-dependent coding of the illumination of the surface-under-test, achieved by dividing the pupil into several subapertures. The reflected light is decoded to perform an angle-resolved analysis by measuring the intensity contributions of each individual subaperture. By comparing their contributions, additional information of surface tilt can be inferred. For our demonstration experiment the subapertures are encoded with specific spectral distribution, which may be called wavelength division encoding. Alternate coding procedures such as temporal or code division encoding are also feasible.

A key feature of Alvarez lenses is that they may be tuned in focal length using lateral rather than axial translation, thus reducing the overall length of a focus-tunable optical system. Nevertheless the bulk of classical microsystems actuators limits further miniaturization. We present here a new, ultrathin focus-tunable Alvarez lens fabricated using molding techniques and actuated using liquid crystal elastomer (LCE) artificial muscle actuators. The large deformation generated by the LCE actuators permits the integration of the actuators in-plane with the mechanical and optical system and thus reduces the device thickness to only 1.6 mm. Movement of the Alvarez lens pair of 178 [my]m results in a focal length change of 3.3 mm, based on an initial focal length of 28.4 mm. This design is of considerable interest for realization of ultraflat focus-tunable and zoom systems.

A compact linear micromechanical stepping drive for positioning a 7 × 5.5 mm^2 optical pinhole array is presented. The system features a step size of 13.2 [my]m and a full displacement range of 200 [my]m. The electrostatic inch-worm stepping mechanism shows a compact design capable of positioning a payload 50% of its own weight. The stepping drive movement, step sizes and position accuracy are characterized. The actuated pinhole array is integrated in a confocal chromatic hyperspectral imaging system, where coverage of the object plane, and therefore the useful picture data, can be multiplied by 14 in contrast to a non-actuated array.

In this paper aberrations in Alvarez-Lohmann lenses are analyzed, and a semi-analytical strategy for compensation is derived. An x-y polynomial model is used to describe the aberrations and classify them into static and dynamic components. The lenses are enhanced by higher-order polynomials, and a numerical optimization process is used to determine the most influential coefficients. Two simulations of corrected systems are presented. The first one is optimized for on-axis imaging. The second system is optimized for multiple field points and shows the limitations of a single Alvarez-Lohmann lens. Two systems overcoming these limitations by introducing additional optical surfaces are presented, and their performance is analyzed in simulations.

The size of the aperture stop of a lens is a major parameter to define, e.g., the depth of focus of an optical imaging system. In conventional systems, totally absorbing apertures are generally assumed. Their optical performance can be easily described by a geometric ray model. We propose an extended model to estimate the depth of focus with respect to a nontotally absorbing circular aperture, which may correspond to new concepts for tunable apertures, in particular for micro-optical systems. We present specifications to analyze and optimize the performance of those systems and verify the theoretical model by experimental depth of focus measurements with a partly transparent aperture.

An efficient algorithm for calculating nonparaxial scalar field distributions in the focal region of a lens is discussed. The algorithm is based on fast Fourier transform implementations of the first RayleighSommerfeld diffraction integral and assumes that the input field at the pupil plane has a larger extent than the field in the focal region. A sampling grid is defined over a finite region in the output plane and referred to as a tile. The input field is divided into multiple separate spatial regions of the size of the output tile. Finally, the input tiles are added coherently to form a summed tile, which is propagated to the output plane. Since only a single tile is propagated, there are significant reductions of computational load and memory requirements. This method is combined either with a subpixel sampling technique or with a chirp z-transform to realize smaller sampling intervals in the output plane than in the input plane. For a given example the resulting methods enable a speedup of approximately 800× in comparison to the normal angular spectrum method, while the memory requirements are reduced by more than 99%.

An innovative optical system for trapping particles in air is presented. We demonstrate an optical system specifically optimized for high precision positioning of objects with a size of several micrometers within a nanopositioning and nanomeasuring machine (NPMM). Based on a specification sheet, an initial system design was calculated and optimized in an iterative design process. By combining optical design software with optical force simulation tools, a highly efficient optical system was developed. Both components of the system, which include a refractive double axicon and a parabolic ring mirror, were fabricated by ultra-precision turning. The characterization of the optical elements and the whole system, especially the force simulations based on caustic measurements, represent an important interim result for the subsequently performed trapping experiments. The caustic of the trapping beam produced by the system was visualized with the help of image processing techniques. Finally, we demonstrated the unique efficiency of the configuration by reproducibly trapping fused silica spheres with a diameter of 10 [my]m at a distance of 2.05 mm from the final optical surface.

Hyperspectral imaging systems record information with high resolution in both the spatial and spectral regimes simultaneously. One central challenge remains distribution of the three- or multi-dimensional data, consisting of the spatial soordinates (x,y,z) and the wavelength ([lamda]) in a suitable way on commonly used two dimensional receiver chips. Tunable optics, used in combination with suitable methods and system concepts, offer solutions that optimize capacity as well as recording dynamics.

We present tunable lenses based on aluminum nitride membranes. The achievable tuning range in the refractive power is 0 to 25 dpt with an external pressure load of ≤20 kPa. The lenses are manufactured using MOEMS technology. For 500-nm-thick membranes with a diameter of 3 mm, a spherical deflection profile is found. The system provides good long-term stability showing no creep or hysteresis. A model for the refractive power versus applied pressure is derived and validated experimentally. Based on this model, design guidelines are discussed. One essential parameter is the residual stress of the aluminum nitride layer that can be controlled during deposition.

Parallelized imaging systems based on the chromatic confocal approach can be used for hyperspectral imaging and optical profilometry.

Automatic differentiation is an often superior alternative to numerical differentiation that is yet unregarded for calculating derivatives in the optimization of imaging optical systems. We show that it is between 8% and 34% faster than numerical differentiation with central difference when optimizing various optical systems.

We demonstrate an integrated optofluidic system for monitoring the particle mass concentration and the Sauter mean diameter of polydisperse flowing suspensions of water and standardized test dust. For optimum integration, a planar emitter-receiver unit is developed and fabricated an Si technology. A vertical cavity surface emitting laser at 850 nm serves as light source, and monolithically integrated segmented photodiodes detect both the attenuation of the primary light beam und the scattered light. The optical system is integrated into a planar transparent polymethylmethacrylate substate by micromilling of an optical freeform surface and electron beam evaporation of reflective layers following the concept of planar integrated free-space optical systems. We are able to detect 1 mg/L of standards ISO 12103-A2 (fine test dust), -A3 (medium test dust), and -A4 (coarse test dust) and to determine the Sauter mean diameter of the particle size distribution.

Ghost image analysis is of interest for optical designers as ghost images may degrade image quality in imaging systems. In a previous paper by Abd El-Maksoud and Sasian (Appl. Opt. 50, 2305 (2011), ghost image analysis for incoherent systems was evaluated using geometrical optics. Some criteria were presented to identify focused ghost images at the nominal image plane. The main goal of this paper is to provide a conceptual understanding of ghost image formation and its impact on the performence of imaging systems using wave theory and Fourier optics. To achieve this goal, a methodology is developed to model ghost images after considering diffration effects. Expressions for the ghost diffraction point spread function and the ghost transfer functions. To provide insights on the developed theory, some simulation examples are provided and discussed.

The performance of optical systems is typically improved by increasing the number of conventionally fabricated optical components (spheres, aspheres, and gratings). This approach is automatically connected to a system enlargement, as well as potentially higher assembly and maintenance costs. Hybrid optical freeform components can help to overcome this trade-off. They merge several optical functions within fewer but more complex optical surfaces, e.g., elements comprising shallow refractive/reflective and high-frequency diffractive structures. However, providing the flexibility and precision essential for their realization is one of the major challenges in the field of optical component fabrication. In this article we present tailored integrated machining techniques suitable for rapid prototyping as well as the fabrication of molding tools for low-cost mass replication of hybrid optical freeform components. To produce the different feature sizes with optical surface quality, we successively combine mechanical machining modes (ultraprecision micromilling and fly cutting) with precisely aligned direct picosecond laser ablation in an integrated fabrication approach. The fabrication accuracy and surface quality achieved by our integrated fabrication approach are demonstrated with profilometric measurements and experimental investigations of the optical performance.

We report the development of a modular and optimized thin-sheet laser imaging microscope (TSLIM) for nondestructive optical sectioning of organisms and thick tissues such as the mouse cochlea, zebrafish brain/inner ear, and rat brain at a resolution that is comparable to wide-field fluorescence microscopy. TSLIM optically sections tissue using a thin sheet of light by inducing a plane of fluorescence in transparent or fixed and cleared tissues. Moving the specimen through the thinnest portion of the light sheet and stitching these image columns together results in optimal resolution and focus across the width of a large specimen. Dual light sheets and aberration-corrected objectives provide uniform section illumination and reduce absorption artifacts that are common in light-sheet microscopy. Construction details are provided for duplication of a TSLIM device by other investigators in order to encourage further use and development of this important technology.

Gerade in den letzten Jahren hat sich die Entwicklung integrierter mikrooptoelektronischer Systeme stark beschleunigt und das Anwendungsspektrum signifikant erweitert.

A novel approach for the fabrication of diffractive optical elements is described. This approach is based on an interferometric phase contrast method that transforms a complex object wavefront into an intensity pattern. The resulting intensity pattern is used to expose a photoresist layer on a substrate. After development, a diffractive phase object with an on-axis diffraction pattern is achieved. We show that the interferometric phase contrast method allows a precise control of the resulting intensity pattern. An array of blazed Fresnel lenses is realized in photoresist by using kinoform or detour-phase computer holograms for the interferometric phase contrast setup.

A modified phase contrast method is described for the fabrication of holographic optical elements with in-line diffraction patterns. In this method, a liquid crystal phase modulator is addressed by computer-generated holograms to achieve variable phase gratings. The phase gratings are imaged onto photosensitive material by a phase contrast method without a Fourier filter. We show the analytical treatment of the modified phase contrast method and demonstrate the application in holographic recording.

We present a compact design for an integrated interconnect based on a hybrid imaging setup combining microchannel and conventional imaging. Within this setup the conventional imaging is performed by an aluminum-coated spherical lens. The aberrations introduced by this spherical mirror to the channels of the interconnect can be compensated by channel-wise adapted microlenses located at the in- and output interfaces. These microlenses are used for collimating or refocusing the beams, respectively. Within this paper we present the design of the microlens array with individually shaped lenses referred to as chirped mircolens array (cMLA) based on numerical optimization and the use of fitting functions. Further on we focus on the fabrication of the chirped microlens arrays by laser lithography and first experimental results of coupling efficiencies of singlemode as well as multimode fibers for the realized prototypes.

Wesentliche Kriterien der Abbildung einer räumlichen Szene sind Perspektivitätsart und perspektivischer Tiefeneindruck. Die Perspektivitätsart einer Abbildung wird von der Lage der Eintrittspupille bestimmt. Der perspektivische Eindruck bei Abbildungen unterliegt spezifischen geometrischen Zusammenhängen.