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Structured light has revolutionized optical particle manipulation, nano-scaled material processing, and high-resolution imaging. In particular, propagation-invariant light fields such as Bessel, Airy, or Mathieu beams show high robustness and have a self-healing nature. To generalize such beneficial features, these light fields can be understood in terms of caustics. However, only simple caustics have found applications in material processing, optical trapping, or cell microscopy. Thus, these technologies would greatly benefit from methods to engineer arbitrary intensity shapes well beyond the standard families of caustics. We introduce a general approach to arbitrarily shape propagation-invariant beams by smart beam design based on caustics. We develop two complementary methods, and demonstrate various propagation-invariant beams experimentally, ranging from simple geometric shapes to complex image configurations such as words. Our approach generalizes caustic light from the currently known small subset to a complete set of tailored propagation-invariant caustics with intensities concentrated around any desired curve., Finanziert durch den Open-Access-Publikationsfonds der Westfälischen Wilhelms-Universität Münster (WWU Münster).

Journal of synchrotron radiation 27(4), 1059 - 1068 (2020). doi:10.1107/S1600577520006864, X-ray imaging is a complementary method to electron and fluorescence microscopy for studying biological cells. In particular, scanning small-angle X-ray scattering provides overview images of whole cells in real space as well as local, high-resolution reciprocal space information, rendering it suitable to investigate subcellular nanostructures in unsliced cells. One persisting challenge in cell studies is achieving high throughput in reasonable times. To this end, a fast scanning mode is used to image hundreds of cells in a single scan. A way of dealing with the vast amount of data thus collected is suggested, including a segmentation procedure and three complementary kinds of analysis, i.e. characterization of the cell population as a whole, of single cells and of different parts of the same cell. The results show that short exposure times, which enable faster scans and reduce radiation damage, still yield information in agreement with longer exposure times., Published by Wiley-Blackwell, [S.l.]

Among the numerous propulsion mechanisms developed in the past to self-propelled nano- and micro particles, light-driven machines are most promising, since they enable a natural spatio-temporal control of the motion. We report a novel fuel-free propulsion mechanism induced by an external light stimulus. The actuation relies on refraction of light, while the net propulsion force emerges from an asymmetric particle shape and a symmetry-broken refractive index profile. Two-photon polymerization is employed for fabrication of the artificial machines, whose geometries and refractive index profiles are optimized with the help of numerical simulations. We demonstrate the directional movement of refractive light-propelled particles, and the increased performance of artificial refractive index machines.

So-called fully-structured light fields are characterized by a spatial variation in their amplitude, phase, and, simultaneously, polarization. Along with the technological progress of light-shaping methods based on q-plates or spatial light modulators, the variety of accessible structures has significantly increased and has proven being advantageous for high resolution imaging [1] , quantum communication [2] and particle manipulation [3] . For the vast majority of implemented light fields, the actually accessible customization is limited to two dimensions, i.e. the transverse plane of the light field to be modulated. Only a few approaches have recently been developed towards a full control of all degrees of freedom of the light field in all three spatial dimensions [4] , [5] . A common approach to structure light along its propagation direction takes advantage of tight focusing [6] . However, the resulting non-paraxial situation as well as the short longitudinal distance of the focal volume make it difficult to obtain an equal partition of fully structured light in all three dimensions. Thus, tailoring the longitudinal direction of a light field in the same effective manner than the transverse direction is an actual unsolved challenge. Counter-propagating light approaches that structure the longitudinal direction by interference at the same scale than transverse dimensions is a promising approach to this question. However, though being theoretically appealing, its experimental realization and detection meets unsolved challenges since the detection of a field in one direction is disturbed by the counterpropagating field.

We customize a transversely structured, tunable light landscape on the basis of orbital angular momentum (OAM)-carrying beams for the purpose of advanced optical manipulation. Combining Laguerre-Gaussian (LG) modes with helical phase fronts of opposite OAM handedness, counter-rotating transfer of OAM is enabled in a concentric intensity structure, creating a dynamic "grinding" scenario on dielectric microparticles. We demonstrate the ability to trap and rotate silica spheres of various sizes and exploit the light fields’ feature to spatially separate trapped objects by their size. We show the adaptability of the light field depending on the chosen LG mode indices, allowing on-demand tuning of the trapping potential and sorting criteria. The versatility of our approach for biomedical application is examined by spatial discriminating yeast cells and silica spheres of distinct diameter.

The study and application of structured light is mature in the transverse extent, whereas the third spatial dimension is rather unexploited though bearing many promising advancements with respect to super resolution and information content. We present complex light volumes that are shaped on-demand in their amplitude, phase and polarization in all spatial extents. By holographic beam shaping, we embed singularities in phase as well as polarization as skeleton of these structure. By transferring the self-imaging property of scalar beams to self-replicating vectorial light fields, we form 3d singular fields that include the creation and annihilation of singularities upon propagation. Furthermore we generate fields of sub-wavelength complexity by counter-propagation of structured light.

Spherical aberration of light is commonly known as an undesired effect in optical trapping. We demonstrate how to take advantage of this effect in order to shape extended light landscapes in a counter-propagating trapping configuration, fully-structured in three-dimensional (3D) space and embedding transverse as well as longitudinal field contributions. We numerically analyze the aberration, presenting controlled intensity and 3D polarization ellipse structures. Experimentally, we demonstrate the versatility of the structured light field using optically guided particles as sensors, analyzing their dynamics by velocimetry and scattered light. Finally, we elucidate the potential of the complex light field for 3D particle assemblies.

Within the past years, structured light has paved the way to advanced optical manipulation. Besides well-established amplitude and phase structuring, we will present the pioneering topical role of polarization modification for innovative trapping landscapes. Beyond, by combining scalar and vectorial modulation, fully-structured light becomes available including sophisticated spin and orbital energy flow topologies – thus, trapping gets full structure. Additionally, we present tight focusing as a tool for 4D light field customization, demonstrate its application as well as identification. These tailored focal fields of 3D polarization will revolutionize trapping of polarization sensitive objects, allowing for the formation of advanced functional matter.

Tailored light represents a fundamental tool for complex optical tweezing and micromanipulation. Despite many successful applications, there is still unexploited potential in fully structuring amplitude, phase and polarization. This is especially true if multiple spatial degrees of freedom are combined as e.g. the transverse plane as well as longitudinal extent, leading to three-dimensional (3d) volumes. This gives easy access to versatile features as spatio-temporal singularities or singularity networks in polarization landscapes. Within this contribution, we employ two fully-structured counter-propagating light fields to enable 3d beam shaping in amplitude, phase and polarization with a sub-wavelength resolution and a dynamic exchange between phase and polarization singularities upon propagation.

We transfer on-demand structuring of three-dimensional scalar amplitude and phase patterns to polarization-structured, vectorial light fields and its singularities. Our approach allows inheriting non-diffracting as well as self-imaging propagation properties to tailored singular ellipse fields, including self-replicating amplitude, polarization, and singularity configurations. It is experimentally realized by amplitude, phase and polarization modulation of the angular spectrum of the light field. We demonstrate the customization of complex singularity formations embedded in three-dimensionally (3d) tailored vectorial field. Our findings show that embedded networks of polarization singularities can be customized to propagate in a robust way along curved trajectories, creating and annihilating during propagation. This 3d structuring of vectorial singular light fields opens new perspectives for in-depth singularity studies and for advancing applications as optical micro-manipulation and material machining.

Magnetic force microscopy (MFM) represents a versatile technique within the manifold methods of scanning probe microscopy (SPM), focusing on the investigation of magnetic phenomena at the nanoscale. Although magnetism is a fundamental element of physics education, educational content at the cutting edge of actual scientific topics and techniques in magnetism, like MFM, is lacking. Therefore, we present a scaled teaching model imparting the core principles of MFM, implementing a macroscopic model operating in dynamic mode. The experimental configuration of the model is based on popular bricks by LEGO and drivers based on LEGO Mindstorms (Lego, Billund, Denmark), as well as on further off the shelf components being easily accessible for schools and universities. Investigations of macroscopic magnetic structures reveal numerical, visual and auditory information based on magnetic forces between an oscillating cantilever and ferromagnetic samples allowing a sensual experience of force microscopy for students. Along these lines, students obtain multiple representations to study the precision measurement process of SPM in general and MFM in particular at a scale that allows experiencing micro- and nanoscopic effects. The magnetic force gradients and spatial resolution of the macroscopic model are in agreement with those of an authentic microscopic magnetic force microscope.

In this work, the optical behavior of Red Blood Cells (RBCs) under an optically-induced mechanical stress was studied. Exploiting the new findings concerning the optical lens-like behavior of RBCs, the variations of the wavefront refracted by optically-deformed RBCs were further investigated. Experimental analysis have been performed through the combination of digital holography and numerical analysis based on Zernike polynomials, while the biological lens is deformed under the action of multiple dynamic optical tweezers. Detailed wavefront analysis provides comprehensive information about the aberrations induced by the applied mechanical stress. By this approach it was shown that the optical properties of RBCs in their discocyte form can be affected in a different way depending on the geometry of the deformation. In analogy to classical optical testing procedures, optical parameters can be correlated to a particular mechanical deformation. This could open new routes for analyzing cell elasticity by examining optical parameters instead of direct but with low resolution strain analysis, thanks to the high sensitivity of the interferometric tool. Future application of this approach could lead to early detection and diagnosis of blood diseases through a single-step wavefront analysis for evaluating different cells elasticity. © 2017 International Society for Advancement of Cytometry.

The promising idea of using self-propelled particles in fields such as microfabrication and environmental remediation has led to the appearance of a plethora of different approaches for their motion during the past years. [1] Among them, the combination of living swimmers and nanocontainer cargos is especially attractive [2]. However, the usual necessity of a fuel in the environment hinders their application in biological tissue and consequently, their bio-medical usage. Here, we propose a novel propulsion mechanism based solely on the refraction of light in the volume of micro-scale particles. In order to understand these artificial swimmers, numerical simulations were undertaken to identify optimized geometries and refractive index distributions of particles. In particles with broken symmetry, a resultant directional photon momentum transfer is intiating a propulsion force, which in turn can be enhanced by the inclusion of a GRadient of refractive INdex (GRIN). We demonstrate fabrication of such artificial swimmers by femtosecond laser lithography based on two photon polymerization (TPP). The versatility of TPP along with the well-suited fabrication polymer Ormocomp allow replicating any of the numerically suggested particles, especially those containing GRIN distributions. We demonstrate the directional motion of the fabricated artificial swimmers under collimated illumination, with the GRIN particles outperforming their homogeneous counterparts. With this first proof-of-principle we pave the way for numerous applications this new generation of fuel-free, refraction-driven self propelled swimmers. [1] M. Guix, S. M. Weiz, O. G. Schmidt, M. Medina‐Sanchez, Part. Part. Syst. Charact. 35, 1700382 (2015). [2] A. Barroso et al., Biomedical Microdevices 17, 26 (2015);A. Barroso et al., SPIE Newsroom (2015); doi:10.1117/2.1201507.006023

We demonstrate optical rogue waves as a consequence of enhanced focusing of caustic networks due to linear propagation in weakly scattering 2D tailored disorder in nonlinear photorefractive systems, and discuss the conditions for extreme amplitudes.

The biomimetic, deterministic aperiodic golden angle Vogel spiral lacks both translational and rotational symmetry and exhibits circularly symmetric scattering rings in Fourier space. We present a versatile and low cost method for maskless fabrication of a Vogel spiral relief grating by illumination of a photoresist with laser light complexly structured using a spatial light modulator and subsequent transfer of its negative into a polymer layer via replica molding. We investigate its diffractive properties and present as one application its implementation as a surface structure, leading to an efficiency enhancement of 18% in an organic solar cell.

Aberrations of light are commonly known as undesired effects in different applications, including optical trapping. However, here we demonstrate how to take advantage of controlled spherical aberration in order to shape extended optical trapping landscapes, fully-structured in three-dimensional (3D) space and embedding transverse as well as longitudinal electric field contributions. We numerically analyze the light field customization by the simple means of including glass plates in a counter-propagating trapping configuration, presenting sculpted intensity as well as 3D polarization ellipse structures. Experimentally, we prove the realized counter-propagating optical structure by particle velocimetry as well as the analysis of scattered light of optically guided micro-particles. Finally, we demonstrate the potential of our approach by creating extended 3D particle assemblies.

We investigate light propagation in a two-dimensional aperiodic refractive index lattice realized using the interference of multiple Mathieu–Gauss beams. We demonstrate experimentally and numerically that such a lattice effectively hinders linear light expansion and leads to light localization, compared to periodic photonic lattices in a photorefractive crystal. Most promisingly, we show that such an aperiodic lattice supports the nonlinear confinement of light in the form of soliton-like propagation that is robust with respect to changes in a wide range of intensities.

This joint issue of Optics Express and Optical Materials Express features 18 state-of-the art articles that witness actual developments in nonlinear optics, including those by authors who participated in the international conference Nonlinear Optics held in Waikoloa, Hawaii from July 15 to 19, 2019. As an introduction, the editors provide a summary of these articles that cover all aspects of nonlinear optics, from basic nonlinear effects and novel frequency windows to innovative nonlinear materials and devices, thereby paving the way for new nonlinear optical concepts and forthcoming applications.

Polarization singularities in vectorial light fields have become an important tool for different cutting-edge applications such as information processing with integer information units. However, even though vectorial singularities naturally form configurations of multiple singular points, up to now only rather simple, mostly cylindrical vector beams including single central singularities, have been considered. Here we demonstrate the customization of extended singularity networks embedded in a class of complex polarization structures based on general Ince-Gaussian modes, namely, Ince-Gaussian vector modes. Contributing to fundamental singular optics, our investigations evince the conservation of tailored singularity arrangements upon 3D propagation, whereby respective modes enlarge the range of stable vectorial fields, paving the way to information technologies with a significantly enhanced number of degrees of freedom.

Nature holds situations in which sudden changes are caused by smooth alterations. The famous Airy and Pearcey beams represent diffraction patterns of corresponding fold and cusp bifurcations, respectively. When classified in a hierarchical order, they are subsequently followed by swallowtail and butterfly beams. These beams are generally characterized as cuspoid beams, accelerated on bent trajectories. They lead to various applications, among them the realization of curved waveguides. Their umbilic counterparts, however, characterized by even more complex diffraction patterns, have up to now only been characterized, but not yet been utilized as functional fabrication templates for applications in photonics. In this contribution, we present our results on embedding higher-order cuspoid and umbilic catastrophes in tailored light. These light structures show versatile curved strands of high intensity during propagation. The elliptic umbilic beam even morphs from a hexagonal transverse intensity pattern to a beam with a single central hot spot to become again the original hexagonal pattern. We thus exploit the dynamics of these caustics to optically induce corresponding photonic lattices in nonlinear media and demonstrate light propagation in elliptic umbilic lattices. Our approach enables fabricating continuously transforming lattices with varying band structure, paving the way for advanced topological photonics.

Light structured in phase and polarization has proven extraordinary benefit for optical tweezing. However, the customization of focal, non-paraxial light landscapes still poses a challenge. We tailor novel focal landscapes including its intensity as well as three-dimensional polarization structure by complex vector beams, even combined with phase vortices. By focusing vector fields, whose polarization is oriented in flower- or web-shape, we fabricate intensity structures resembling bright flowers or dark stars. These structures are formed by transverse Ex,y as well as longitudinal Ez electric field components. We tune the z-polarization components as well as threedimensional focal volume E (x, y, z) = [Ex, Ey, Ez]T on demand, optimizing its structure for optical trapping applications. Beyond, we prove their benefit for advanced optical trapping schemes by tweezing and orienting zeolite-L nano-containers.

We realize a portfolio of higher-order cuspoid and umbilic caustics, exploit them as fabricating light for advanced nonlinear photonic structures, and demonstrate nonlinear light propagation therein.

We fabricate elliptic umbilic caustics photonic structures in photosensitive media. The lattice morphs from hexagonal patterns to a single hot spot. We demonstrate experimentally the formation of a soliton in these elliptic umbilic lattices.

We present the realization and analysis of tailored vector fields including polarization singularities. The fields are generated by a holographic method based on an advanced system including a spatial light modulator. We demonstrate our systems capabilities realizing specifically customized vector fields including stationary points of defined polarization in its transverse plane. Subsequently, vectorial flowers and spider webs as well as unique hybrid structures of these are introduced, and embedded singular points are characterized. These sophisticated light fields reveal attractive properties that pave the way to advanced application in e.g. optical micromanipulation. Beyond particle manipulation, they contribute essentially to actual questions in singular optics.

The mechanical loading of dental restorations and hard tissue is often investigated numerically. For validation and optimization of such simulations, comparisons with measured deformations are essential. We combine digital holographic interferometry and digital speckle photography for the determination of microscopic deformations with a photogrammetric method that is based on digital image correlation of a projected laser speckle pattern. This multimodal workstation allows the simultaneous acquisition of the specimen’s macroscopic 3D shape and thus a quantitative comparison of measured deformations with simulation data. In order to demonstrate the feasibility of our system, two applications are presented: the quantitative determination of (1) the deformation of a mandible model due to mechanical loading of an inserted dental implant and of (2) the deformation of a (dental) bridge model under mechanical loading. The results were compared with data from finite element analyses of the investigated applications. The experimental results showed close agreement with those of the simulations.

Controlling artificial Pearcey and swallowtail beams allows realizing caustic lattices in nonlinear photosensitive media at very low light intensities. We examine their functionality as 2D and 3D waveguiding structures, and show the potential of exploiting these lattices as a linear beam splitter, which we name a Pearcey-Y-splitter. For symmetrized Pearcey beams as auto-focusing beams, the formation of solitons in focusing nonlinearity is observed. Our original approach represents the first realization of caustic photonic lattices and can directly be applied in signal processing, microscopy and material lithography.

Developing the recently discovered concept of RBCs as microlenses, we demonstrate further applications in wavefront analysis and diagnostics. Correlation between RBC’s morphology and its behavior as a refractive optical element has been established. In fact, any deviation from the healthy RBC morphology can be seen as additional aberration in the optical wavefront passing through the cell. By this concept, accurate localization of focal spots of RBCs can become very useful in blood disorders identification. Moreover, By modelling RBC as bio-lenses through Zernike polynomials it is possible to identify a series of orthogonal parameters able to recognise RBC shapes. The main improvement concerns the possibility to combine such parameters because of their independence conversely to standard image-based analysis where morphological factors are dependent each-others. We investigate the three-dimensional positioning of such focal spots over time for samples with two different osmolarity conditions, i.e. discocytes and spherocytes. Finally, Zernike polynomials wavefront analysis allows us to study the optical behavior of RBCs under an optically-induced mechanical stress. Detailed wavefront analysis provides comprehensive information about the aberrations induced by the deformation obtained using optical tweezers. This could open new routes for analyzing cell elasticity by examining optical parameters instead of direct but with low resolution strain analysis, thanks to the high sensitivity of the interferometric tool.

We report the fabrication of distributed feedback lasers in Rhodamin 6G doped SU-8 by direct femtosecond laser writing. The devices are fabricated by point-by-point exposure and consist of a core layer with a low-order Bragg grating on the surface. A quarter-wavelength phase shift is introduced in the middle of the grating section to obtain single-mode emission. Lasing at different wavelengths is demonstrated by changing the grating pitch. Spectral emission, threshold and lifetime characterization is performed by pumping the laser with a pulsed, frequency double Nd:YAG laser.

A major current challenge in the field of structured light represents the development from three- (3d) to four-dimensional (4d) electric field structures, in which one exploits the transverse as well as longitudinal field components in 3d space. For this purpose, non-paraxial fields are required in order to be able to access visionary 3d topological structures as optical cones, ribbons and Möbius strips formed by 3d polarization states. We numerically demonstrate the customization of such complex topological structures by controlling generic polarization singularities in non-paraxial light fields. Our approach is based on tightly focusing tailored higher-order vector beams in combination with phase vortices. Besides demonstrating the appearance of cones and ribbons around the optical axis, we evince sculpting arrays of Möbius strips realized around off-axis generic singularities.

Beneath being a well-established tool in optical manipulation for spinning and eventually sorting trapped objects, transverse energy flow (TEF) of light represents a versatile, illustrative and instructive approach to understand singular light fields and their properties. Up to now, these transverse flows were mainly investigated in its orbital part, for example, as in optical vortices, whereas the study of complex spin flow density (SFD) topologies has not yet reached the same mature state. Especially, the defined customization of energy flow and respective SFD fields has not yet been initiated. By the application of higher-order Laguerre– as well as Ince–Gaussian modes, being well-studied tools in optical manipulation, we demonstrate for the first time to our knowledge the development of sophisticated SFD configurations. We show shaping the flow field and controlling the type, number, spatial location and order of formed singular points in these TEFs. Our approach facilitates on-demand sculpting energy flow fields and their complex topologies for future applications in, for example, optical manipulation based on tailored polarization structures.

Discrete optical gratings are essential components to customize structured light waves, determined by the band structure of the periodic potential. Beyond fabricating static devices, light-driven diffraction management requires nonlinear materials. Up to now, nonlinear self-action has been limited mainly to discrete spatial solitons. Discrete solitons, however, are restricted to the eigenstates of the photonic lattice. Here, we control light formation by nonlinear discrete diffraction, allowing for versatile output diffraction states. We observe morphing of diffraction structures for discrete Mathieu beams propagating nonlinearly in photosensitive media. The self-action of a zero-order Mathieu beam in a nonlinear medium shows characteristics similar to discrete diffraction in one-dimensional waveguide arrays. Mathieu beams of higher orders show discrete diffraction along curved paths, showing the fingerprint of respective two-dimensional photonic lattices.

Optical tweezers, a simple and robust implementa- tion of optical micromanipulation technologies, have become a standard tool in biological, medical and physics research labo- ratories. Recently, with the utilization of holographic beam shap- ing techniques, more sophisticated trapping configurations have been realized to overcome current challenges in applications. Holographically generated higher-order light modes, for exam- ple, can induce highly structured and ordered three-dimensional optical potential landscapes with promising applications in op- tically guided assembly, transfer of orbital angular momentum, or acceleration of particles along defined trajectories. The non- diffracting property of particular light modes enables the op- tical manipulation in multiple planes or the creation of axially extended particle structures. Alongside with these concepts which rely on direct interaction of the light field with particles, two promising adjacent approaches tackle fundamental limita- tions by utilizing non-optical forces which are, however, induced by optical light fields. Optoelectronic tweezers take advantage of dielectrophoretic forces for adaptive and flexible, massively parallel trapping. Photophoretic trapping makes use of thermal forces and by this means is perfectly suited for trapping ab- sorbing particles. Hence the possibility to tailor light fields holo- graphically, combined with the complementary dielectrophoretic and photophoretic trapping provides a holistic approach to the majority of optical micromanipulation scenarios.

In this contribution we demonstrate for the first time to our knowledge a three-dimensional monitoring of the evolution of the spontaneous polarization (Ps) during electrical poling and temperature annealing in multi-domain strontium barium niobate using Cerenkov-type second-harmonic generation.

We transfer higher order caustic catastrophes to optics, thus extending Airy and Pearcey by paraxial swallowtail and butterfly beams. Their caustics provide outstanding light trajectories for optical induction of refractive index structures.

We propose a method for efficient second-harmonic beam shaping in lithium niobate employing volume diffraction holograms induced by femtosecond laser lithography. The amplitude of the fundamental wave is shaped by the diffraction at the hologram. The zeroth and one higher diffraction order of the fundamental wave fulfill the noncollinear phase matching condition leading to second-harmonic generation in a large wavelengths range between 1074 nm and 2500 nm. We demonstrate the generation of SHG vortices with desired topological charges.

In photonics, the investigation of structured nonlinear systems is an active and vivid research area. Their ability to control the dispersion and diffraction properties of light allows tailoring light in its spectral, temporal, and spatial features. Manipulating the spatial features, i.e., overcoming diffraction, is an actual and challenging issue that is of utmost importance for further information processing tasks. With this contribution, we highlight some of the most recent publications in this field. Among others, we discuss the progress in complex photonic lattice generation as well as the guiding and control of discrete spatial solitons-entities that no longer spread during propagation. We also emphasize the importance of photonic lattices as optical analogies to quantum mechanical systems and review current results on Anderson localization of light.

The increasing importance of optical 3D measurement techniques and the growing number of available methods and systems require a fast and simple method to characterize the measurement accuracy. However, the conventional approach of comparing measured coordinates to known reference coordinates of a test target faces two major challenges: the precise fabrication of the target and – in case of pattern projecting systems – finding the position of the reference points in the obtained point cloud. The modulation transfer function (MTF) on the other hand is an established instrument to describe the resolution characteristics of 2D imaging systems. Here, the MTF concept is applied to two different topometric systems based on fringe and speckle pattern projection to obtain a 3D transfer function. We demonstrate that in the present case fringe projection provides typically 3.5 times the 3D resolution achieved with speckle pattern projection. By combining measurements of the 3D transfer function with 2D MTF measurements the dependency of 2D and 3D resolutions are characterized. We show that the method allows for a simple comparison of the 3D resolution of two 3D sensors using a low cost test target, which is easy to manufacture.

In the earlier study, effective mixing in a simple design of crossing channels has been demonstrated. The mixer makes use of the combined mechanisms of the split-and-recombine and chaotic mixing elements of stretching and folding. In the present study, geometrical factors were further explored to enhance the mixing performance of the mixers. The elbow length parameter l at the turning corners and the aspect ratio AR of the channel cross section are examined. Numerical simulations were employed first and the experiments followed to support the results. In addition to the light intensity method, particle-counting technique was utilized to quantify the mixing performance. It was found that shorter elbow length obviously improved the mixing. Meanwhile, optimized AR might also enhance the mixing performance. In general, mixing can be further improved with simple geometry adjustment and mixing index above 0.9 can be achieved in four crossings.

article i nfo We investigated the influence of edges and corners on the Anderson localization of light in disordered two- dimensional photonic lattices that are optically induced in nonlinear saturable photorefractive media. A sys- tematic quantitative study of gradual transition from corner to bulk Anderson localization in truncated two- dimensional photonic lattices was carried out. We analyzed numerically the localization at several corners and edges of the square and triangular photonic lattices and compared them with the localization in bulk me- dium. We found that, for strong disorder, corners and edges effectively suppress Anderson localization, as

Physics of counterpropagating optical beams and spatial optical solitons is reviewed, including the formation of stationary states and spatiotemporal instabilities. First, several models describing the evolution and interactions between optical beams and spatial solitons are discussed, that propagate in opposite directions in nonlinear media. It is shown that coherent collisions between counterpropagating beams give rise to an interesting focusing mechanism resulting from the interference between the beams, and that interactions between such beams are insensitive to the relative phase between them. Second, recent experimental observations of the counterpropagation effects and instabilities in waveguides and bulk geometries, as well as in one- and two-dimensional photonic lattices are discussed. A variety of different generalizations of this concept are summarized, including the counterpropagating beams of complex structures, such as multipole beams and optical vortices, as well as the beams in different media, such as photorefractive materials and liquid crystals.

Fully-structured light is an emerging approach to sculpt light in all its degrees of freedom, i.e. amplitude, phase and polarization with a high transverse resolution and complex modulation patterns. Such an attractive and versatile approach for advanced optical trapping or fabricating novel materials still poses fundamental as well as technical challenges. Though the implementation of spatial light modulators (SLMs) has opened up a promising path for this task, up to now, fully-structured light can only be achieved using interferometric methods, multiple SLMs, or split-screen techniques reducing spatial resolution. We present a sophisticated single-beam approach based on spatial multiplexing, which does not only allow joint customization of phase and polarization combined with natural and prospectively even on-demand amplitude modulation, but also ensures high spatial resolution by the use of a single standard SLM in the full-screen mode. We demonstrate the capabilities of our approach realizing double phase modulated light fields, as well as first- and higher-order vector modes with additional global phase modulation and natural amplitude shaping. These findings open new perspectives to optically trap polarization-sensitive, i.e. magnetic particles and advance laser material machining in anisotropic, birefingent matter.

Reliable methods for the optical acquisition of three-dimensional (3D) coordinates like the fringe projec- tion technique or 3D laser scanning are sensible to object movements because they require the recording of a sequence of images. In contrast, techniques using the projection of a random pattern reduce the measurement time to a single ex- posure time. A method is presented which allows the 3D acquisition of a surface from a single stereo image pair by projecting a laser speckle pattern. The use of a laser enables a simple design of the projection device as well as a suppression of ambient light by narrow band-pass filtering. Corresponding image points are determined by a digital image correlation algorithm. In order to optimize the introduced method, the influence of various parameters on the number and accuracy of the determined 3D coordinates is analyzed on the basis of comparative measurements with the fringe projection tech

A nonlinear dynamic phase-contrast stereoscope has been developed for the measurement of all three velocity components of a microfluidic flow field. The stereoscope system captures simultaneously two images of different off-axis views of the same region of interest in a microflow, seeded with tracer particles. Two independent photorefractive two-beam coupling novelty filters, one in each stereoscope channel, are employed to enhance the contrast of tracer particle images. A subsequently applied particle tracking algorithm extracts the velocity information from the images, and in first experiments the axial velocity components could be determined with an error of less than 5%. Finally we report on the determination of the velocity field in a rectangular microchannel with a 170 μm high microstep with the dynamic phase-contrast stereoscope.