Experimental characterization of quasiperiodic route to chaos in a VECSEL with SESAM.

We analyze the temporal dynamics of an optically-pumped quantum well vertical external-cavity surface-emitting laser (VECSEL) with a Semiconductor Saturable Absorber Mirror (SESAM) using the time series obtained when varying the pump power. We unveil the quasiperiodic route to chaos in the system by characterizing the Fourier spectra, the attractors in phase space, and the Lyapunov exponents for each temporal behavior observed: periodicity, quasiperiodicity, and chaos. Thus, we provide a complete description of this experimental observation of the route to chaos in a VECSEL-SESAM system.

Digital light processing 3D printing of microfluidic devices targeting high-pressure liquid-phase separations.

This paper focuses on 3D printing using digital light processing (DLP) to create microchannel devices with inner diameters of 100, 200, and 500 µm and cater flow-through applications within the realm of analytical chemistry, in particular high-pressure liquid chromatographic separations. Effects of layer thickness and exposure time on channel dimensions and surface roughness were systematically investigated. Utilizing a commercially accessible 3D printer and acrylate resin formulation, we fabricated 100-500 µm i.d. squared and circular channel designs minimizing average surface roughness (< 20%) by applying a 20-µm layer thickness and exposure times ranging from 1.1 to 0.7 s. Pressure resistance was measured by encasing microdevices in an aluminum chip holder that integrated flat-bottom polyetheretherketon (PEEK) nanoports allowing to establish the micro-to-macro interface to the HPLC instrument. After thermal post-curing and finetuning the clamping force of the chip holder, a maximum pressure resistance of 650 bar (1.5% RSD) was reached (n = 3). A polymer monolithic support structure was successfully synthesized in situ with the confines of a 500 µm i.d. 3D printed microchannel. A proof-of-concept of a reversed-phase chromatographic gradient separation of intact proteins is demonstrated using an aqueous-organic mobile-phase with isopropanol as organic modifier.

Freeform optical system design with differentiable three-dimensional ray tracing and unsupervised learning.

Optical systems have been crucial for versatile applications such as consumer electronics, remote sensing and biomedical imaging. Designing optical systems has been a highly professional work due to complicated aberration theories and intangible rules-of-thumb, hence neural networks are only coming into this realm until recent years. In this work, we propose and implement a generic, differentiable freeform raytracing module, suitable for off-axis, multiple-surface freeform/aspheric optical systems, paving the way toward a deep learning-based optical design method. The network is trained with minimal prior knowledge, and it can infer numerous optical systems after a one-time training. The presented work unlocks great potential for deep learning in various freeform/aspheric optical systems, and the trained network could serve as an effective, unified platform for generating, recording, and replicating good initial optical designs.

Miniaturized cost-effective broadband spectrometer employing a deconvolution reconstruction algorithm for resolution enhancement.

We demonstrate a miniaturized broadband spectrometer employing a reconstruction algorithm for resolution enhancement. We use an opto-digital co-design approach, by firstly designing an optical system with certain residual aberrations and then correcting these aberrations with a digital algorithm. The proposed optical design provides an optical resolution less than 1.7 nm in the VIS-channel (400-790 nm) and less than 3.4 nm in the NIR-channel (760-1520 nm). Tolerance analysis results show that the components are within a commercial class, ensuring a cost-efficient design. We build the prototype with a size of 37x30x26 mm3 and demonstrate that by applying a restoration algorithm, the optical resolution can be further improved to less than 1.3 nm (VIS-channel) and less than 2.3 nm (NIR-channel).

Automated freeform imaging system design with generalized ray tracing and simultaneous multi-surface analytic calculation.

Recently, freeform optics has been widely used due to its unprecedented compactness and high performance, especially in the reflective designs for broad-wavelength imaging applications. Here, we present a generalized differentiable ray tracing approach suitable for most optical surfaces. The established automated freeform design framework simultaneously calculates multi-surface coefficients with merely the system geometry known, very fast for generating abundant feasible starting points. In addition, we provide a "double-pass surface" strategy with desired overlap (not mutually centered) that enables a component reduction for very compact yet high-performing designs. The effectiveness of the method is firstly demonstrated by designing a wide field-of-view, fast f-number, four-mirror freeform telescope. Another example shows a two-freeform, three-mirror, four-reflection design with high compactness and cost-friendly considerations with a double-pass spherical mirror. The present work provides a robust design scheme for reflective freeform imaging systems in general, and it unlocks a series of new 'double-pass surface' designs for very compact, high-performing freeform imaging systems.

Compact conical beam shaper and freeform segmented reflector for SERS analysis.

We present a Raman spectroscopy setup containing a conical beam shaper in combination with a freeform segmented reflector for surface enhanced Raman scattering (SERS) analysis. The freeform segmented reflector and the conical beam shaper are designed by numerical approaches and fabricated by means of ultra-precision diamond tooling. The segmented reflector has a numerical aperture of 0.984 and a working distance of 1mm for SERS measurements. We perform systematic simulations using non-sequential ray tracing to assess the detecting abilities of the designed SERS-based system. We implement a proof-of-concept setup and demonstrate the confocal behavior by measuring the SERS signal of 10µM rhodamine B solution. The experimental results agree well with the simulations concerning the misalignment tolerances of the beam shaper with respect to the segmented reflector and the misalignment tolerances of the collecting fiber. In addition, we conduct benchmark SERS measurements by using a 60× objective lens with a numerical aperture of 0.85. We find that the main Raman intensity of rhodamine B at 1502 cm-1 obtained by our segmented reflector working together with the conical beam shaper is approximately 30% higher compared to the commercial objective lens.

Performance of laterally elongated pillar array columns in capillary electrochromatography mode.

In the present study, cylindrical and laterally elongated pillar array columns were investigated for use in capillary electrochromatography. Minimal theoretical plate heights of H = 1.90 and 1.46 µm (in absence of sidewall effect) were obtained for coumarin C440 under unretained conditions for cylindrical and rectangular (laterally elongated, aspect ratio 4) pillar array columns, respectively. By comparing dispersion at the entire channel width to that at the central zone only, it appears that sidewall related dispersion significantly contributes to overall dispersion. A 40% reduction of the plate height was observed by taking into account only the central channel zone. A kinetic plot analysis was performed to evaluate the potential of the studied geometries by considering a maximum operating voltage of 20 kV as limiting parameter. It was demonstrated that rectangular radially elongated pillars produce a higher efficiency than cylindrical pillars and other microfabricated column structures for microchip capillary electrochromatography previously studied.

High-Resolution 3D Bioprinting of Photo-Cross-linkable Recombinant Collagen to Serve Tissue Engineering Applications.

Various biopolymers, including gelatin, have already been applied to serve a plethora of tissue engineering purposes. However, substantial concerns have arisen related to the safety and the reproducibility of these materials due to their animal origin and the risk associated with pathogen transmission as well as batch-to-batch variations. Therefore, researchers have been focusing their attention toward recombinant materials that can be produced in a laboratory with full reproducibility and can be designed according to specific needs (e.g., by introducing additional RGD sequences). In the present study, a recombinant protein based on collagen type I (RCPhC1) was functionalized with photo-cross-linkable methacrylamide (RCPhC1-MA), norbornene (RCPhC1-NB), or thiol (RCPhC1-SH) functionalities to enable high-resolution 3D printing via two-photon polymerization (2PP). The results indicated a clear difference in 2PP processing capabilities between the chain-growth-polymerized RCPhC1-MA and the step-growth-polymerized RCPhC1-NB/SH. More specifically, reduced swelling-related deformations resulting in a superior CAD-CAM mimicry were obtained for the RCPhC1-NB/SH hydrogels. In addition, RCPhC1-NB/SH allowed the processing of the material in the presence of adipose tissue-derived stem cells that survived the encapsulation process and also were able to proliferate when embedded in the printed structures. As a consequence, it is the first time that successful HD bioprinting with cell encapsulation is reported for recombinant hydrogel bioinks. Therefore, these results can be a stepping stone toward various tissue engineering applications.

A Tunable Freeform-Segmented Reflector in a Microfluidic System for Conventional and Surface-Enhanced Raman Spectroscopy.

We present a freeform-segmented reflector-based microfluidic system for conventional Raman and Surface-Enhanced Raman Scattering (SERS) analysis. The segmented reflector is directly designed by a numerical approach. The polymer-based Raman system strongly suppresses the undesirable background because it enables confocal detection of Raman scattering through the combination of a freeform reflector and a microfluidic chip. We perform systematic simulations using non-sequential ray tracing with the Henyey-Greenstein model to assess the Raman scattering behavior of the substance under test. We fabricate the freeform reflector and the microfluidic chip by means of ultra-precision diamond turning and laser cutting respectively. We demonstrate the confocal behavior by measuring the Raman spectrum of ethanol. Besides, we calibrate the setup by performing Raman measurements on urea and potassium nitrate solutions with different concentrations. The detection limit of our microfluidic system is approximately 20 mM according to the experiment. Finally, we implement a SERS microfluidic chip and discriminate 100 µM urea and potassium nitrate solutions.

Achieving a Peak Capacity of 1800 Using an 8 m Long Pillar Array Column.

In the present study, the peak capacity potential of ultralong porous cylindrical pillar array columns is investigated. Coupling 4 columns of 2 m long allows for working near the minimal separation impedance of small molecules under retained conditions at a maximal pressure load of 250 bar. Minimal plate heights of H = 5.0 µm, H = 6.3 µm, and H = 7.7 µm were obtained for uracil (unretained), butyrophenone (k = 0.85), and valerophenone (k = 1.94), respectively, corresponding to a number of theoretical plates of N = 1.6 × 106, N = 1.2 × 106, and N = 1.0 × 106. The optimal linear velocities were 0.60 mm/s for a retained compound and 0.74 mm/s for an unretained compound. Based on a mixture of 9 compounds, the peak capacity nc was determined as a function of gradient time (tG). Peak capacities (tG-based) of 1103 and 1815 were obtained when applying 650 min and 2050 min gradients (tG/t0 = 4.5 and 14, respectively, with tG as the gradient time and t0 as the void time). These values are much higher than earlier reported peak capacity values for small molecules.

Study of peak capacities generated by a porous layered radially elongated pillar array column coupled to a nano-LC system.

The performance of a porous-layered radially elongated pillar (PLREP) array column in a commercial nano-LC system was examined by performing separation of alkylphenones and peptides. The mesoporous silica layer was prepared by sol-gel processing of a mixture of tetramethoxysilane and methyltrimethoxysilane on REPs filling a 16.5 cm long, 1 mm wide channel (three lanes of 5.5 cm long channels connected by turns). The minimum plate height of 1.4 µm for octanophenone (k = 2.21) observed in isocratic mode is 5 times smaller than the smallest off-column plate height previously reported for porous pillar array columns for a retained component. This advantage is related to the earlier introduced shape of the radially elongated pillar bed that outperforms the cylindrically shaped pillar bed in terms of the plate height. In gradient mode, maximum conditional peak capacities of 220 (for a mixture of thiourea and 7 alkylphenones, tG = 180 min) and 160 (for a cytochrome c digest, tG = 150 min) were obtained. These results indicate excellent potential for implementation of this sol-gel layer in pillar array column formats.

Regularized non-convex image reconstruction in digital holographic microscopy.

Inverse problem approaches for image reconstruction can improve resolution recovery over spatial filtering methods while reducing interference artifacts in digital off-axis holography. Prior works implemented explicit regularization operators in the image space and were only able to match intensity measurements approximatively. As a consequence, convergence to a strictly compatible solution was not possible. In this paper, we replace the non-convex image reconstruction problem for a sequence of surrogate convex problems. An iterative numerical solver is designed using a simple projection operator in the data domain and a Nesterov acceleration of the simultaneous Kaczmarz method. For regularization, the complex-valued object wavefield image is represented in the multiresolution CDF 9/7 wavelet domain and an energy-weighted preconditioning promotes minimum-norm solutions. Experiments demonstrate improved resolution recovery and reduced spurious artifacts in reconstructed images. Furthermore, the method is resilient to additive Gaussian noise and subsampling of intensity measurements.

Studies on the sparsifying operator in compressive digital holography.

In compressive digital holography, we reconstruct sparse object wavefields from undersampled holograms by solving an ℓ1-minimization problem. Applying wavelet transformations to the object wavefields produces the necessary sparse representations, but prior work clings to transformations with too few vanishing moments. We put several wavelet transformations belonging to different wavelet families to the test by evaluating their sparsifying properties, the number of hologram samples that are required to reconstruct the sparse wavefields perfectly, and the robustness of the reconstructions to additive noise and sparsity defects. In particular, we recommend the CDF 9/7 and 17/11 wavelet transformations, as well as their reverse counter-parts, because they yield sufficiently sparse representations for most accustomed wavefields in combination with robust reconstructions. These and other recommendations are procured from simulations and are validated using biased, noisy holograms.

Efficient multiscale phase unwrapping methodology with modulo wavelet transform.

Many robust phase unwrapping algorithms are computationally very time-consuming, making them impractical for handling large datasets or real-time applications. In this paper, we propose a generic framework using a novel wavelet transform that can be combined with many types of phase unwrapping algorithms. By inserting reversible modulo operators in the wavelet transform, the number of coefficients that need to be unwrapped is significantly reduced, which results in large computational gains. The algorithm is tested on various types of wrapped phase imagery, reporting speedup factors of up to 500. The source code of the algorithm is publicly available.

Polydopamine-Gelatin as Universal Cell-Interactive Coating for Methacrylate-Based Medical Device Packaging Materials: When Surface Chemistry Overrules Substrate Bulk Properties.

Despite its widespread application in the fields of ophthalmology, orthopedics, and dentistry and the stringent need for polymer packagings that induce in vivo tissue integration, the full potential of poly(methyl methacrylate) (PMMA) and its derivatives as medical device packaging material has not been explored yet. We therefore elaborated on the development of a universal coating for methacrylate-based materials that ideally should reveal cell-interactivity irrespective of the polymer substrate bulk properties. Within this perspective, the present work reports on the UV-induced synthesis of PMMA and its more flexible poly(ethylene glycol) (PEG)-based derivative (PMMAPEG) and its subsequent surface decoration using polydopamine (PDA) as well as PDA combined with gelatin B (Gel B). Successful application of both layers was confirmed by multiple surface characterization techniques. The cell interactivity of the materials was studied by performing live-dead assays and immunostainings of the cytoskeletal components of fibroblasts. It can be concluded that only the combination of PDA and Gel B yields materials possessing similar cell interactivities, irrespective of the physicochemical properties of the underlying substrate. The proposed coating outperforms both the PDA functionalized and the pristine polymer surfaces. A universal cell-interactive coating for methacrylate-based medical device packaging materials has thus been realized.

Merging Open-Tubular and Packed Bed Liquid Chromatography.

Experimental and theoretical proof is provided for the fact that a microfabricated packed bed column, which is uniformly filled with radially elongated pillars (REPs), can produce the same separation performance as nonpacked, open-tubular columns. These are generally recognized as the best possible chromatographic column format, offering the highest conceivable separation speed and efficiency. It is also demonstrated both experimentally and theoretically that, as long as pressure is not a limiting factor, the REP column format can even outperform the open-tubular column format, with significant gains in either speed or efficiency proportional to the tortuosity, τ, of the bed. Conducting chromatographic experiments on 4 cm long micromachined packed bed columns filled with radially elongated pillars, separation efficiencies corresponding to N = 160,000 theoretical plates (unretained analytes) and N = 70,000 theoretical plates were achieved, despite the relatively large interpillar distance (2.5 µm).

Algorithms for determining the radial profile of the photoelastic coefficient in glass and polymer optical fibers.

We discuss two algorithms to determine the value and the radial profile of the photoelastic coefficient C in glass and polymer optical fibers. We conclude that C is constant over the fiber cross-sections, with exception of silica glass fibers containing a fluorine-doped depressed cladding. In the undoped and Ge-doped parts of these silica glass fibers we find a consistent value for C that is slightly larger than in bulk silica. In the fluorine-doped trenches of the absolute value of C decreases with about 27%. In polymethyl methacrylate optical fibers, the value of C significantly varies from fiber to fiber.

Mass-manufacturable polymer microfluidic device for dual fiber optical trapping.

We present a microfluidic chip in Polymethyl methacrylate (PMMA) for optical trapping of particles in an 80µm wide microchannel using two counterpropagating single-mode beams. The trapping fibers are separated from the sample fluid by 70µm thick polymer walls. We calculate the optical forces that act on particles flowing in the microchannel using wave optics in combination with non-sequential ray-tracing and further mathematical processing. Our results are compared with a theoretical model and the Mie theory. We use a novel fabrication process that consists of a premilling step and ultraprecision diamond tooling for the manufacturing of the molds and double-sided hot embossing for replication, resulting in a robust microfluidic chip for optical trapping. In a proof-of-concept demonstration, we show the trapping capabilities of the hot embossed chip by trapping spherical beads with a diameter of 6µm, 8µm and 10µm and use the power spectrum analysis of the trapped particle displacements to characterize the trap strength.

Nonequilibrium Gibbs' criterion for completely wetting volatile liquids.

During the spreading of a liquid over a solid substrate, the contact line can stay pinned at sharp edges until the contact angle exceeds a critical value. At (or sufficiently near) equilibrium, this is known as Gibbs' criterion. Here, we show both experimentally and theoretically that, for completely wetting volatile liquids, there also exists a dynamically-produced contribution to the critical angle for depinning, which increases with the evaporation rate. This suggests that one may introduce a simple modification of the Gibbs' criterion for (de)pinning that accounts for the nonequilibrium effect of evaporation.

Integration of uniform porous shell layers in very long pillar array columns using electrochemical anodization for liquid chromatography.

Electrochemical anodization has been applied to grow porous shell layers of 300 nm (30 nm pores) in 5 µm diameter pillar array columns (PACs) with a spacing of 2.5 µm. Using turn structures preceded and followed by the flow distributor structures recently introduced by our group and filled with radially elongated pillars, columns with quasi unlimited channel lengths could be conceived. The uniformity of the porous PAC was assessed by determining local plate heights along the channel, which appeared to be constant. Minimal (absolute) plate heights (H) between 4 and 6 µm were obtained at optimal flow rates when imposing increasing retention factors. Upon measuring the surface area involved in chromatographic retention as an indicator of the available surface area, an increase in the surface area by a factor of about 30 compared to that of non-anodized pillars was found. On reconfiguring a commercial HPLC instrument to enable on-chip injections, 90% of the performance (expressed in theoretical plates) could be maintained for a 1 m column, while for a 25 cm column severe losses were still observed. As the corresponding pressure drop for optimal operation of retained components is on the order of 10 bar per m only, portable and cheaper HPLC devices with high efficiencies become realistically conceivable.