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Implementation of hydrogel precursors in two-photon polymerization (2PP) technology provides promising opportunities in the tissue engineering field thanks to their soft characteristics and similarity to extracellular matrix. Most of the hydrogels, however, are prone to post-fabrication deformations, leading to a mismatch between the computer-aided design and the printed structure. In the present work, we have developed novel synthetic hydrogel precursors to overcome the limitations associated with 2PP processing of conventional hydrogel precursors such as post-processing deformations and a narrow processing window. The precursors are based on a poly(ethylene glycol) backbone containing urethane linkers and are, on average, functionalized with six acrylate terminal groups (three on each terminal group). As a benchmark material, we exploited a precursor with an identical backbone and urethane linkers, albeit functionalized with two acrylate groups, that were reported as state-of-the-art. An in-depth characterization of the hexafunctional precursors revealed a reduced swelling ratio (36 MPa Young's modulus) compared to their difunctional analogs. The superior physical properties of the newly developed hydrogels lead to 2PP-based fabrication of stable microstructures with excellent shape fidelity at laser scanning speeds up to at least 90 mm s-1, in contrast with the distorted structures of conventional difunctional precursors. The hydrogel films and microscaffolds revealed a good cell interactivity after functionalization of their surface with a gelatin methacrylamide-based coating. The proposed synthesis strategy provides a one-pot and scalable synthesis of hydrogel building blocks that can overcome the current limitations associated with 2PP fabrication of hydrogel microstructures

Despite the rapid developments in the field of two-photon polymerization-based direct laser writing, limited attention has been paid to the efficient design of optical waveguide-based building blocks. To fill that gap, we have numerically investigated air-clad waveguides, tapers, and S-bends, with the aim to minimize insertion losses, whilst reducing the device sizes. We have first demonstrated waveguides with square and circular cross-sections that are mode-matched with single-mode optical fibers featuring insertion losses below −0.6 dB and −1.5 dB around 1550 nm for lengths of respectively 0.2 mm and 1 mm. We have also identified parabolic tapers that allow for adiabatic transition between a wide range of input and output waveguide sizes. These shapes allow, for example, tapering down from 15 µm to 2 µm diameter waveguides over a length as short as 43.2 µm. We have fabricated a series of such components and confirmed their nearly lossless performance with insertion loss measurements. Finally, we have designed and optimized S-bends with Bezier curve shapes. As a proof-of-principle demonstration, we have fabricated a 160 µm long S-bend that offsets the waveguide axis by 50 µm. The insertion loss of the resulting 400 µm long component, which also included two parabolic tapers, was less than −1.7 dB. Apart from providing design rules and ready-to-use recipes for fabricating low-loss 3D-printed waveguide-based building blocks, we project that our work will spark the development of a series of efficient photonic devices that rely on these components and that can be exploited in diverse application fields.

We present the characterization of micro-lenses that are fabricated with the technology of two-photon polymerization (2PP)-based direct laser writing. We fabricate multiple lenses with different exposure parameters, after which we use interferometric characterization tools to investigate their geometry and optical performance to optimize the fabrication strategy.

Stem cells are characterized by their self-renewal capacity and their ability to differentiate into multiple cell types of the human body. Using directed differentiation strategies, stem cells can now be converted into hepatocyte-like cells (HLCs) and therefore, represent a unique cell source for toxicological applications in vitro. However, the acquired hepatic functionality of stem cell-derived HLCs is still significantly inferior to primary human hepatocytes. One of the main reasons for this is that most in vitro models use traditional two-dimensional (2D) setups where the flat substrata cannot properly mimic the physiology of the human liver. Therefore, 2D-setups are progressively being replaced by more advanced culture systems, which attempt to replicate the natural liver microenvironment, in which stem cells can better differentiate towards HLCs. This review highlights the most recent cell culture systems, including scaffold-free and scaffold-based three-dimensional (3D) technologies and microfluidics that can be employed for culture and hepatic differentiation of stem cells intended for hepatotoxicity testing. These methodologies have shown to improve in vitro liver cell functionality according to the in vivo liver physiology and allow to establish stem cell-based hepatic in vitro platforms for the accurate evaluation of xenobiotics.

Measuring the nonlinear coefficient γ of any guiding medium, regardless of the sign and magnitude of its group-velocity dispersion parameter β 2 , is challenging because of the lack of general solutions of the nonlinear Schrodinger equation (NLSE). Indeed, existing approaches typically need to disregard chromatic-dispersion effects to determine γ [1] . Here we propose an all-encompassing approach to measure the ratio β 2 /γ and prove our method in polarization-maintaining (PM) and single-mode (SM) fibers with positive and negative β 2 .

Direct laser writing with two-photon polymerization technique allows high-speed and single-step manufacturing of complex 3D optical components [1] . 3D-printed waveguiding components and devices can be separated into a standalone category, where optical communication and sensing applications are the main targets. Yet the actively developing domain is lacking well developed design and fabrication rules and in-depth study for different building blocks. In our study, we have adopted a bottom-up research approach and first numerically studied and then experimentally demonstrated a series of low-loss fiber-coupled waveguiding components.

We present our latest results on the design and fabrication of mode-field conversion tapers for low-loss optical interconnects. These structures are fabricated by means of two-photon polymerization-based 3D nanoprinting. We experimentally demonstrate that our 3D nanoprinted downtapers outperform conventional lensed fibers for low-loss edge coupling of single-mode fibers with SOI, Si3N4 and InP-based photonic integrated circuits. They are also more robust as they allow butt coupling rather than free-space coupling. Non-linear taper profiles allow shortening the length of the downtapers while keeping their performance. We also demonstrate 3D nanoprinted uptapers that allow for relaxation of the lateral misalignment tolerances.

We present the design, fabrication and characterization of mode conversion down- tapers printed directly on single-mode fiber tips, enabling high-efficiency physical contact edge- coupling connections to various photonic integrated chips.

Connections between standard single-mode fibers and waveguides in photonic integrated circuits tend to have relatively high coupling losses due to a difference in mode size and mode profile between both light guiding media. Edge coupling strategies involving specialty fibers are frequently used to obtain the best performance in terms of coupling efficiency, bandwidth and polarization independence. We propose the fabrication of free-standing down-taper structures on top of cleaved fiber facets by two-photon direct laser writing and show their performance for silicon, silicon nitride and indium phosphide generic chip platforms. We present a comprehensive analysis of the design of such taper structures that show unprecedented flexibility. We demonstrate their fabrication and fully characterize them in terms of output modal fields and coupling efficiencies. Furthermore, we experimentally compare the fabricated down-tapers with commercially available lensed fibers and demonstrate equal or better coupling efficiency for four out of the five investigated photonic integrated circuit platforms, with a measured improvement in coupling efficiency up to 1.43 dB.

No general approach is available yet to measure directly the ratio between chromatic dispersion and the nonlinear coefficient, and hence the soliton number for a given optical pulse, in an arbitrary guiding medium. Here we solve this problem using continuum generation. We experimentally demonstrate our method in polarization-maintaining and single-mode fibers with positive and negative chromatic dispersion. Our technique also offers new opportunities to determine the chromatic dispersion of guiding media over a broad spectral range while pumping at a fixed wavelength. (C) 2020 Optical Society of America

Photocrosslinkable gelatin hydrogels are excellent bioinks or biomaterial ink components to serve biofabrication applications. Especially the widely investigated gelatin-methacroyl hydrogels hold an impressive track record. However, over the past decade, increasing attention is being paid to thiol-ene photo-click chemistry to obtain hydrogel networks benefitting from a faster reactivity (i.e. seconds vs minutes) along with superior biocompatibility and processability. In order to exploit this photo-click chemistry, often an ene-functionality (e.g.. Norbornene) is introduced onto gelatin followed by crosslinking in the presence of a multifunctional thiol (e.g.. DTT). To date, very limited research has been performed on the influence of the applied thiolated crosslinker on the final hydrogel properties. Therefore, the present work assesses the influence of different thiolated crosslinkers on the crosslinking kinetics, mechanical properties and biological performance of the hydrogels upon encapsulation of primary adipose tissue-derived stem cells in which indicated a cell viability exceeding 70%. Furthermore, the different formulations were processed using two-photon polymerisation which indicated, in addition to differences in processing window and swelling ratio, a previously unreported phenomenon. At high intensities (i.e. ≥ 150 mW), the laser results in cleavage of the gelatin backbone even in the absence of distinct photo-cleavable functionalities. This can have potential to introduce channels or softer regions in gels to result in zones characterized by different degradation speeds or the formation of blood vessels. Consequently, the present study can be used to provide guidance towards tailoring the thiol-ene system towards the desired applications. Creative Commons Attribution license.

Improved chemical- and bio-sensing with Surface Enhanced Raman Spectroscopy (SERS) requires nanostuctures that can be flexibly designed and fabricated with different physical and optical properties. Here, we present nano-pillar arrays ranging from 200 nm to 600 nm as SERS substrates for mycotoxin detection that are fabricated by means of two-photon polymerization. We built a nominal shape and a voxel-based model for simulating the enhancement of the electric field of the nano-pillar arrays using the Finite-Difference Time-Domain (FDTD) method. A new model was built based on the Atomic Force Microscopy (AFM) data obtained from the fabricated nanostructures and introduced into a FDTD model. We demonstrated the enhancement behavior by measuring the Raman spectrum of Rhodamine B solutions. Both the simulations and experimental results suggest that the 200 nm nano-pillar array has the highest Enhancement Factor (EF). Besides, we determined the limit of detection of the 200 nm pillar array by performing Raman measurements on Rhodamine B solutions with different concentrations. The detection limit of our 200 nm nano-pillar array is 0.55 μM. Finally we discriminated 1 ppm deoxynivalenol and 1.25 ppm fumonisin b1 in acetonitrile solutions by our SERS substrate in combination with principal component analysis. This versatile approach for SERS substrates fabrication gives new opportunities for material characterization in chemical and biological applications.

Corneal blindness is the fourth leading cause of visual impairment. Of specific interest is blindness due to a dysfunctional corneal endothelium which can only be treated by transplanting healthy tissue from a deceased donor. Unfortunately, corneal supply does not meet the demand with only one donor for every 70 patients. Therefore, there is a huge interest in tissue engineering of grafts consisting of an ultra-thin scaffold seeded with cultured endothelial cells. The present research describes the fabrication of such artificial Descemet membranes based on the combination of a biodegradable amorphous polyester (poly (d,l-lactic acid)) and crosslinkable gelatins. Four different crosslinkable gelatin derivatives are compared in terms of processing, membrane quality, and function, as well as biological performance in the presence of corneal endothelial cells. The membranes are fabricated through multi-step spincoating, including a sacrificial layer to allow for straightforward membrane detachment after production. As a consequence, ultrathin (90%), semi-permeable membranes could be obtained with high biological potential. The membranes supported the characteristic morphology and correct phenotype of corneal endothelial cells while exhibiting similar proliferation rates as the positive control. As a consequence, the proposed membranes prove to be a promising synthetic alternative to donor tissue.

We report on the design, fabrication, and characterization of a $\text{1550 nm}$ / $\text{1950 nm}$ directional coupler based on single-crystal diamond waveguides. The input and output ports of the waveguides are connected with $\text{1550 nm}$ and $\text{1950 nm}$ grating couplers to allow out-of-plane light coupling. The devices are fabricated using electron-beam lithography and reactive ion etching technologies. We obtain a good performance for both the grating couplers (efficiency of $-\text{10.9 dB}$ at $\text{1550 nm}$ and $-\text{11.6 dB}$ at $\text{1950 nm}$ ) and waveguides (attenuation of $\text{2.3 dB/mm}$ at $\text{1550 nm}$ and $\text{4.5 dB/mm}$ at $\text{1950 nm}$ ). Furthermore, the actual directional coupler component is found to be quasi-lossless. By means of directional coupler calibration devices, we measure high extinction ratios between the “cross” and “through” ports ( $\text{22 dB}$ at $\text{1565 nm}$ and $\text{21 dB}$ at $\text{1949 nm}$ ), hence showing a high-efficiency wave-combining functionality. Taking into account that integrated directional couplers are essential for allowing light routing between different on-chip devices, our results pave the way for larger-scale and more complex photonic integrated circuits in single-crystal diamond thin films.

Graphene is considered a record-performance nonlinear-optical material on the basis of numerous experiments. The observed strong nonlinear response ascribed to the refractive part of graphene’s electronic third-order susceptibility χ(3) cannot, however, be explained using the relatively modest χ(3) value theoretically predicted for the 2D material. Here we solve this long-standing paradox and demonstrate that, rather than χ(3)-based refraction, a complex phenomenon which we call saturable photoexcited-carrier refraction is at the heart of nonlinear-optical interactions in graphene such as self-phase modulation. Saturable photoexcited-carrier refraction is found to enable self-phase modulation of picosecond optical pulses with exponential-like bandwidth growth along graphene-covered waveguides. Our theory allows explanation of these extraordinary experimental results both qualitatively and quantitatively. It also supports the graphene nonlinearities measured in previous self-phase modulation and self-(de)focusing (Z-scan) experiments. This work signifies a paradigm shift in the understanding of 2D-material nonlinearities and finally enables their full exploitation in next-generation nonlinear-optical devices., Graphene enables extraordinary nonlinear-optical refraction, far exceeding predictions based on conventional nonlinear-susceptibility theory. Here, Vermeulen et al. show that rather than the nonlinear susceptibility, a complex saturable refraction process is central to graphene’s unusual behavior.

We present the development of a compact (about 1.3 x 2.0 x 20 mm(3)) freeform optical lightguide for sensing applications, from the conceptual design to the fabrication through injection molding. The design of the optic is based on the flow-line method from Nonimaging Optics, which allows the desired optical functionalities (45 degrees half-acceptance and 40 degrees beam steering) while meeting particularly tight mechanical and geometrical constraints. An extensive analysis of the effects of fabrication parameters on the performances demonstrates the importance of minimizing the fillet radius. This requisite inspired a special procedure for designing the mold, which is built as a "3D puzzle" assembly of separate pieces, each one dedicated to one specific side surface of the lightguide. This technique enables uniform optical quality on all the optic's surfaces and removes the need of a fillet radius in the mold. At present, the first lightguide prototypes have been fabricated; after the coating phase, they will be ready for the validation stage. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Surface Plasmon Resonance devices have the promise to become the prevailing device technology for miniaturized point-of-care diagnosis. We have designed, fabricated and tested a disposable freeform SPR device outperforming traditional prism devices.

Over the recent decades gelatin has proven to be very suitable as an extracellular matrix mimic for biofabrication and tissue engineering applications. However, gelatin is prone to dissolution at typical cell culture conditions and is therefore often chemically modified to introduce (photo-)crosslinkable functionalities. These modifications allow to tune the material properties of gelatin, making it suitable for a wide range of biofabrication techniques both as a bioink and as a biomaterial ink (component). The present review provides a non-exhaustive overview of the different reported gelatin modification strategies to yield crosslinkable materials that can be used to form hydrogels suitable for biofabrication applications. The different crosslinking chemistries are discussed and classified according to their mechanism including chain-growth and step-growth polymerization. The step-growth polymerization mechanisms are further classified based on the specific chemistry including different (photo-)click chemistries and reversible systems. The benefits and drawbacks of each chemistry are also briefly discussed. Furthermore, focus is placed on different biofabrication strategies using either inkjet, deposition or light-based additive manufacturing techniques, and the applications of the obtained 3D constructs. STATEMENT OF SIGNIFICANCE: Gelatin and more specifically gelatin-methacryloyl has emerged to become one of the gold standard materials as an extracellular matrix mimic in the field of biofabrication. However, also other modification strategies have been elaborated to take advantage of a plethora of crosslinking chemistries. Therefore, a review paper focusing on the different modification strategies and processing of gelatin is presented. Particular attention is paid to the underlying chemistry along with the benefits and drawbacks of each type of crosslinking chemistry. The different strategies were classified based on their basic crosslinking mechanism including chain- or step-growth polymerization. Within the step-growth classification, a further distinction is made between click chemistries as well as other strategies. The influence of these modifications on the physical gelation and processing conditions including mechanical properties is presented. Additionally, substantial attention is put to the applied photoinitiators and the different biofabrication technologies including inkjet, deposition or light-based technologies.

We report on the design, fabrication, and characterization of millimeter-length strip waveguides with monolithic grating couplers in commercially available synthetic single-crystal diamond. To minimize the device footprint and the influence of the wafer wedge of the single-crystal diamond thin plate, we adopt a curled waveguide layout. The devices are fabricated using electron-beam lithography and reactive-ion etching. To improve the e-beam patterning accuracy of the grating etch masks, we apply proximity-effect compensation on the gratings and tapers. The linear characterization results indicate a waveguide attenuation of 6.5 dB/mm and a grating transmission of -6.3 dB in the fiber-optic communication C band. These results demonstrate the feasibility of fabricating long waveguides and integrated grating couplers in single-crystal diamond. Our research findings would be beneficial for further exploring quantum and nonlinear optics in integrated single-crystal diamond devices.

Compact and broadband spectrometers are very useful in various application domains such as agriculture, food, health and security. In minimally-invasive image-guided procedures spectral tissue sensing helps for screening and diagnostic purposes aiming at discriminating between healthy and tumorous tissue at point-of-care locations and outpatient centers. We designed a compact spectrometer based on a three-segment diffraction grating which operates from 300 nm to 1700 nm. The first two segments of the grating cover spectral ranges from ultraviolet to visible and near infrared (UV, VIS/NIR), whereas the third segment covers the short-wave infrared (SWIR) region from 830 nm to 1700 nm. The spectrometer has a resolution of 6 nm in the UV-VIS/NIR ranges and 10 nm in the SWIR range. The smallest signal-to-noise ratio (SNR) of the spectrometer achieved in the VIS range is 650 and in the SWIR range 9300. Afterwards, the designed three-segment grating was fabricated in-house with ultra-precision diamond tooling followed by hot embossing and quantitatively characterized. The experimental results show that the three-segment grating improves the diffraction efficiency in the NIR-SWIR wavelength range by at least a factor of 2 compared to a Richardson grating which only operates from 300 nm to 1100 nm. This result paves the way towards a new approach for making compact and low-cost spectrometers which could be integrated with hand held devices such as tablets and smartphones.

The use of graphene in optical and photonic applications has gained much attention in recent years. To maximize the exploitation of graphene's extraordinary optical properties, precise control over its Fermi level (e.g. by means of chemical doping) will be of vital importance. In this work, we show the usage of a versatile p-doping strategy based on the incorporation of bis(trifluoromethanesulfonyl)amide (TFSA), functioning as an active p-dopant molecule, into a poly(2,2,3,3,4,4,5,5-octafluoropentyl methacrylate) (POFPMA) polymer matrix. The TFSA/POFPMA dopant can be utilized both onto large size graphene regions via spin coating and on small predefined spatial zones of micrometer dimension by localized inkjet printing. Whereas pure TFSA suffers from a clustered layer deposition combined with environmental instability, the application of the POFPMA polymer matrix yields doping layers revealing superior properties counteracting the existing shortcomings of pure TFSA. A first key finding relates to the optical quality of the dopant layer. We obtain a layer with an extremely low surface roughness (0.4-0.8 nm/25 μm 2) while exhibiting very high transparency (absorbance

Optical fiber technology is the driving force behind the ever-increasing data transport and internet usage in today's connected society. The tight alignment tolerance required when connecting single-mode telecom fibers become even more tight when multiple fiber connectors are being used in the optical link. To alleviate this, we expand the mode field of the fiber and use 3D nanoprinting to print taper structures that can relax alignment tolerances in physical contact expanded beam connectors. We present the design and fabrication of a linear taper which expands the fundamental mode of a single-mode telecom fiber adiabatically with a factor of 3. The taper itself was fabricated on top of a cleaved fiber facet with the two-photon polymerization-based 3D nanoprinting technique, which allows fabrication of high aspect-ratio structures with submicrometer resolution. A proof-of-concept demonstrator was built to measure the obtained misalignment tolerance relaxation. Experimental results for lateral misalignment show excellent agreement with simulated values, but the beam expansion with an air-cladding taper also induces an excess loss of about 0.22 dB compared to a standard physical contact connection without beam expansion. This shows the compromise that has to be made between insertion loss and misalignment tolerance relaxation. The use of additive manufacturing techniques in fiber beam expansion applications makes it possible to fabricate taper structures with full 3D design freedom.

In the present work, gelatin type B is modified with highly reactive norbornene functionalities (Gel-NB) following a one-pot synthesis approach to enable subsequent thiol-ene photo-click crosslinking. The modification strategy displays close control over the amount of introduced functionalities. Additionally, Gel-NB exhibits considerably improved processing capabilities in terms of two-photon polymerization when benchmarked to earlier-reported crosslinkable gelatin derivatives (e.g., gelatin-methacrylamide (Gel-MOD) and gelatin-methacrylamide-aminoethylmethacrylate (Gel-MOD-AEMA)). The improvement is especially apparent in terms of minimally required laser power (20 mW vs >= 60 mW (Gel-MOD) vs >= 40 mW (Gel-MOD-AEMA) at 100 mm s(-1) scan speed) and processable concentration range (>= 5 w/v% vs >= 10 w/v% (Gel-MOD/Gel-MOD-AEMA)). Furthermore, the proposed functionalization scheme maintains the excellent biocompatibility and cell interactivity of gelatin. Additionally, the norbornene functionalities have potential for straightforward postprocessing thiol-ene surface grafting of active molecules. As a consequence, a very promising material toward tissue engineering applications and more specifically, biofabrication, is presented.

The appearance or printing quality of paper surfaces is mostly characterized by their glossiness, measured with a glossmeter as specular reflectance. The gloss properties of a base paper substrate can be improved after application of a poly(styrene-co-maleimide) nanoparticle coating under pure conditions or in the presence of different vegetable oils. The specular gloss properties of 11 different nanoparticle paper coatings have been determined under 75 degrees and 85 degrees incident light angles, with good relation between values along parallel (machine) and perpendicular (transverse) direction. The gloss properties for the different coatings have been further related to the surface topography. Therefore, the statistical and spatial surface roughness parameters have been studied in detail at two length scales including non-contact profilometry (1 x 1 mm(2)) and atomic force microscopy (2 x 2 m(2)). Based on values of the Rayleigh parameter for non-contact profilometry, the surfaces can be considered as optically rough. The gloss values cannot be directly related to statistical surface roughness parameters. Otherwise, an experimental power-law model for gloss has been proposed as a function of (/Sq) with correlation length and root-mean-square roughness Sq. A best-fit model illustrates that gloss properties of various nanoparticle paper coatings mainly relate to the spatial surface roughness parameters determined from non-contact profilometry. (c) 2015 Wiley Periodicals, Inc. Col Res Appl, 41, 596-610, 2016

We present an optofluidic lab-on-chip for confocal Raman spectroscopy, which can be used for the analysis of substances. The device strongly suppresses unwanted background signals because it enables confocal detection of Raman scattering thanks to a free-form reflector embedded in the optofluidic chip. We design the system using non-sequential ray-tracing combined with a mathematical code to simulate the Raman scattering behavior of the substance under test. We prototype the device in polymethyl methacrylate by means of ultraprecision diamond tooling. In a proof-of-concept demonstration, we first show the confocal behavior of our Raman lab-on-chip system by measuring the Raman spectrum of ethanol. In a next step, we compare Raman spectra measured in our lab-on-chip with spectra measured with a commercial Raman spectrometer. Finally, to calibrate the system we perform Raman measurements on urea solutions with different concentrations with our proposed experimental proof-of-concept setup. We achieved a detection limit that corresponds to the noise equivalent concentration of 20 mM.

We present our technology supply chain for freeform polymer optics. We show through recent examples how this technology supply chain is a key-enabler for frontier applied research and demonstrate how it paves the way towards efficient technology take-up and effective industrial innovation.

We present an optofluidic chip in polymethyl methacrylate (PMMA) that combines optical trapping of single particles with confocal Raman spectroscopy. We introduce the design of the optofluidic chip and the ray-tracing simulations combined with mathematical calculations used to determine the optical forces exerted on the particles and to model the excitation and collection of Raman scattering. The optical trapping is done using a single-beam gradient trap realized by a high numerical aperture free-form reflector, monolithically embedded in the optofluidic chip. The focused beam functions both as the excitation beam as well as the trapping beam. The embedded free-form reflector is also used to collect the Raman scattered light generated from the trapped particle. We discuss the fabrication process for the prototyping of the chip, which consists of an ultraprecision diamond turning step and a sealing step. Finally, we demonstrate the functionality of the optofluidic chip in a proof-of-concept experimental setup and trap polystyrene beads with diameters from 6 to 15 $\ \mu \text{m}$ . We characterize the maximal transverse optical trap strength in the sample flow direction using the drag force method, measuring average efficiencies that lie between 0.11 and 0.36, and perform confocal Raman measurements of these particles.

We present the design and fabrication of an optofluidic chip combining optical trapping with confocal Raman spectroscopy. Next, we demonstrate the functionality of the chip while trapping polystyrene beads with diameters from 6 to 15µm.

This paper describes a three-channel multiresolution imaging system which contains hybrid lenses to reduce the chromatic aberrations of the system. We present the optical design, fabrication and experimental characterization of the optical system.

We present a comprehensive mathematical formalism, together with its numerical implementation, for optical time-domain reflectometry (OTDR) simulations with the ability to produce OTDR traces of passive optical point-to-multipoint networks (PONs). According to our knowledge, the proposed OTDR simulator is the first one with this feature, thus progressing beyond the existing state of the art of OTDR simulations. In our approach, the optical fiber network under test is treated as a linear time-invariant single-input/single-output system, which allows us to include the effects of the OTDR pulse shape. Furthermore, the limitations of current OTDR equipment such as (nonlinear) power saturation of the detector, limited dynamic range due to OTDR noise, and the finite bandwidth of the detector, are also incorporated into the simulation model. Our simulation results are experimentally verified with OTDR measurements, and we show an excellent agreement between the simulated traces and the measured traces. Especially, we demonstrate that the simulations properly handle PONs, that may contain a cascade of splitters, providing an important tool for telecom operators to investigate cost-effective centralized monitoring techniques in fiber-to-the-home networks.

The World Health Organization accepts chromatographic fingerprints as a tool for identification and quality control of herbal medicines. This is the first study in which the distinction, identification and quality control of four different Artemisia species, i.e. Artemisia vulgaris, A. absinthium, A. annua and A. capillaris samples, is performed based on the evaluation of entire chromatographic fingerprint profiles developed with identical experimental conditions. High-Performance Liquid Chromatography (HPLC) with Diode Array Detection (DAD) was used to develop the fingerprints. Application of factorial designs leads to methanol/water (80:20 (v/v)) as the best extraction solvent for the pulverised plant material and to a shaking bath for 30 min as extraction method. Further, so-called screening, optimisation and fine-tuning phases were performed during fingerprint development. Most information about the different Artemisia species, i.e. the highest number of separated peaks in the fingerprint, was acquired on four coupled Chromolith columns (100 mm × 4.6 mm I.D.). Trifluoroacetic acid 0.05% (v/v) was used as mobile-phase additive in a stepwise linear methanol/water gradient, i.e. 5, 34, 41, 72 and 95% (v/v) methanol at 0, 9, 30, 44 and 51 min, where the last mobile phase composition was kept isocratic till 60 min. One detection wavelength was selected to perform data analysis. The lowest similarity between the fingerprints of the four species was present at 214 nm. The HPLC/DAD method was applied on 199 herbal samples of the four Artemisia species, resulting in 357 fingerprints. The within- and between-day variation of the entire method, as well as the quality control fingerprints obtained during routine analysis, were found acceptable. The distinction of these Artemisia species was evaluated based on the entire chromatographic profiles, developed by a shared method, and visualised in score plots by means of the Principal Component Analysis (PCA) exploratory data-analysis technique. Samples of different quality could be indicated on the score plots. No multi-component analysis was required to reach the goal. Furthermore, differences related to the origin of some of the not-certified samples were shown. The importance of the specific herbal part used for its identification was also presented. In addition, no differences were observed among fingerprints of lyophilised or conditioned-air dried samples. Finally, a classification technique, Soft Independent Modelling by Class Analogy (SIMCA), was successfully evaluated as identification technique for unknown samples. Six additional Artemisia species (29 herbal samples) were identified as not belonging to any of the four modelled classes. The developed chromatographic fingerprints and the evaluation of the entire profiles provide an added value to the distinction, identification and quality control of the simultaneously investigated Artemisia species.

We experimentally demonstrate a negative Kerr nonlinearity for quasiundoped graphene. Hereto, we introduce the method of chirped-pulse-pumped self-phase modulation and apply it to graphene-covered silicon waveguides at telecom wavelengths. The extracted Kerr-nonlinear index for graphene equals n(2,gr) = -10(-13) m(2) = W. Whereas the sign of n(2,gr) turns out to be negative in contrast to what has been assumed so far, its magnitude is in correspondence with that observed in earlier experiments. Graphene's negative Kerr nonlinearity strongly impacts how graphene should be exploited for enhancing the nonlinear response of photonic (integrated) devices exhibiting a positive nonlinearity. It also opens up the possibility of using graphene to annihilate unwanted nonlinear effects in such devices, to develop unexplored approaches for establishing Kerr processes, and to extend the scope of the "periodic poling" method often used for second-order nonlinearities towards third-order Kerr processes. Because of the generic nature of the chirped-pulse-pumped self-phase modulation method, it will allow fully characterizing the Kerr nonlinearity of essentially any novel (2D) material.

Deep proton writing ( DPW) is a fabrication technology developed for the rapid prototyping of polymer microstructures. We use polymethylmethacrylate (PMMA) substrates, which act as a positive resist, for irradiation with a collimated 12-MeV energy proton beam. Using 12 MeV enables the irradiation of increasingly thick PMMA substrates with less conicity of the sidewalls compared to the lower energies used in previous work. A microhole of 47.7 mu m diameter over a depth of 1 mm is achieved, leading to a maximum aspect ratio of 21:1. The sidewalls of the irradiated structures show a slightly conical shape and their root-mean-square surface roughness is lower than 50 nm averaged over 72 measured areas of 56 mu m x 44 mu m. This means that DPW components have optical surface quality sidewalls for wavelengths larger than 400 nm. Based on the trade-off among the sidewall roughness, conicity, and the development time, we determine that the optimal proton fluence for 12-MeV DPW in PMMA is 7.75 x 10(6) mu m(- 2.) Finally, we discuss some high aspect ratio microstructures with optical surface quality that were created with DPW to be used for a myriad of applications, such as micromirrors, microlenses, optofluidic devices, and high-precision alignment structures for single-mode optical fiber connectors. (C) 2016 Society of Photo-Optical Instrumentation Engineers (SPIE)

This paper presents the hot-embossing replication of self-centering fiber alignment structures for high-precision, single-mode optical fiber connectors. To this end, a metal mold insert was fabricated by electroforming a polymer prototype patterned by means of deep proton writing (DPW). To achieve through-hole structures, we developed a postembossing process step to remove the residual layer inherently present in hot-embossed structures. The geometrical characteristics of the hot-embossed replicas are compared, before and after removal of the residual layer, with the DPW prototypes. Initial measurements on the optical performance of the replicas are performed. The successful replication of these components paves the way toward low-cost mass replication of DPW-fabricated prototypes in a variety of high-tech plastics. (C) 2016 Society of Photo-Optical Instrumentation Engineers (SPIE).

In parallel with the development of nanoparticle coatings for protection of paper substrates, detailed descriptions of the surface topography with micro-to nanoscale roughness features are needed. In this work, papers have been coated with poly(styrene-co-maleic anhydride) nanoparticles including different types of vegetable oils and the surface roughness was evaluated at 2000 x 2000 mu m(2) to 0.2 x 0.2 mu m(2) length scales by combining non-contact optical profilometry (NCP) and atomic force microscopy (AFM). The relationships between roughness data were studied for statistical roughness parameters, spatial roughness parameters and in the frequency domain. In order to compare AFM roughness more accurately, the original images were flattened to remove effects of the underlaying fibrous substrate and highlight features of the nanoparticle coating. More detailed information on the coating topography could be obtained by considering bearing ratio curves and histograms, where it was concluded that the oil-filledcoatings form a rather thin and continuous coating that closely follows the shape of the cellulose fibers. The relation between statistical roughness parameters from NCP and AFM follows an exponential trend with relatively low coefficient of determination. The increase in surface roughness with length scale showed a transition point attributed to short-and long-range surface features. Therefore, the correlation length was used as a spatial roughness parameter that provides a successful extrapolation of the average roughness over different length scales in a double logarithmic diagram with very high coefficient of determination. Based on the power spectral density, it was difficult to exactly distinguish between the different types of SMI/oil coatings, as they include similar nanoscale features. The frequency roughness parameters were better suited for extrapolation than statistical roughness parameters but little less efficient than the spatial roughness parameters. (C) 2015 Elsevier Ltd. All rights reserved.

Over the past decades, solid freeform fabrication (SFF) has emerged as the main technology for the production of scaffolds for tissue engineering applications as a result of the architectural versatility. However, certain limitations have also arisen, primarily associated with the available, rather limited range of materials suitable for processing. To overcome these limitations, several research groups have been exploring novel methodologies through which a construct, generated via SFF, is applied as a sacrificial mould for production of the final construct. The technique combines the benefits of SFF techniques in terms of controlled, patient-specific design with a large freedom in material selection associated with conventional scaffold production techniques. Consequently, well-defined 3D scaffolds can be generated in a straightforward manner from previously difficult to print and even "unprintable" materials due to thermomechanical properties that do not match the often strict temperature and pressure requirements for direct rapid prototyping. These include several biomaterials, thermally degradable materials, ceramics and composites. Since it can be combined with conventional pore forming techniques, indirect rapid prototyping (iRP) enables the creation of a hierarchical porosity in the final scaffold with micropores inside the struts. Consequently, scaffolds and implants for applications in both soft and hard tissue regeneration have been reported. In this review, an overview of different iRP strategies and materials are presented from the first reports of the approach at the turn of the century until now.

We report on the design, fabrication and experimental demonstration of single-crystal diamond (SCD) waveguide devices with integrated grating couplers optimized for 850 nm, 635 nm, and 405 nm wavelengths, respectively. The devices are fabricated on SCD thin films using electron-beam lithography and reactive-ion etching technologies. To reduce the wafer wedge typically present in commercial diamond plates, we introduce a novel tilted-etching technique in the preparation of the thin films. We obtain 60% reduction of the wafer wedge, namely from 300 to 120 nm mm(-1), enabling us to properly fabricate the devices designed for the short to ultra-short wavelengths considered here. Using light in- and out-coupling with 50 mu m core tapered fibers, we measure total (input plus output) grating coupling losses of 20.6 dB and 22.7 dB, and waveguide losses of 11 dB mm(-1)and 20.5 dB mm(-1)for the 850 nm and 635 nm wavelength devices, respectively. The 405 nm wavelength devices, tested with a lensed 9 mu m core input fiber and with the same tapered output fiber as employed for the other devices, also demonstrate light guidance, and feature total grating coupling and waveguide losses on the order of 33.1 dB and 46.7 dB mm(-1), respectively. These results showcase the possibility of down-scaling grating-coupled SCD devices for VIS-UV operation, and pave the way for exploiting diamond's properties on photonic chips at extremely short wavelengths.

We present a microfluidic chip in Polymethyl methacrylate (PMMA) for optical trapping of particles in an 80 mu m wide microchannel using two counterpropagating single-mode beams. The trapping fibers are separated from the sample fluid by 70 mu m 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 6 mu m, 8 mu m and 10 mu m and use the power spectrum analysis of the trapped particle displacements to characterize the trap strength. (c) 2015 Optical Society of America

We present a new approach to remove monolayer graphene transferred on top of a silicon-on-insulator (SOI) photonic integrated chip. Femtosecond laser ablation is used for the first time to remove graphene from SOI waveguides, whereas oxygen plasma etching through a metal mask is employed to peel off graphene from the grating couplers attached to the waveguides. We show by means of Raman spectroscopy and atomic force microscopy that the removal of graphene is successful with minimal damage to the underlying SOI waveguides. Finally, we employ both removal techniques to measure the contribution of graphene to the loss of grating-coupled graphene-covered SOI waveguides using the cut-back method. (C) 2015 Optical Society of America

The present work describes for the first time the production of self-supporting low gelatin density (

For mass production of multiscale-optical components, microstructured moulding tools are needed. Metal tools are used for hot embossing or injection moulding of microcomponents made of a thermoplastic polymer. Microstructures with extremely tight specifications, e.g. low side wall roughness and high aspect ratios are generally made by lithographic procedures such as x-ray lithography or deep proton writing. However, these processes are unsuitable for low-cost mass production. An alternative manufacturing method of moulding tools has been developed at the Karlsruhe Institute of Technology (KIT). This article describes a mould insert fabrication and a new replication process for self-centring fibre alignment structures for low loss field installable single-mode fibre connectors, developed and fabricated by the Vrije Universiteit Brussel (VUB) in collaboration with TE Connectivity. These components are to be used in fibre-to-the-home networks and support the deployment and maintenance of fibre optic links. The special feature of this particular fibre connector is a self-centring fibre alignment, achieved by means of a through hole with deflectable cantilevers acting as micro-springs. The particular challenge is the electroforming of through holes with a centre hole diameter smaller than 125 mu m. The fibre connector structure is prototyped by deep proton writing in polymethylmethacrylate and used as a sacrificial part. Using joining, physical vapour deposition and electroforming technology, a negative copy of the prototyped connector is transferred into nickel to be used as a moulding tool. The benefits of this replication technique are a rapid and economical fabrication of moulding tools with high-precision microstructures and a long tool life. With these moulding tools low-cost mass production is possible. We present the manufacturing chain we have established. Each individual manufacturing step of the mould insert fabrication will be shown in this report. The process reliability and suitability for mass production was tested by hot embossing.

To boost the deployment of fiber-to-the-home networks in order to meet the ever-increasing demand for bandwidth, there is a strong need for single-mode fiber (SMF) connectors which combine low insertion loss with field installability. Shifting from ferrule-based to ferruleless connectors can reduce average insertion losses appreciably and minimize modal noise interference. We propose a ferruleless connector and adaptor in which physical contact between two inline fibers is ensured by at least one fiber being in a buckled state. To this end, we design a buckling cavity in which the SMF can buckle in a controlled way to ensure good optical performance as well as mechanical stability. This design is based on both mechanical and optical considerations. Finite element analysis suggests that mechanically a minimal buckling cavity length of 17 mm is required, while the height of the cavity should be chosen such that the buckled SMF is not mechanically confined to ensure buckling in a first-order mode. The optical bending loss in the buckled SMF is calculated using a fully vectorial mode solver, showing that a minimal buckling cavity length of 20 mm is necessary to keep the excess optical loss from bending below 0.1 dB. Both our optical and mechanical simulation results are experimentally verified. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)

We present our polymer micro-optics technology supply chain and its key constituents. We show how it is a key-enabler for frontier applied research and demonstrate how it paves the way towards efficient technology take-up and effective industrial innovation.

In conventional multichannel imaging systems, all channels have similar imaging properties [field-of-view (FOV) and angular resolution]. In our approach, channels are designed to have different imaging properties which add multiresolution capability to the system. We have experimentally demonstrated, for the first time to our knowledge, a three-channel imaging system which simultaneously captures multiple images having different magnifications and FOVs on an image sensor. Each channel consists of four aspherical lens surfaces fabricated from four PMMA plates by ultraprecision diamond tooling and of a baffle made from a titanium (Ti) and aluminum (Al) based metal alloy. The integrated imaging system is able to record a FOV of 7.6 degrees with the first channel and 73 degrees with the third channel while having a magnification ratio of about 6 between them. The experimental and simulation results, specifically the FOV and magnification ratios, are comparable, and this paves a way for low-cost, compact imaging systems which can embed smart imaging functionalities. (c) 2013 Optical Society of America

The use of photonics in data communication and numerous other industrial applications brought plenty of prospects for innovation and opened up different unexplored market opportunities. This is a major driving force for the fabrication of micro-optical and micro-mechanical structures and their accurate alignment and integration into opto-mechanical modules and systems. To this end, we present Deep Proton Writing (DPW) as a powerful rapid prototyping technology for such micro-components. The DPW process consists of bombarding polymer samples (PMMA or SU-8) with swift protons, which results after chemical processing steps in high-quality micro-optical components. One of the strengths of the DPW micro-fabrication technology is the ability to fabricate monolithic building blocks that include micro-optical and mechanical functionalities which can be precisely integrated into more complex photonic systems. In this paper we comment on how we shifted from using 8.3 to 16.5 MeV protons for DPW and give some examples of micro-optical and micro-mechanical components recently fabricated through DPW, targeting applications in optical interconnections and in optofluidics. (C) 2012 Elsevier B.V. All rights reserved.

The European Commission's goal of providing each European household with at least a 30 Mb s(-1) Internet connection by 2020 would be facilitated by a widespread deployment of fibre-to-the-home, which would in turn be sped up by the development of connector essential components, such as high-precision alignment features. Currently, the performance of state-of-the-art physical contact optical fiber connectors is limited by the tolerance on the cladding of standard telecom-grade single-mode fiber (SMF), which is typically smaller than +/- 1 mu m. We propose to overcome this limit by developing micro-spring-based self-centering alignment structures (SCAS) for SMF-connectors. We design these alignment structures with robustness and low-cost replication in mind, allowing for large-scale deployment. Both theoretical and finite element analysis (FEA) models are used to determine the optimal dimensions of the beams of which the micro-springs of the SCAS are comprised. Two topologies of the SCAS, consisting of three and four micro-springs respectively, are investigated for two materials: polysulfone (PSU) and polyetherimide (PEI). These materials hold great potential for high-performance fiber connectors while being compatible with low-cost production and with the harsh environmental operation conditions of those connectors. The theory and FEA agree well (

Transmitting ultra-high symbol rate optical signals remains a challenge due to their high sensitivity to fluctuations of GVD and higher orders of dispersion in the transmission link. Being able to cancel the impairments due to those fluctuations is a key requirement to make transmission of ultrashort optical pulses practical. We demonstrate an automatic compensation scheme able to keep an Optical Time Division Multiplexed (OTDM) signal stable at a bandwidth of up to 1.28 Tbaud in spite of external perturbations. Our approach is based on monitoring the signal with a photonic-chip-based all-optical RF-spectrum analyzer. The measurement of a single parameter extracted from the RF-spectrum is used to drive a multidimensional optimization algorithm. We apply the method to the real time simultaneous compensation for 2 nd , 3 rd and 4 th order dispersion using an LCOS spectral pulse shaper (SPS) as a tunable dispersion compensator.