Force-displacement relationship in micro-metric pantographs: experiments and numerical simulations

International audienceIn this paper, we reveal that the mathematical discrete model of Hencky type, introduced in [1], is appropriate for describing the mechanical behavior of micro-metric pantographic elementary modules. This behavior does not differ remarkably from what has been observed for milli-metric modules, as we prove with suitably designed experiments. Therefore, we conclude that the concept of pantographic microstructure seems feasible for micro-metrically architected microstructured (meta)materials as well. These results are particularly indicative of the possibility of fabricating materials that can have an underlying pantographic microstructure at micrometric scale, so that its unique behavior can be exploited in a larger range of technological applications

Optical trapping with superfocused high-M2 laser diode beam

Many applications of high-power laser diodes demand tight focusing. This is often not possible due to the multimode nature of semiconductor laser radiation possessing beam propagation parameter M2 values in double-digits. We propose a method of 'interference' superfocusing of high-M2 diode laser beams with a technique developed for the generation of Bessel beams based on the employment of an axicon fabricated on the tip of a 100 μm diameter optical fiber with highprecision direct laser writing. Using axicons with apex angle 140º and rounded tip area as small as 10 μm diameter, we demonstrate 2-4 μm diameter focused laser 'needle' beams with approximately 20 μm propagation length generated from multimode diode laser with beam propagation parameter M2=18 and emission wavelength of 960 nm. This is a few-fold reduction compared to the minimal focal spot size of 11 μm that could be achieved if focused by an 'ideal' lens of unity numerical aperture. The same technique using a 160º axicon allowed us to demonstrate few-μm-wide laser 'needle' beams with nearly 100 μm propagation length with which to demonstrate optical trapping of 5-6 μm rat blood red cells in a water-heparin solution. Our results indicate the good potential of superfocused diode laser beams for applications relating to optical trapping and manipulation of microscopic objects including living biological objects with aspirations towards subsequent novel lab-on-chip configurations

Superfocusing of high-M2 semiconductor laser beams:experimental demonstration

The focusing of multimode laser diode beams is probably the most significant problem that hinders the expansion of the high-power semiconductor lasers in many spatially-demanding applications. Generally, the 'quality' of laser beams is characterized by so-called 'beam propagation parameter' M2, which is defined as the ratio of the divergence of the laser beam to that of a diffraction-limited counterpart. Therefore, M2 determines the ratio of the beam focal-spot size to that of the 'ideal' Gaussian beam focused by the same optical system. Typically, M2 takes the value of 20-50 for high-power broad-stripe laser diodes thus making the focal-spot 1-2 orders of magnitude larger than the diffraction limit. The idea of 'superfocusing' for high-M2 beams relies on a technique developed for the generation of Bessel beams from laser diodes using a cone-shaped lens (axicon). With traditional focusing of multimode radiation, different curvatures of the wavefronts of the various constituent modes lead to a shift of their focal points along the optical axis that in turn implies larger focal-spot sizes with correspondingly increased values of M2. In contrast, the generation of a Bessel-type beam with an axicon relies on 'self-interference' of each mode thus eliminating the underlying reason for an increase in the focal-spot size. For an experimental demonstration of the proposed technique, we used a fiber-coupled laser diode with M2 below 20 and an emission wavelength in ~1μm range. Utilization of the axicons with apex angle of 140deg, made by direct laser writing on a fiber tip, enabled the demonstration of an order of magnitude decrease of the focal-spot size compared to that achievable using an 'ideal' lens of unity numerical aperture

Graphene-doped photo-patternable ionogels: tuning of conductivity and mechanical stability of 3D microstructures

This work reports for the first time the development of enhanced conductivity, graphene- doped photo-patternable hybrid organic-inorganic ionogels and the effect of the subsequent materials condensation on the conductivity and mechanical stability of three- dimensional microstructures fabricated by multi-photon polymerisation (MPP). Ionogels were based on photocurable silicon/zirconium hybrid sol-gel materials and phosphonium (trihexyltetradecylphosphonium dicyanamide [P6,6,6,14][DCA] ionic liquid (IL). To optimise the dispersion of graphene within the ionogel matrices, aqueous solutions of graphene were prepared, as opposed to the conventional graphene powder approach, and employed as catalysts of hydrolysis and condensation reactions occurring in the sol-gel process. Ionogels were prepared via a two step process by varying the hydrolysis degree from 25 to 50%, IL content between 0-50 w/w%, and the inorganic modifier (zirconate complex) concentration from 30 to 60 mol.% against the photocurable ormosil and they were characterised via Raman, Electrochemical Impedance Spectroscopy and Transmission Electron Microscopy. MPP was performed on the hybrid ionogels, resulting in three- dimensional microstructures that were characterised using scanning electron microscopy. It is clearly demonstrated that the molecular formulation of the ionogels, including the concentration of graphene and the zirconate network modifier, play a critical role in the conductivity of the ionogels and influence the resulting mechanical stability of the fabricated three-dimensional microstructures. This work aims to establish for the first time the relationship between the molecular design and condensation of materials in the physico-chemistry and dynamic of ionogels

3D laser nano-printing on fibre paves the way for super-focusing of multimode laser radiation

Multimode high-power laser diodes suffer from inefficient beam focusing, leading to a focal spot 10–100 times greater than the diffraction limit. This inevitably restricts their wider use in ‘direct-diode’ applications in materials processing and biomedical photonics. We report here a ‘super-focusing’ characteristic for laser diodes, where the exploitation of self-interference of modes enables a significant reduction of the focal spot size. This is achieved by employing a conical microlens fabricated on the tip of a multimode optical fibre using 3D laser nano-printing (also known as multi-photon lithography). When refracted by the conical surface, the modes of the fibre-coupled laser beam self-interfere and form an elongated narrow focus, usually referred to as a ‘needle’ beam. The multiphoton lithography technique allows the realisation of almost any optical element on a fibre tip, thus providing the most suitable interface for free-space applications of multimode fibre-delivered laser beams. In addition, we demonstrate the optical trapping of microscopic objects with a super-focused multimode laser diode beam thus rising new opportunities within the applications sector where lab-on-chip configurations can be exploited. Most importantly, the demonstrated super-focusing approach opens up new avenues for the ‘direct-diode’ applications in material processing and 3D printing, where both high power and tight focusing is required

High laser induced damage threshold photoresists for nano-imprint and 3D multi-photon lithography

Optics manufacturing technology is predicted to play a major role in the future production of integrated photonic circuits. One of the major drawbacks in the realization of photonic circuits is the damage of optical materials by intense laser pulses. Here, we report on the preparation of a series of organic-inorganic hybrid photoresists that exhibit enhanced laser-induced damage threshold. These photoresists showed to be candidates for the fabrication of micro-optical elements (MOEs) using three-dimensional multiphoton lithography. Moreover, they demonstrate pattern ability by nanoimprint lithography, making them suitable for future mass production of MOEs

Nerve guides manufactured from photocurable polymers to aid peripheral nerve repair

The peripheral nervous system has a limited innate capacity for self-repair following injury, and surgical intervention is often required. For injuries greater than a few millimeters autografting is standard practice although it is associated with donor site morbidity and is limited in its availability. Because of this, nerve guidance conduits (NGCs) can be viewed as an advantageous alternative, but currently have limited efficacy for short and large injury gaps in comparison to autograft. Current commercially available NGC designs rely on existing regulatory approved materials and traditional production methods, limiting improvement of their design. The aim of this study was to establish a novel method for NGC manufacture using a custom built laser-based microstereolithography (μSL) setup that incorporated a 405 nm laser source to produce 3D constructs with ∼50 μm resolution from a photocurable poly(ethylene glycol) resin. These were evaluated by SEM, in vitro neuronal, Schwann and dorsal root ganglion culture and in vivo using a thy-1-YFP-H mouse common fibular nerve injury model. NGCs with dimensions of 1 mm internal diameter × 5 mm length with a wall thickness of 250 μm were fabricated and capable of supporting re-innervation across a 3 mm injury gap after 21 days, with results close to that of an autograft control. The study provides a technology platform for the rapid microfabrication of biocompatible materials, a novel method for in vivo evaluation, and a benchmark for future development in more advanced NGC designs, biodegradable and larger device sizes, and longer-term implantation studies

Aισθητήριες διατάξεις αντιχείων συντονισμού εγγεγραμμένες σε οπτικές ίνες με χρήση μη γραμμικού φωτοπολυμερισμού

The aim of this thesis is to combine standard single mode optical fibers and multi-photon lithography in order to produce robust and functional fiber-optic devices that will be exploited as fiber-optic gas or liquid sensors in the time being; also constituting basic platforms for other sensing and switching devices. Further, this study investigates in depth the advantages and disadvantages of the direct laser fabrication of micro-optical light resonating cavities onto the optical fibers. For these reasons two different kinds of light resonating structures were fabricated by multi-photon polymerization technique directly onto single mode, telecom optical fibers. In particular, (i) fiber-optic devices based on Fabry-Perot interferometers fabricated on the endface of optical fibers and (ii) whispering-gallery mode micro-ring resonators fabricated directly onto tapered optical fibers will be demonstrated. In both cases hybrid organic-inorganic photosensitive materials were used for the fabrication of the light resonating components. For the fabrication of these fiber-optic interferometric devices new processes for the sample preparation had to be explored and developed. Furthermore, the designs of the interferometric components and their fabrication parameters had to be studied in order to produce functional structures. The spectral performance of both resonating devices was tested in the wavelength region of 1550 nm. Initially, three different designs of fiber-optic devices based on Fabry-Perot interferometers were fabricated on the endface of optical fibers, namely, microdrum, microfunnel and microprism. Their fringe visibilities that were recorded were up to ~22 dB, while the free spectral range (FSR) was differ according to the length cavity, for example for a length cavity of ~18 μm the corresponding FSR was ~68 nm. The microprism endface FP devices were tested as vapor sensors for the detection of ethanol and chlorinated organic solvents vapors with sensitivities down to 4 ppm. Their sensitivities with respect to the surface porosity of the fabricated micro-prism sensing head were studied by reducing the pores size of the photopolymerized reflection surface using annealing treatment processes. Results show the dependence of the sensitivity and selectivity/specificity upon the porosity of the fabricated FP cavity. Moreover, the studies revealed that the operation mechanism of the fabricated sensing device did not depended only on the optical and vapor pressure characteristics of the solvents but also on their molecular structure and affinity to the sensing surfaces, thus revealing distinct spectral response versus sensing time. In addition, sensing mechanism of the devices was based on physisorption phenomena, allowing the full recover of the sensing probes to their initial spectral response (after pumping with nitrogen) and their reuse for several times. Sensitivities of ~1.5 × 10^3 nm/RIU were recorded in case of ethanol vapors. For the liquid sensor the device was used for the probing of substances of oily media. Experiments revealed that each of the oily liquids exhibited a distinct spectral response versus sensing time, depending on the diffusion process of the oily liquid into the photopolymerized porous reflection surface.Regarding to the second part of this thesis, we proposed and realized for the first time the fabrication of micro-ring resonators directly onto tapered single mode telecommunication fibers by employing the multi-photon polymerization technique. Such an approach allowed the demonstration of integrated and robust tapered optical fiber WGM resonators of straightforward packaging capabilities. A fabricated ring of 20 μm diameter and thickness of ~900 nm onto a taper of 2.4 μm diameter, revealed fringe visibility of 20 dB and Q factor of 3.55 x 10^3 operating in TM polarization. Parallel coupled microresonators with different center to center distances were also fabricated resulting fringe visibility of 38.5 dB and Q factor of 3.82 x 10^3 operating in TE polarization. The single and coupled microring resonators were tested as vapor sensors for ethanol species. The parallel coupled ring sensor was also tested as a pressure sensor for the detection of inert gases (N2 and Kr). Pressures down to 50 mbar were recorded.Στόχος της παρούσας διδακτορικής διατριβής είναι ο συνδιασμός της τεχνικής της πολυφωτονικής λιθογραφίας και τον οπτικών ινών με σκοπό την κατασκευή εύρωστων και λειτουργικών διατάξεων οπτικών ινών, οι οποίες θα χρησιμοποιηθούν ως αισθητήριες διατάξεις για την ανίχνευση αέριων ή υγρών μέσων. Επιπλέον η παρούσα διατριβή μελετά και παρουσιάζει τα πλεονεκτήματα και τα μειονεκτήματα της απευθείας εγγραφής με Laser, μικρο-οπτικών αντιχείων εγκλωβισμού του φωτός επάνω σε οπτικές ίνες.Δυο διαφορετικά είδη αντιχείων εγκλωβισμού του φωτός κατασκευάστηκαν απευθείας επάνω σε οπτικές ίνες, με την τεχνικής του πολυφωτονικού πολυμερισμού. Συγκεκριμένα θα παρουσιαστούν (i) διατάξεις οπτικών ινών βασισμένες σε Fabry-Perot συμβολόμετρα, κατασκευασμένα στην διατομή οπτικών ινών καθώς επίσης και (ii) αντιχεία τρόπων ψιθυρισμού σε σχήμα δαχτυλιδιού κατασκευασμένα απευθείας επάνω σε θερμικά εφελκυσμένες οπτικές ίνες. Και οι δύο διατάξεις αντιχείων λειτουργούν στην περιοχή των 1550 nm μηκών κύματος. Για την κατασκευή των μικρο-οπτικών αντιχείων χρησιμοποιήθηκαν υβριδικά οργανικά-ανόργανα φωτοευαίσθητα υλικά. Επιπλέον, νέες μέθοδοι αναπτύχθηκαν και χρησιμοποιήθηκαν για την προετοιμασία των δειγμάτων.Αρχικά θα παρουσιαστούν τρεις διαφορετικές διατάξεις οπτικών ινών βασισμένες σε Fabry-Perot συμβολόμετρα, κατασκευασμένες στην διατομή οπτικών ινών, καθώς επίσης και τα αποτελέσματα από την χρήση τους ως αισθητήριες διατάξεις για τον εντοπισμό ατμών οργανικών διαλυτών όπως η αιθανόλη και χλωριωμένων οργανικών διαλυτών. Επιπλέον, οι διατάξεις αυτές χρησιμοποιήθηκαν και για την ανίχνευση ελαιωδών υγρών μέσων. Λόγω της διέγερσης των οπτικών αντιχείων μέσω της διάδοσης του φωτός, μέσω του πυρήνα των οπτικών ινών, μπορούν να καταγραφούν οι διαφορές στον δείκτη διάθλασης που δημιουργούνται λόγω της παρουσίας των ατμών ή των υγρών μέσα στην οπτική κοιλότητα των αντιχείων, μέσω των φασμάτων ανάκλασης.Στην συνέχεια θα παρουσιαστούν διάφορα αντιχεία τρόπων ψιθυρισμού σε σχήμα δαχτυλιδιού κατασκευασμένα απευθείας επάνω σε θερμικά εφελκυσμένες οπτικές ίνες, καθώς επίσης και συζευγμένα αντιχεία τρόπων ψιθυρισμού. Οι διατάξεις αυτές χρησιμοποιήθηκαν ως αισθητήρες για τον εντοπισμό ατμών αιθανόλης, καθώς επίσης και για την ανίχνευση αδρανών αερίων όπως το διατομικό Άζωτο (N2) και το Κρυπτό (Kr), μέσω της καταγραφής των φασμάτων διέλευσης