259 research outputs found
Dielectric and optical properties of organic photorefractive materials
The work presented in this thesis is derived from experimentation in the field of polymeric photorefractive materials. Low T(_g) polymeric composites were prepared, based on the well-known photoconductive polymer PVK (maximum 50% w/w), sensitized with TNF (2% w/w) and C(_60) (0.2% w/w), plasticized with ECZ (maximum 49.3% w/w) and doped with the nonlinear optical materials NPP (50% w/w), DAN (20% w/w), DED (5% w/w), DCNQI (0.5% w/w), ULTRA-DEMI (5% w/w) and DI-DEMI (2% w/w), and their dielectric, linear and non linear optical properties were investigated. All the materials, except DCNQI, exhibited good solubility and sample processibility. The dielectric properties of the composites at 1 KHz and 1 MHz were determined using a parallel-plate capacitance bridge. The dielectric constant and loss at 10 GHz were measured using a novel adaptation of the resonant cavity technique, which was designed for measurements at ambient and elevated temperatures. The method was used to measure of the dielectric constant and loss of two novel, high T(_g), electro-optic polymers at temperatures up to 100 ºC. The dielectric properties measured were typical of polymeric materials. The absorption coefficient and the refractive index at different wavelengths were measured using a spectrophotometer. For the refractive index, an interference fringe analysis was used. The nonlinear measurements consisted of second harmonic generation, to prove the nonhnearity of the composites, two-beam coupling measurements, to prove their photorefractivity and degenerate four-wave mixing to measure their diffraction efficiency. The NPP, DAN, DED and ULTRA-DEMI doped investigated composites exhibited second order nonlinearity with highest the one of ULTRA-DEMI, at 292 pm/V for 19 kV of corona poling field. The photorefractivity of the NPP, DAN and DED doped composites was proven at 632.8 nm, while ULTRA-DEMI doped composites photooxidized before any measurements were possible. The two-beam coupling coefficients measured were lower than 20 cm(^-3), while net gain was observed only in the NPP doped composite. The diffraction efficiencies of the NPP, DAN and DED doped composites were measured at 632.8 nm, and were found to be l0(^-5)-l0(^-6)
Additive Manufacturing: Applications and Directions in Photonics and Optoelectronics
The combination of materials with targeted optical properties and of complex, 3D architectures, which can be nowadays obtained by additive manufacturing, opens unprecedented opportunities for developing new integrated systems in photonics and optoelectronics. The recent progress in additive technologies for processing optical materials is here presented, with emphasis on accessible geometries, achievable spatial resolution, and requirements for printable optical materials. Relevant examples of photonic and optoelectronic devices fabricated by 3D printing are shown, which include light-emitting diodes, lasers, waveguides, optical sensors, photonic crystals and metamaterials, and micro-optical components. The potential of additive manufacturing applied to photonics and optoelectronics is enormous, and the field is still in its infancy. Future directions for research include the development of fully printable optical and architected materials, of effective and versatile platforms for multimaterial processing, and of high-throughput 3D printing technologies that can concomitantly reach high resolution and large working volumes
Understanding sustainable tourism policy : conceptual framework and cognitive mapping
EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Advances in functional assemblies for regenerative medicine
The ability to synthesise bioresponsive systems and selectively active biochemistries using polymer-based materials with supramolecular features has led to a surge in research interest directed towards their development as next generation biomaterials for drug delivery, medical device design and tissue engineering
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
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
Bio-Interfaces Engineering Using Laser-Based Methods for Controlled Regulation of Mesenchymal Stem Cell Response In Vitro
The controlled interfacial properties of materials and modulated behaviours of cells and biomolecules on their surface are the requirements in the development of a new generation of high-performance biomaterials for regenerative medicine applications. Roughness, chemistry and mechanics of biomaterials are all sensed by cells. Organization of the environment at the nano- and the microscale, as well as chemical signals, triggers specific responses with further impact on cell fate. Particularly, human mesenchymal stem cells (hMSCs) hold a great promise in both basic developmental biology studies and regenerative medicine, as progenitors of bone cells. Their fate can be affected by various key regulatory factors (e.g. soluble growth factors, intrinsic, extrinsic environmental factors) that can be delivered by a fabricated scaffold. For example, when cultured on engineered environments that reproduce the physical features of the bone, hMSCs express tissue-specific transcription factors and consequently undergo an osteogenic fate. Therefore, producing smart bio-interfaces with targeted functionalities represents the key point in effective use of hierarchically topographical and chemical bioplatforms. In this chapter, we review laser-based approaches (e.g. Matrix-Assisted Pulsed Laser Evaporation (MAPLE), Laser-Induced Forward Transfer (LIFT), laser texturing and laser direct writing) used for the design of bio-interfaces aimed at controlling stem cell behaviour in vitro
Smart photochromic gratings with switchable wettability realized by green-light interferometry
We demonstrate the enhancement of the wetting properties of smart photochromic surfaces by a specifically developed, gentle interferometric patterning employing green light. We realized photochromic gratings with 2.5–10.0μm period by a blend consisting of a green-curable matrix and spiropyran molecules. The structured surfaces exhibit photocontrolled and reversible wettability, and enhanced hydrophilicity with respect to the native substrates. The dynamics of liquid spreading onto the gratings was also investigated, and the wetting behavior analyzed according to Wenzel's [Ind. Eng. Chem. 28, 988 (1936)] model for rough surfaces. These results indicate switchable gratings as promising functional components for microfluidics and modulation, and green-light interferometry as a reliable lithographic method for patterning organics without degradation or photochemical reactions
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
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