93 research outputs found
Polymer nanofibers as novel light-emitting sources and lasing material
Polymer micro- and nano-fibers, made of organic light-emitting materials with
optical gain, show interesting lasing properties. Fibers with diameters from
few tens of nm to few microns can be fabricated by electrospinning, a method
based on electrostatic fields applied to a polymer solution. The morphology and
emission properties of these fibers, composed of optically inert polymers
embedding laser dyes, are characterized by scanning electron and fluorescence
microscopy, and lasing is observed under optical pumping for fluences of the
order of 10^2 microJ cm^-2. In addition, light-emitting fibers can be
electrospun by conjugated polymers, their blends, and other active organics,
and can be exploited in a range of photonic and electronic devices. In
particular, waveguiding of light is observed and characterized, showing optical
loss coefficient in the range of 10^2-10^3 cm^-1. The reduced size of these
novel laser systems, combined with the possibility of achieving wavelength
tunability through transistor or other electrode-based architectures embedding
non-linear molecular layers, and with their peculiar mechanical robustness,
open interesting perspectives for realizing miniaturized laser sources to
integrate on-chip optical sensors and photonic circuits.Comment: 7 pages, 3 figures, 27 references. Invited contribution. Copyright
(2013) Society of Photo Optical Instrumentation Engineers. One print or
electronic copy may be made for personal use only. Systematic reproduction
and distribution, duplication of any material in this paper for a fee or for
commercial purposes, or modification of the content of the paper are
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Computational homogenization of fibrous piezoelectric materials
Flexible piezoelectric devices made of polymeric materials are widely used
for micro- and nano-electro-mechanical systems. In particular, numerous recent
applications concern energy harvesting. Due to the importance of computational
modeling to understand the influence that microscale geometry and constitutive
variables exert on the macroscopic behavior, a numerical approach is developed
here for multiscale and multiphysics modeling of thin piezoelectric sheets made
of aligned arrays of polymeric nanofibers, manufactured by electrospinning. At
the microscale, the representative volume element consists in piezoelectric
polymeric nanofibers, assumed to feature a piezoelastic behavior and subjected
to electromechanical contact constraints. The latter are incorporated into the
virtual work equations by formulating suitable electric, mechanical and
coupling potentials and the constraints are enforced by using the penalty
method. From the solution of the micro-scale boundary value problem, a suitable
scale transition procedure leads to identifying the performance of a
macroscopic thin piezoelectric shell element.Comment: 22 pages, 13 figure
A multiscale-multiphysics strategy for numerical modeling of thin piezoelectric sheets
Flexible piezoelectric devices made of polymeric materials are widely used
for micro- and nano-electro-mechanical systems. In particular, numerous recent
applications concern energy harvesting. Due to the importance of computational
modeling to understand the influence that microscale geometry and constitutive
variables exert on the macroscopic behavior, a numerical approach is developed
here for multiscale and multiphysics modeling of piezoelectric materials made
of aligned arrays of polymeric nanofibers. At the microscale, the
representative volume element consists in piezoelectric polymeric nanofibers,
assumed to feature a linear piezoelastic constitutive behavior and subjected to
electromechanical contact constraints using the penalty method. To avoid the
drawbacks associated with the non-smooth discretization of the master surface,
a contact smoothing approach based on B\'ezier patches is extended to the
multiphysics framework providing an improved continuity of the
parameterization. The contact element contributions to the virtual work
equations are included through suitable electric, mechanical and coupling
potentials. From the solution of the micro-scale boundary value problem, a
suitable scale transition procedure leads to the formulation of a macroscopic
thin piezoelectric shell element.Comment: 11 pages, 6 pages, 21 reference
Cooperativity in the enhanced piezoelectric response of polymer nanowires
We provide a detailed insight into piezoelectric energy generation from
arrays of polymer nanofibers. For sake of comparison, we firstly measure
individual poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFe)) fibers
at well-defined levels of compressive stress. Under an applied load of 2 mN,
single nanostructures generate a voltage of 0.45 mV. We show that under the
same load conditions, fibers in dense arrays exhibit a voltage output higher by
about two orders of magnitude. Numerical modelling studies demonstrate that the
enhancement of the piezoelectric response is a general phenomenon associated to
the electromechanical interaction among adjacent fibers, namely a cooperative
effect depending on specific geometrical parameters. This establishes new
design rules for next piezoelectric nano-generators and sensors.Comment: 31 pages, 11 figures, 1 tabl
Polarization mode splitting in monolithic polymer microcavities
We demonstrate the mode splitting of the resonant emission from a symmetric monolithic organic semiconductor microcavity. The device, realized by low-temperature reactive electron-beam evaporation and deposition of a conjugated polymer, exhibits a 100 meV polarization-induced splitting of the transmission and emission resonances for angles larger than 45°. This opens the way for the realization of novel polarized-emitting optoelectronic devices based on plastic materials
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
Registration accuracy in multilevel soft lithography
We investigate the registration accuracy achievable by multilevel soft lithography. By a specifically designed soft lithography aligner, we obtain, for the average misalignment between two registered patterned organic layers, values decreasing from (4.96 ? 0.02) to (0.50 ? 0.01)??m upon increasing the Young's modulus of the stamp materials from 1.8 to 2600?MPa. This clearly identifies in the stamp distortions the main factor limiting the registration accuracy. The potentiality to achieve registration within 500?nm over areas of 50 ? 50??m2 is demonstrated, opening the way for soft lithographies with high overlay alignment accuracy
Amplified spontaneous emission and waveguiding properties of the colored merocyanine form of (1',3'-dihydro-1',3',3'-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2'-(2H)-indole] molecules
We report a complete study of the properties of amplified spontaneous emission (ASE) of the merocyanine form of the photochromic system, 1',3'-dihydro-1',3',3'-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2'-(2H)-indole], under nanosecond excitation conditions. ASE line narrowing is clearly observed for excitation densities larger than 400 μJ cm-2 with a threshold pumping length in the range 0.5−1.2 mm. Remarkable waveguiding properties were observed, with losses throughout the organic slab of about 5.7 cm-1. The observation of ASE is discussed in terms of a possible S1-dominated photoconversion and excitation/de-excitation dynamics of the photochromic system. These results are important in view of the application of merocyanine-based films as active layers for potentially gateable laser devices
Amplified spontaneous emission from a conjugated polymer undergone a high-temperature lithography cycle
We investigated the amplified spontaneous emission (ASE) and waveguiding properties of a conjugated polymer film after a heating cycle typical of soft lithography procedures. We found a maximum gain coefficient of 8cm−1, with excitation density and length thresholds for ASE-induced line narrowing of 200μJcm−2 and 0.9 mm, respectively. Importantly, we found a loss coefficient of the organic slab as low as 5.4cm−1, which is among the best results reported for organic waveguide amplifiers. These results are important in view of the application of polymer films as active layers for laser devices realized by patterning with high-temperature mechanical lithographies
Multiscale modeling of piezoelectric materials
This paper focuses on numerical strategies to predict the behavior of piezoelectric materials and devices characterized by heterogenous microstructural features. Several of these materials are attractive for technological applications including mechanical energy harvesting and pressure/force sensors. After a general introduction on the linear piezoelastic problem, two multiscale strategies are presented and applied to the solution of simple but significant problems frequently encountered in nanotechnology test setups. The first strategy consists in classical homogenization based on the choice of a representative volume element and on the classical micro-macro work equality known as Hill’s lemma. The second strategy is based on the so called FE2 method, where the microscale average response resulting from an homogenization procedure is directly used as a constitutive model at each quadrature point at the macroscale. Both strategies have been implemented within an advanced numeric framework based on the authomatic differentiation technique
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