862 research outputs found
Computer-controlled vibration testing
System features quickly achieved steady state, increased accuracy of spectrum definition, and true Gaussian amplitude distribution of resulting signals. Controlled shock-tests might also be tried with this system
Hopf Bifurcations in a Watt Governor With a Spring
This paper pursues the study carried out by the authors in "Stability and
Hopf bifurcation in a hexagonal governor system", focusing on the codimension
one Hopf bifurcations in the hexagonal Watt governor differential system. Here
are studied the codimension two, three and four Hopf bifurcations and the
pertinent Lyapunov stability coefficients and bifurcation diagrams, ilustrating
the number, types and positions of bifurcating small amplitude periodic orbits,
are determined. As a consequence it is found an open region in the parameter
space where two attracting periodic orbits coexist with an attracting
equilibrium point.Comment: 30 pages and 7 figure
Mechanical oscillations in lasing microspheres
We investigate the feasibility of activating coherent mechanical oscillations
in lasing microspheres by modulating the laser emission at a mechanical
eigenfrequency. To this aim, 1.5% Nd3+:Barium-Titanium-Silicate microspheres
with diameters around 50 {\mu}m were used as high quality factor (Q>10^6)
whispering gallery mode lasing cavities. We have implemented a pump-and-probe
technique in which the pump laser used to excite the Nd3+ ions is focused on a
single microsphere with a microscope objective and a probe laser excites a
specific optical mode with the evanescent field of a tapered fibre. The studied
microspheres show monomode and multi-mode lasing action, which can be modulated
in the best case up to 10 MHz. We have optically transduced thermally-activated
mechanical eigenmodes appearing in the 50-70 MHz range, the frequency of which
decreases with increasing the size of the microspheres. In a pump-and-probe
configuration we observed modulation of the probe signal up to the maximum pump
modulation frequency of our experimental setup, i.e., 20 MHz. This modulation
decreases with frequency and is unrelated to lasing emission, pump scattering
or thermal effects. We associate this effect to free-carrier-dispersion induced
by multiphoton pump light absorption. On the other hand, we conclude that, in
our current experimental conditions, it was not possible to resonantly excite
the mechanical modes. Finally, we discuss on how to overcome these limitations
by increasing the modulation frequency of the lasing emission and decreasing
the frequency of the mechanical eigenmodes displaying a strong degree of
optomechanical coupling.Comment: 17 pages, 5 figure
A novel high resolution contactless technique for thermal field mapping and thermal conductivity determination: Two-Laser Raman Thermometry
We present a novel high resolution contactless technique for thermal
conductivity determination and thermal field mapping based on creating a
thermal distribution of phonons using a heating laser, while a second laser
probes the local temperature through the spectral position of a Raman active
mode. The spatial resolution can be as small as nm, whereas its
temperature accuracy is K. We validate this technique investigating the
thermal properties of three free-standing single crystalline Si membranes with
thickness of 250, 1000, and 2000 nm. We show that for 2-dimensional materials
such as free-standing membranes or thin films, and for small temperature
gradients, the thermal field decays as in the diffusive
limit. The case of large temperature gradients within the membranes leads to an
exponential decay of the thermal field, . The
results demonstrate the full potential of this new contactless method for
quantitative determination of thermal properties. The range of materials to
which this method is applicable reaches far beyond the here demonstrated case
of Si, as the only requirement is the presence of a Raman active mode
Integrability and explicit solutions in some Bianchi cosmological dynamical systems
The Einstein field equations for several cosmological models reduce to
polynomial systems of ordinary differential equations. In this paper we shall
concentrate our attention to the spatially homogeneous diagonal G_2
cosmologies. By using Darboux's theory in order to study ordinary differential
equations in the complex projective plane CP^2 we solve the Bianchi V models
totally. Moreover, we carry out a study of Bianchi VI models and first
integrals are given in particular cases
Two-Dimensional Phononic Crystals: Disorder Matters
The design and fabrication of phononic crystals (PnCs) hold the key to
control the propagation of heat and sound at the nanoscale. However, there is a
lack of experimental studies addressing the impact of order/disorder on the
phononic properties of PnCs. Here, we present a comparative investigation of
the influence of disorder on the hypersonic and thermal properties of
two-dimensional PnCs. PnCs of ordered and disordered lattices are fabricated of
circular holes with equal filling fractions in free-standing Si membranes.
Ultrafast pump and probe spectroscopy (asynchronous optical sampling) and Raman
thermometry based on a novel two-laser approach are used to study the phononic
properties in the gigahertz (GHz) and terahertz (THz) regime, respectively.
Finite element method simulations of the phonon dispersion relation and
three-dimensional displacement fields furthermore enable the unique
identification of the different hypersonic vibrations. The increase of surface
roughness and the introduction of short-range disorder are shown to modify the
phonon dispersion and phonon coherence in the hypersonic (GHz) range without
affecting the room-temperature thermal conductivity. On the basis of these
findings, we suggest a criteria for predicting phonon coherence as a function
of roughness and disorder.Comment: 19 pages, 4 figures, final published version, Nano Letters, 201
Orthotropic Piezoelectricity in 2D Nanocellulose
The control of electromechanical responses within bonding regions is
essential to face frontier challenges in nanotechnologies, such as molecular
electronics and biotechnology. Here, we present I\b{eta}-nanocellulose as a
potentially new orthotropic 2D piezoelectric crystal. The predicted in-layer
piezoelectricity is originated on a sui-generis hydrogen bonds pattern. Upon
this fact and by using a combination of ab-initio and ad-hoc models, we
introduce a description of electrical profiles along chemical bonds. Such
developments lead to obtain a rationale for modelling the extended
piezoelectric effect originated within bond scales. The order of magnitude
estimated for the 2D I\b{eta}-nanocellulose piezoelectric response, ~pm V-1,
ranks this material at the level of currently used piezoelectric energy
generators and new artificial 2D designs. Such finding would be crucial for
developing alternative materials to drive emerging nanotechnologies.Comment: 5 figures included. Supp. Mat. available on the online version:
https://www.nature.com/articles/srep34616, Others on:
http://www.nanowerk.com/nanotechnology-news/newsid=44806.ph
A cost-effective methodology applied to videoconference services over hybrid clouds
This paper tackles the optimization of applications in multi-provider hybrid cloud scenarios from an economic point of view. In these scenarios the great majority of solutions offer the automatic allocation of resources on different cloud providers based on their current prices. However our approach is intended to introduce a novel solution by making maximum use of divide and rule. This paper describes a methodology to create cost aware cloud applications that can be broken down into the three most important components in cloud infrastructures: computation, network and storage. A real videoconference system has been modified in order to evaluate this idea with both theoretical and empirical experiments. This system has become a widely used tool in several national and European projects for e-learning and collaboration purposes
Bottom-up growth of fully transparent contact layers of indium tin oxide nanowires for light-emitting devices
Thin layers of indium tin oxide are widely used as transparent coatings and electrodes in solar energy cells, flat-panel displays, antireflection coatings, radiation protection and lithium-ion battery materials, because they have the characteristics of low resistivity, strong absorption at ultraviolet wavelengths, high transmission in the visible, high reflectivity in the far-infrared and strong attenuation in the microwave region. However, there is often a trade-off between electrical conductivity and transparency at visible wavelengths for indium tin oxide and other transparent conducting oxides. Here, we report the growth of layers of indium tin oxide nanowires that show optimum electronic and photonic properties and demonstrate their use as fully transparent top contacts in the visible to near-infrared region for light-emitting devices
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