423 research outputs found
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
Dynamical response of the Galileo Galilei on the ground rotor to test the equivalence principle: Theory, simulation, and experiment. II. The rejection of common mode forces
"Galileo Galilei on the ground" (GGG) is a fast rotating differential accelerometer designed to test the equivalence principle (EP). Its sensitivity to differential effects, such as the effect of an EP violation, depends crucially on the capability of the accelerometer to reject all effects acting in common mode. By applying the theoretical and simulation methods reported in Part I of this work, and tested therein against experimental data, we predict the occurrence of an enhanced common mode rejection of the GGG accelerometer. We demonstrate that the best rejection of common mode disturbances can be tuned in a controlled way by varying the spin frequency of the GGG rotor. (c) 2006 American Institute of Physics
Dynamical response of the "GGG" rotor to test the Equivalence Principle: theory, simulation and experiment. Part I: the normal modes
Recent theoretical work suggests that violation of the Equivalence Principle
might be revealed in a measurement of the fractional differential acceleration
between two test bodies -of different composition, falling in the
gravitational field of a source mass- if the measurement is made to the level
of or better. This being within the reach of ground based
experiments, gives them a new impetus. However, while slowly rotating torsion
balances in ground laboratories are close to reaching this level, only an
experiment performed in low orbit around the Earth is likely to provide a much
better accuracy.
We report on the progress made with the "Galileo Galilei on the Ground" (GGG)
experiment, which aims to compete with torsion balances using an instrument
design also capable of being converted into a much higher sensitivity space
test.
In the present and following paper (Part I and Part II), we demonstrate that
the dynamical response of the GGG differential accelerometer set into
supercritical rotation -in particular its normal modes (Part I) and rejection
of common mode effects (Part II)- can be predicted by means of a simple but
effective model that embodies all the relevant physics. Analytical solutions
are obtained under special limits, which provide the theoretical understanding.
A simulation environment is set up, obtaining quantitative agreement with the
available experimental data on the frequencies of the normal modes, and on the
whirling behavior. This is a needed and reliable tool for controlling and
separating perturbative effects from the expected signal, as well as for
planning the optimization of the apparatus.Comment: Accepted for publication by "Review of Scientific Instruments" on Jan
16, 2006. 16 2-column pages, 9 figure
2,2':6',2''-Terpyridine-functionalized redox-responsive hydrogels as a platform for multi responsive amphiphilic polymer membranes
Nanophase-separated amphiphilic polymer co-networks are ideally suited as responsive membranes due to their stable co-continuous structure. Their functionalization with redox-responsive 2,2âČ:6âČ,2âČâČ-terpyridineâmetal complexes and light-responsive spiropyran derivatives leads to a novel material with tunable optical, redox and permeability properties. The versatility of the system in complexing various metal ions, such as cobalt or iron at different concentrations, results in a perfect monitoring over the degree of crosslinking of the hydrophilic poly(2-hydroxyethyl acrylate) channels. The reversibility of the complexation, the redox state of the metal and the isomerization to the merocyanine form upon UV illumination was evidenced by cyclic voltammetry, UV-Vis and permeability measurements under sequential conditions. Thus, the membrane provides light and redox addressable functionalities due to its adjustable and mechanically stable hydrogel network
Enhanced Piezoelectricity of Electrospun Polyvinylidene Fluoride Fibers for Energy Harvesting
Piezoelectric polymers are promising energy materials for wearable and implantable applications for replacing bulky batteries in small and flexible electronics. Therefore, many research studies are focused on understanding the behavior of polymers at a molecular level and designing new polymer-based generators using polyvinylidene fluoride (PVDF). In this work, we investigated the influence of voltage polarity and ambient relative humidity in electrospinning of PVDF for energy-harvesting applications. A multitechnique approach combining microscopy and spectroscopy was used to study the content of the ÎČ-phase and piezoelectric properties of PVDF fibers. We shed new light on ÎČ-phase crystallization in electrospun PVDF and showed the enhanced piezoelectric response of the PVDF fiber-based generator produced with the negative voltage polarity at a relative humidity of 60%. Above all, we proved that not only crystallinity but also surface chemistry is crucial for improving piezoelectric performance in PVDF fibers. Controlling relative humidity and voltage polarity increased the d33 piezoelectric coefficient for PVDF fibers by more than three times and allowed us to generate a power density of 0.6 ÎŒW·cm-2 from PVDF membranes. This study showed that the electrospinning technique can be used as a single-step process for obtaining a vast spectrum of PVDF fibers exhibiting different physicochemical properties with ÎČ-phase crystallinity reaching up to 74%. The humidity and voltage polarity are critical factors in respect of chemistry of the material on piezoelectricity of PVDF fibers, which establishes a novel route to engineer materials for energy-harvesting and sensing applications
Conductive Hybrid Cu-HHTP-TCNQ MetalâOrganic Frameworks for Chemiresistive Sensing
Electrically conductive metalâorganic frameworks (MOFs) and MOF-like coordination polymers are an emerging class of materials that combine good electrical charge transport with unique properties such as nanoporosity. The combination of different metal nodes and organic linkers allows tailoring MOFs to specific properties and applications in electronics, like selective chemiresistive sensing. The intrinsic crystallinity of MOFs, which usually promotes efficient charge transport, makes them also difficult to integrate into flexible systems, as crystalline MOFs are often brittle. The present study reports on a fast and reliable interfacial synthesis of conductive MOF films composed of two different organic ligands, 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and 7,7,8,8-tetracyanoquinodimethane (TCNQ), lacking long-range periodic order while preserving good electrical conductivity of 0.033 S cmâ1 at room temperature and chemiresistive response toward ambient changes. The hybrid nature of the discontinuous film is investigated multiparametrically by electron and atomic force microscopy as well as by Raman spectroscopy. This study demonstrates that including different types of MOFs is a good compromise between structural order and conductivity, thus making hybrid framework architectures to a promising active material for chemiresistive sensors without the need for high crystallinity
The variable finesse locking technique
Virgo is a power recycled Michelson interferometer, with 3 km long Fabry-Perot cavities in the arms. The locking of the interferometer has been obtained with an original lock acquisition technique. The main idea is to lock the instrument away from its working point. Lock is obtained by misaligning the power recycling mirror and detuning the Michelson from the dark fringe. In this way, a good fraction of light escapes through the antisymmetric port and the power build-up inside the recycling cavity is extremely low. The benefit is that all the degrees of freedom are controlled when they are almost decoupled, and the linewidth of the recycling cavity is large. The interferometer is then adiabatically brought on to the dark fringe. This technique is referred to as variable finesse, since the recycling cavity is considered as a variable finesse Fabry-Perot. This technique has been widely tested and allows us to reach the dark fringe in few minutes, in an essentially deterministic way
Astrophysically Triggered Searches for Gravitational Waves: Status and Prospects
In gravitational-wave detection, special emphasis is put onto searches that
focus on cosmic events detected by other types of astrophysical observatories.
The astrophysical triggers, e.g. from gamma-ray and X-ray satellites, optical
telescopes and neutrino observatories, provide a trigger time for analyzing
gravitational wave data coincident with the event. In certain cases the
expected frequency range, source energetics, directional and progenitor
information is also available. Beyond allowing the recognition of gravitational
waveforms with amplitudes closer to the noise floor of the detector, these
triggered searches should also lead to rich science results even before the
onset of Advanced LIGO. In this paper we provide a broad review of LIGO's
astrophysically triggered searches and the sources they target
Directional limits on persistent gravitational waves using LIGO S5 science data
The gravitational-wave (GW) sky may include nearby pointlike sources as well
as astrophysical and cosmological stochastic backgrounds. Since the relative
strength and angular distribution of the many possible sources of GWs are not
well constrained, searches for GW signals must be performed in a
model-independent way. To that end we perform two directional searches for
persistent GWs using data from the LIGO S5 science run: one optimized for
pointlike sources and one for arbitrary extended sources. The latter result is
the first of its kind. Finding no evidence to support the detection of GWs, we
present 90% confidence level (CL) upper-limit maps of GW strain power with
typical values between 2-20x10^-50 strain^2 Hz^-1 and 5-35x10^-49 strain^2
Hz^-1 sr^-1 for pointlike and extended sources respectively. The limits on
pointlike sources constitute a factor of 30 improvement over the previous best
limits. We also set 90% CL limits on the narrow-band root-mean-square GW strain
from interesting targets including Sco X-1, SN1987A and the Galactic Center as
low as ~7x10^-25 in the most sensitive frequency range near 160 Hz. These
limits are the most constraining to date and constitute a factor of 5
improvement over the previous best limits.Comment: 10 pages, 4 figure
Reconstruction of the gravitational wave signal during the Virgo science runs and independent validation with a photon calibrator
The Virgo detector is a kilometer-scale interferometer for gravitational wave
detection located near Pisa (Italy). About 13 months of data were accumulated
during four science runs (VSR1, VSR2, VSR3 and VSR4) between May 2007 and
September 2011, with increasing sensitivity.
In this paper, the method used to reconstruct, in the range 10 Hz-10 kHz, the
gravitational wave strain time series from the detector signals is
described. The standard consistency checks of the reconstruction are discussed
and used to estimate the systematic uncertainties of the signal as a
function of frequency. Finally, an independent setup, the photon calibrator, is
described and used to validate the reconstructed signal and the
associated uncertainties.
The uncertainties of the time series are estimated to be 8% in
amplitude. The uncertainty of the phase of is 50 mrad at 10 Hz with a
frequency dependence following a delay of 8 s at high frequency. A bias
lower than and depending on the sky direction of the GW is
also present.Comment: 35 pages, 16 figures. Accepted by CQ
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