Broad Frequency Spectrum Research Articles

Comparison of Electronic and Vibrational Coherence Measured by Two-Dimensional Electronic Spectroscopy

The short pulse durations and broad frequency spectra of femtosecond laser pulses allow coherent superpositions of states to be prepared and probed. Two-dimensional electronic spectroscopy (2D ES) has the potential to identify more clearly the origin and evolution of such coherences. In this report we examine how electronic and vibrational coherences can be distinguished by decomposing the total 2D ES signal into rephasing and nonrephasing components. We investigate and identify differences between the cross peak oscillations measured in two laser dyes with those measured in the PC645 light-harvesting antenna protein of the cryptophyte alga Chroomonas sp. strain CCMP270 at ambient temperature.

Illusory Centrifugal Motion Direction Observed in Brief Stimuli: Psychophysics and Energy Model

All stationary stimuli of fixed duration have motion energy and the amount of motion energy increases with decreasing duration. Consequently, perception of motion direction could be biased if the readout mechanisms are unbalanced. Previous physiological study showed prefered direction of MT neurons in peripheral tend to be oriented away from fovea(Albright, 1989). Given the broadening of motion energy in brief stimuli, such effect should increase as the stimulus duration decreases. Here, we tested this hypothesis by presenting vertical gratings (0.5c/deg, raised cosine spatial envelope, radius = 5deg, 98% contrast) with different speeds(2,4,8 16deg/sec) and direction(moving towards fovea or moving away from fovea). And Stimuli were presented in a temporal Gaussian envelope with durations ranging between 5 and 500ms. Observers' task was to identify perceived motion direction (guessing when unsure). Results showed that as predicted, the observers were biased to perceive these stimuli as moving away from fovea. In summary, briefly presented stationary stimuli are perceived as moving in centrifugal direction when presented in visual periphery. One possible explanation for this illusion is that these stimuli, by virtue of their broad temporal frequency spectrum, stimulate centrifugally biased motion mechanisms in area MT.

Voltage-sensitive dye imaging reveals tonotopic organization of auditory cortex spontaneous activity

AbstractImaging neural activity across a large (several mm) cortical area with high temporal and spatial resolution is desirable, for example in the auditory system to measure cortical processing across a broad frequency spectrum. Voltage-sensitive dye imaging (VSDI) has a unique combination of properties making this possible, but so far studies have been limited to studying simple sparsely-presented sensory stimuli. We demonstrate the feasibility of long-acquisition VSDI (using the dye RH-1691) in auditory cortex while presenting complex time-varying acoustic stimuli or silence. Using a dense array of partially-overlapping 50 ms tone pips (8 frequencies per octave spanning six octaves), we obtained high-resolution spectrotemporal receptive fields (STRFs) simultaneously across the majority of the guinea pig primary auditory cortical fields (A1 and DC). Long epochs of spontaneous activity were also measured, permitting a comparison of spontaneous activity patterns with functional architecture. By grouping all pixels in areas A1 and DC according to sound frequency preference (obtained from STRFs), we reveal that spontaneous activity (such as cortical spindles) show complex spatial patterns, which are organized according to sound frequency preference within and across cortical areas. More specifically, spontaneous activity correlation decreases as frequency preference diverges within A1 or DC; but additionally, pixels in A1 are also highly correlated with (even far-away) pixels in DC sharing similar frequency preference. These properties of patterned cortical spontaneous activity constrain mechanistic hypotheses regarding their genesis. Beyond these observations, the feasibility of VSDI with continuous stimulation or silence permits measuring population activity during long-lasting sound patterns, which is necessary for examining cortical dynamics and sensory-context dependent processing.

Performance of disturbance augmented control design in turbulent wind conditions

This paper investigates the use of disturbance models in the design of wind turbine individual pitch controllers. Previous work has used individual pitch control and disturbance models with the Multiblade Coordinate Transformation to design controllers that reduce the blade loads at the frequencies associated with the rotor speed. This paper takes a similar approach of using a disturbance model within the H ∞ design framework to account for periodic loading effects. The controller is compared with a baseline design that does not include the periodic disturbance model. In constant wind speeds, the disturbance model design is significantly better than the baseline design at canceling blade loads at the rotor frequencies. However, these load reduction improvements become negligible even under low turbulent wind conditions. The two controllers perform similarly in turbulent wind conditions because disturbance augmentation improves load reduction only at the multiples of the rotor frequency in the yaw and tilt moment channels whereas turbulence creates strong collective bending moments. In addition, turbulent wind contains energy across a broad frequency spectrum and improvements at multiples of the rotor frequency are less important in these conditions. Therefore inclusion of periodic disturbance models in the control design may not lead to the expected load reduction in fielded wind turbines.

Electromagnetic pulses generated by meteoroid impacts on spacecraft

Meteoroid impacts on spacecraft are known to cause mechanical damage, but their electrical effect on spacecraft systems are not well characterized. Several reported spacecraft anomalies are suggestive of an electrical failure associated with meteoroid impact. We present a theory to explain plasma production and subsequent electric fields occurring when a meteoroid strikes a spacecraft, ionizing itself and part of the spacecraft. This plasma, with a charge separation commensurate with different specie mobilities, can produce a strong electromagnetic pulse (EMP) at broad frequency spectra, potentially causing catastrophic damage if the impact is relatively near an area with low shielding or an open umbilical. Anomalies such as gyrostability loss can be caused by an EMP without any detectable momentum transfer due to small (<1 μg) particle mass. Subsequent plasma oscillations can also emit significant power and may be responsible for many reported satellite anomalies. The presented theory discusses both a dust‐free plasma expansion with coherent electron oscillation and a dusty plasma expansion with macroscopic charge separation.

Quantum Enhancement of the Zero-Area Sagnac Interferometer Topology for Gravitational Wave Detection

Only a few years ago, it was realized that the zero-area Sagnac interferometer topology is able to perform quantum nondemolition measurements of position changes of a mechanical oscillator. Here, we experimentally show that such an interferometer can also be efficiently enhanced by squeezed light. We achieved a nonclassical sensitivity improvement of up to 8.2dB, limited by optical loss inside our interferometer. Measurements performed directly on our squeezed-light laser output revealed squeezing of 12.7dB. We show that the sensitivity of a squeezed-light enhanced Sagnac interferometer can surpass the standard quantum limit for a broad spectrum of signal frequencies without the need for filter cavities as required for Michelson interferometers. The Sagnac topology is therefore a powerful option for future gravitational-wave detectors, such as the Einstein Telescope, whose design is currently being studied.

Adaptin' endosomes for synaptic vesicle recycling, learning and memory

Adaptin' endosomes for synaptic vesicle recycling, learning and memory

Using White Noise to Probe Actomyosin Cycling Kinetics During Shortening and Lengthening in Drosophila Flight Muscle Fibers

Our laboratory routinely measures demembranated muscle fiber mechanics using small amplitude sinusoidal length perturbation analysis and estimates kinetics of actomyosin cycling and myosin attachment time. We have developed a complementary measurement technique using small, random changes in muscle length (white-noise length stimuli) that simultaneously cover a broad frequency spectrum, comparable with the sinusoidal length perturbations. We find the white-noise technique provides a description of the viscoelastic properties that is consistent with sinusoidal analysis measurements, rapidly capturing much of the physiological behavior associated with contracting muscle fibers. The white-noise technique samples a vast range of system behavior in a fraction of the time required to complete sinusoidal analysis, and does not require the linear response underlying sinusoidal analysis methods. Thus, we combined the white-noise stimuli with a linear shortening and lengthening transients to probe cross-bridge cycling behavior during these periods of varied load. Preliminary measurements using demembranated dorsal longitudinal flight muscle fibers from Drosophila melanogaster indicate faster cross-bridge cycling kinetics during lengthening and slower cycling during shortening, compared to isometric contraction. During isometric contraction we estimate a myosin attachment time of 5.0 ms, and 4.6 versus 5.5 ms during the lengthening and shortening transients, respectively. These initial applications of the white-noise system analysis technique show promise for future studies probing molecular processes that underlie complicated length transients associated with normal muscle contraction.

Simulations of Frequency-dependent Impedance of Ground Rods Considering Multi-layered Soil Structures

Lightning has a broad frequency spectrum from DC to a few MHz. Consequently, the high frequency performance of grounding systems for protection against lightning should be evaluated, with the distributed parameter circuit model in a uniform soil being used to simulate grounding impedances. This paper proposes a simulation method which applies the distributed parameter circuit model for the frequency-dependent impedance of vertically driven ground rods by considering multi-layered soil structures where ground rods are buried. The Matlab program was used to calculate the frequency-dependent ground impedances for two ground rods of different lengths. As a result, an increase of the length of ground rod is not always followed by a decrease of grounding impedance, at least at a high frequency. The results obtained using the newly proposed simulation method considering multi-layered soil structures are in good agreement with the measured results.

Implications of Both Statistical Equilibrium and Global Warming Simulations with CCSM3. Part II: On the Multidecadal Variability in the North Atlantic Basin

Abstract The nature of the multidecadal variability in the North Atlantic basin is investigated through detailed analysis of multicentury integrations performed using the low-resolution version of the Community Climate System Model, version 3 (CCSM3), a modern atmosphere–ocean coupled general circulation model. Specifically, the results of control simulations under both preindustrial and present-day perpetual seasonal cycle conditions are compared to each other and also to the results of five simulations with increasing CO2 concentration scenarios. In the absence of greenhouse gas–induced warming, the meridional overturning circulation (MOC) variability is shown to be dependent on the details of the simulation. In the present-day control simulation, the MOC is characterized by a broad spectrum of low frequencies, whereas, in preindustrial control simulations, MOC variability is characterized either by a well-defined periodicity of 60 yr or by a broad spectrum of low frequencies. In all the control simulations, the MOC appears to respond with a delay of 10 yr to synchronous temperature and salinity anomalies in the deep water formation sites located in the subpolar gyre, but salinity dominates the density anomalies. The explanation of the modeled MOC periodicity is therefore sought in the creation of these density anomalies. The influence of increased sea ice coverage under cold/preindustrial conditions is shown to modify the salinity variability, but it is not a sufficient condition for the support of the MOC periodicity. Instead, its source appears to be a modified subpolar gyre circulation resulting from interaction with the bottom bathymetry, which is able to sustain strong coupling between the horizontal and overturning circulations. Based on the global warming analyses, for the simulations initialized from the cold/preindustrial statistical equilibrium run, the North Atlantic variability continues to be dominated by strong coupling between the horizontal and overturning circulations if the imposed forcing is weak. More generally, the delayed response of the MOC to surface density anomalies in the deep water formation regions is preserved under weak forcing.

Effects of the plenum chamber volume and distributor geometry on fluidized bed hydrodynamics

Most hydrodynamic fluidized bed models, including CFD codes, neglect any effects of the plenum chamber volume. Experiments were performed in a 0.13 m ID fluidization column to determine plenum chamber volume effects on fluidized bed hydrodynamics for FCC and glass particles. Two low-pressure-drop distributors were used, one with a single orifice, and the other with 33 orifices and the same total open area as the single orifice. The results show two peaks in the frequency spectra for the single-orifice distributor, one representing bubble eruption at the bed surface and the other of higher frequency corresponding to the bubbling frequency at the distributor. The latter decreased slightly with increasing plenum volume and with increasing bed depth. For the multi-orifice distributor, broad frequency spectra from pressure measurements became narrower and moved towards higher frequency with decreasing plenum volume.

PC1 with a broad frequency spectrum — “Goose pulsations”

Geomagnetic pulsations in the frequency range of Pc1 pearl waves with the dynamic spectra having a very narrow spectrum width at the beginning of the event and a very broad spectrum width (Δf/f0 ∼ 1) in the later part of the event are analyzed. One of the observed events shown by the dynamic spectrum resembles a goose with the beak at the beginning of the event and with the wing in the later part of the event. Various interpretations of these geomagnetic pulsations are presented taking into account nonlinear effects, quasilinear interaction of electromagnetic ion-cyclotron waves with energetic, anisotropic protons and modulation of plasma parameters in the magnetosphere by Pc3–5 hydromagnetic waves. The ionospheric effect in the signal formation is determined by the ionospheric Alfven resonator. It can control the frequency range of the dynamic spectra, but not the internal structure of the signal.

High Frequency Grounding Impedances of Vertically-Driven Ground Rods

Grounding impedance depends on the frequency of current flowing into a grounding system. Lightning in particular has a broad frequency spectrum from some tens of ㎐ to a few ㎒. So the grounding impedance related to transient currents such as lightning should be measured. In this paper, the grounding impedances of vertically-driven ground rods of 10, 30 and 48[m] long are measured and analyzed as functions of the frequency of injected current and the feeding point. As a result, the longer the ground rod is, the lower the steady-state ground resistance is. However the grounding impedance of a vertically-driven ground rod at a high frequency is significantly increased. It is not always true that low grounding impedance follows from a low steady-state ground resistance. It is important to evaluate the high frequency performance of grounding systems for protection against lightning.

Nonlinear cyclotron resonance of a massless quasiparticle in graphene

We consider the classical motion of a massless quasiparticle in a magnetic field and under a weak electromagnetic radiation with the frequency $\ensuremath{\omega}$. Due to the nonparabolic linear energy dispersion, the particle responds not only at the frequency $\ensuremath{\omega}$ but generates a broad frequency spectrum around it. The linewidth of the cyclotron resonance turns out to be very broad even in a perfectly pure material which allows one to explain recent experimental data in graphene.

Targeted ultrasound contrast agents for the imaging of biofilm infections.

Targeted ultrasound contrast agents (UCA) offer a novel, potential method for diagnostic imaging of biofilm infections. Currently, there is no established method for molecular imaging of in vitro biofilm infections with any type of modality. For infective endocarditis, the case of biofilm formation on damaged or diseased heart valves, an early diagnostic method might greatly reduce the associated high mortality rate. Fluorescently labeled lectins were used with targeted ultrasound contrast agents in conjunction with epifluorescence microscopy and time-resolved scanning acoustic microscopy for visualization and characterization of the extracellular polymeric substances (EPS) of two types of infectious biofilms (Staphylococcus aureus and Pseudomonas aeruginosa). Noninvasive acoustic measurements of the biofilms were conducted at high-frequency (100 MHz) with a time-resolved scanning acoustic microscope. Measurements of the reflection and transmission coefficients, over a broad frequency spectrum for a range of biofilm growth stages in both, the presence and absence of targeted UCA were obtained. The biofilm morphology and structure were also investigated with acoustic microscopy as a function of growth. [This work was supported by the National Institutes of Health Grants NIH 2 P20 RR016453-05A1 and NIH 2 G12 RR0030161-21.]

Boundary instability of a two-dimensional electron fluid

It was shown previously that the current-carrying state of a Field Effect Transistor with asymmetric source and drain boundary conditions may become unstable against spontaneous generation of plasma waves [1]. By extending the analysis to the two-dimensional case we find that the dominant instability modes correspond to waves propagating in the direction perpendicular to the current and localized near the boundaries. This new type of instability should result in plasma turbulence with a broad frequency spectrum. More generally, it is shown that a similar instability might exist, when a strong enough current goes through a single boundary between the gated and ungated regions.

Sonic diaspora, vibrations, and rhythm: thinking through the sounding of the Jamaican dancehall session

The propagation of vibrations may provide a better way of understanding the spread of diasporas than the conventional focus on the circulation of products (Hall 1980, Appadurai 1986, 1996, Gilroy 1993a, Brah 1996). Jamaican sound systems operate as a broadcast medium and a source of CDs, DVDs, and other commercial products (Henriques 2007a). But the dancehall sound system session also propagates a broad spectrum of frequencies diffused through a range of media and activities – described as ‘sounding’ (following Small's 1998 concept of ‘musicking’). These include the material vibrations of the signature low-pitched auditory frequencies of Reggae as a bass culture (Johnson 1980), at the loudness of ‘sonic dominance’ (Henriques 2003). Secondly a session propagates the corporeal vibrations of rituals, dance routines, and bass-line ‘riddims’ (Veal 2007). Thirdly it propagates the ethereal vibrations (Henriques 2007b), ‘vibes’ or atmosphere of the sexually charged popular subculture by which the crowd (audience) appreciate each dancehall session as part of the Dancehall scene (Cooper 2004). The paper concludes that thinking though vibrating frequencies makes it easier to appreciate how audiences with no direct or inherited connection with a particular music genre can be energetically infected and affected – to form a sonic diaspora.

Analysis and Prediction of Vibration-Induced Solder Joint Failure for a Ceramic Column Grid Array Package

Solder joint fatigue failure under vibration loading continues to be a concern in microelectronic industry. Existing literature has not adequately addressed high-cycle fatigue failure of high-lead solder joints, especially under a broad spectrum of vibration frequencies. Also, damage mapping across solder joints in an area-array package has not been effectively studied using numerical models and experimental cross sectioning. This paper aims to develop an experimental and modeling approach that can accurately determine the solder joint behavior of electronic components under vibration conditions. In particular, this paper discusses the out-of-plane sinusoidal vibration experiments at 1G, numerical modeling, and fatigue life prediction for a 42.5×42.5×4mm3 1089 input∕output ceramic column grid array (CCGA) package on a 133×56×2.8mm3 FR4 board. Detailed investigation and characterization involving dye-and-pry analysis, microstructural examination, and numerical modeling enabled the development of a high-cycle stress-based equation for lead-containing CCGA under sinusoidal loading. The developed approach has been applied to a number of cases including a CCGA package with a heat sink as well as a CCGA package subjected to frequency sweeps. It is seen that the predictions from the developed model agree well with experimental data and that the developed model can map the evolution of solder damage across all solder joints and can also provide important design recommendations in terms of solder joint location as well as heat sink attachment.

Plasma fluctuations as Markovian noise

Noise theory is used to study the correlations of stationary Markovian fluctuations that are homogeneous and isotropic in space. The relaxation of the fluctuations is modeled by the diffusion equation. The spatial correlations of random fluctuations are modeled by the exponential decay. Based on these models, the temporal correlations of random fluctuations, such as the correlation function and the power spectrum, are calculated. We find that the diffusion process can give rise to the decay of the correlation function and a broad frequency spectrum of random fluctuations. We also find that the transport coefficients may be estimated by the correlation length and the correlation time. The theoretical results are compared with the observed plasma density fluctuations from the tokamak and helimak experiments.

Surface gravity wave effects in the oceanic boundary layer: large-eddy simulation with vortex force and stochastic breakers

The wind-driven stably stratified mid-latitude oceanic surface turbulent boundary layer is computationally simulated in the presence of a specified surface gravity-wave field. The gravity waves have broad wavenumber and frequency spectra typical of measured conditions in near-equilibrium with the mean wind speed. The simulation model is based on (i) an asymptotic theory for the conservative dynamical effects of waves on the wave-averaged boundary-layer currents and (ii) a boundary-layer forcing by a stochastic representation of the impulses and energy fluxes in a field of breaking waves. The wave influences are shown to be profound on both the mean current profile and turbulent statistics compared to a simulation without these wave influences and forced by an equivalent mean surface stress. As expected from previous studies with partial combinations of these wave influences, Langmuir circulations due to the wave-averaged vortex force make vertical eddy fluxes of momentum and material concentration much more efficient and non-local (i.e. with negative eddy viscosity near the surface), and they combine with the breakers to increase the turbulent energy and dissipation rate. They also combine in an unexpected positive feedback in which breaker-generated vorticity seeds the creation of a new Langmuir circulation and instigates a deep strong intermittent downwelling jet that penetrates through the boundary layer and increases the material entrainment rate at the base of the layer. These wave effects on the boundary layer are greater for smaller wave ages and higher mean wind speeds.