Excitation Region Research Articles

Toward High-Contrast Atomic Force Microscopy-Tip-Enhanced Raman Spectroscopy Imaging: Nanoantenna-Mediated Remote-Excitation on Sharp-Tip Silver Nanowire Probes.

The tip-enhanced Raman spectroscopy (TERS) imaging technique is designed to provide correlated morphological and chemical information with a nanoscale spatial resolution by utilizing the plasmonic resonance supported by metallic nanostructures at the tip apex of a scanning probe. However, limited by the scattering cross sections of these nanostructures, only a small fraction of the incident light can be coupled to the plasmonic resonance to generate Raman signals. The uncoupled light then directly excites background spectra with a diffraction-limited resolution, which becomes the background noise that often blurs the TERS image. Here, we demonstrate how this problem can be solved by physically separating the light excitation region from the Raman signal generation region on the scanning probe. The remote-excitation TERS (RE-TERS) probe, which can be fabricated with a facile, robust and reproducible method, utilizes silver nanoparticles as nanoantennas to mediate the coupling of free-space excitation light to propagating surface plasmon polaritons (SPPs)in a sharp-tip silver nanowire to excite Raman signals remotely. With this RE-TERS probe, a 10 nm spatial resolution was demonstrated on a single-walled carbon nanotube sample, and the strain distribution in a monolayer molybdenum disulfide (MoS2) was mapped.

A preliminary study on the potential of front face fluorescence spectroscopy for Italian mono-cultivar extra virgin olive oil discrimination

Front-face fluorescence method has been used to obtain fluorescence excitation emission matrix (EEM) characteristic of various Italian monocultivar extra virgin olive oil (EVOO), Mixed EVOO, and lower grade virgin olive oil. Specific region of EEM has been explored using principle component analysis. It was found that EEM region of excitation 250-400 and emission 280-620 nm could be used to discriminate each sample type of different cultivar, mixed sample, and virgin olive oil sample. The important fluorescence peaks for the discrimination belong to unique combination of tocopherols, tocotrienols, phenolic compounds, oxidation products, and vitamin E. These results show that frontface fluorescence EEM spectroscopy has a potential to be used for monocultivar EVOO discrimination.

A preliminary study on the potential of front face fluorescence spectroscopy for Italian mono-cultivar extra virgin olive oil discrimination

Front-face fluorescence method has been used to obtain Fluorescence Excitation Emission Matrix (EEM) characteristic of various Italian monocultivar Extra Virgin Olive Oil (EVOO), Mixed EVOO, and lower grade Virgin Olive Oil (VOO). Specific region of EEM has been explored using Principle Component Analysis (PCA). It was found that EEM region of excitation 250-400 and emission 280-620 nm could be used to discriminate each sample type of different cultivar, mixed sample, and virgin olive oil sample. The important fluorescence peaks for the discrimination belong to unique combination of Tocopherols, Tocotrienols, Phenolic compounds, Oxidation products, and Vitamin E. These results show that front-face Fluorescence EEM spectroscopy has a potential to be used for monocultivar EVOO discrimination.

Design of beam scanning metamaterial antenna

A beam scanning metamaterial planar reflective array antenna is designed by combining square and octagonal rings. Compared with the traditional array antenna design, this array antenna adopts a new phase compensation method thus by combining the phase curve obtained by the reflective array unit when the material of the dielectric substrate is different, the phase compensation from 0~360 is realized, so that the phase curve of the array unit need not to completely cover 0~360, and Hermite interpolation way is used to make up for the poor linearity of the phase characteristics. The advantage of this method is its universality, which reduces the design requirements for the array unit. Using this method, several single-layer plane reflectarray antennas were designed. The simulation results show that the direction of the reflected beam is consistent with the expected setting, and the side lobe and the main lobe are at least 15 dB apart. By adjusting the range of the solid-state plasma excitation region of the metamaterial to change the resonance structure of the array unit, beam scanning in different spatial frequencies is achieved, which provides a new method for the design of the planar reflectarray antenna.

Analysis of elastic waves generated in frozen grounds by means of the electromagnetic pulse source “Yenisei”

Computational technology for multiprocessor computing systems of cluster architecture is developed, the ultimate goal of which is numerical modeling of wave fields generated by the electromagnetic pulse source “Yenisei” in blocky-layered geomedia. To describe the wave processes, mathematical models of the dynamics of elastic and elastic-plastic media, of porous and granular materials are applied. The algorithms of numerical realization are constructed based on the method of two-cyclic splitting with respect to spatial variables. Computational experiments showed that the proposed technology allows reproducing the system of waves near the region of excitation of seismic oscillations in 3D setting with a high degree of details and accuracy. The results of computations can be used in working out the optimal modes of functioning the source “Yenisei”, when mechanical characteristics of the layers vary in a wide range from solid and frozen grounds with inclusions of rock till granular and clayey water-saturated grounds. Numerical analysis makes also possible to obtain the averaged data, necessary for the adequate simulation of the localized pulse action from the source, using simplified mathematical models for calculating the synthetic seismograms of reflected waves over large scale and at great depth of layers bedding in complex geomedia.

Characteristic Regions of the Fluorescence Excitation-Emission Matrix (EEM) To Identify Hydrophobic/Hydrophilic Contents of Organic Matter in Membrane Bioreactors.

This study systematically investigated the correlations between fluorescence distributions characterized by the excitation-emission matrix (EEM) and hydrophobic/hydrophilic composition of dissolved organic matter (DOM) in membrane bioreactors (MBRs). On the basis of samples from 10 full-scale MBRs, we performed point-to-point comparisons among different components using an EEM fluorescence quotient (FQ) method and obtained a hydrophobic/hydrophilic fluorophore distribution map via Wilcoxon signed rank test. Hydrophobic acids/bases (HOA/HOB) concentrated in the low-wavelength region [excitation wavelength (Ex) < 235 nm], while hydrophilic substances (HIS) were enriched in the region of Ex > 235 nm [especially with emission wavelength (Em) = 300-360 nm]. Quantitatively, EEM regional contribution to whole wavelength fluorescence was found to significantly correlate with the hydrophobic/hydrophilic proportions of DOM, with Pearson's coefficients of 0.94 and 0.78 ( p < 0.01) for HOA and HIS, respectively. We established a linear regression model showing the HOA proportion as a function of the EEM regional contribution at (Ex, Em) = (200-285, 340-465 nm), with R2 = 0.876, which was validated via leave-one-out cross-validation and Monte Carlo simulation. This study shows a statistically hydrophobicity-dependent fluorescence property across different MBRs, and it might be applied to provide a quick estimation of hydrophobic/hydrophilic composition of DOM in wastewater treatment systems based on EEM monitoring.

In Vivo Viscoelastic Response (VisR) Ultrasound for Characterizing Mechanical Anisotropy in Lower-Limb Skeletal Muscles of Boys with and without Duchenne Muscular Dystrophy

Our group has previously found that in silico, mechanical anisotropy may be interrogated by exciting transversely isotropic materials with geometrically asymmetric acoustic radiation force excitations and then monitoring the associated induced displacements in the region of excitation. We now translate acoustic radiation force-based anisotropy assessment to human muscle in vivo and investigate its clinical relevance to monitoring muscle degeneration in Duchenne muscular dystrophy (DMD). Clinical anisotropy assessments were performed using Viscoelastic Response ultrasound, with a degree of anisotropy reflected by the ratios of Viscoelastic Response relative elasticity (RE) or relative viscosity (RV) measured with the asymmetric radiation force oriented parallel versus perpendicular to muscle fiber alignment. In vivo results from rectus femoris and gastrocnemius muscles of boys aged ∼7.9–10.4 y indicate that RE and RV anisotropy ratios in rectus femoris muscles of boys with DMD were significantly higher than those of healthy control boys (RE: DMD = 1.51 ± 0.87, control = 0.99 ± 0.69, p = 0.04, Wilcoxon rank sum test; RV: DMD = 1.04 ± 0.71, control = 0.74 ± 0.22, p = 0.02). In the gastrocnemius muscle, only the RV anisotropy ratio was significantly higher in dystrophic than control patients (DMD = 1.23 ± 0.35, control = 0.88 ± 0.31, p = 0.04). In the dystrophic rectus femoris muscle, the RE anisotropy ratio was inversely correlated (slope = –0.03/lbf, r = –0.43, p = 0.07, Pearson correlation) with quantitative muscle testing functional output measures but was not correlated with quantitative muscle testing in the dystrophic gastrocnemius. These results suggest that Viscoelastic Response RE and RV measures reflect differences in mechanical anisotropy associated with functional impairment with dystrophic degeneration that are relevant to monitoring DMD clinically.

Vortices in self-bound dipolar droplets

Quantized vortices have been observed in a variety of superfluid systems, from $^4$He to condensates of alkali-metal bosons and ultracold Fermi gases along the BEC-BCS crossover. In this article we study the stability of singly quantized vortex lines in dilute dipolar self-bound droplets. We first discuss the energetic stability region of dipolar vortex excitations within a variational ansatz in the generalized nonlocal Gross-Pitaevskii functional that includes quantum fluctuation corrections. We find a wide region where stationary solutions corresponding to axially-symmetric vortex states exist. However, these singly-charged vortex states are shown to be unstable, either by splitting the droplet in two fragments or by vortex-line instabilities developed from Kelvin-wave excitations. These observations are the results of large-scale fully three-dimensional simulations in real time. We conclude with some experimental considerations for the observation of such states and suggest possible extensions of this work.

Heavy-mass magnetic modes in pyrochlore iridates due to dominant Dzyaloshinskii-Moriya interaction

Materials with strong spin-orbit interactions are presently a main target in the search for systems with novel magnetic properties. Magnetic anisotropies can be very large in such compounds, ranging from strongly frustrated Kitaev exchange in honeycomb iridates and the associated spin-liquid states to robust antisymmetric couplings in square-lattice Sr_2IrO_4 . Here we predict from ab initio quantum chemistry calculations that another highly unusual regime is realized in pyrochlore iridium oxides: the nearest-neighbor Heisenberg interaction can vanish whilst the antisymmetric Dzyaloshinskii- Moriya exchange reaches values as large as 5 meV, a result which challenges common notions and existing phenomenological models of magnetic superexchange. The resulting spin excitation spectra reveal a very flat magnon dispersion in the Nd- and Tb-based pyrochlore iridates, suggesting the possibility of using these modes to store magnetic information. Indeed the magnetization dynamics indicates that these modes are unable to propagate out of the excitation region.

Spectral Ripples in Round-Window Cochlear Microphonics: Evidence for Multiple Generation Mechanisms.

The cochlear microphonic (CM) results from the vector sum of outer hair cell transduction currents excited by a stimulus. The classical theory of CM generation-that the response measured at the round window is dominated by cellular sources located within the tail region of the basilar membrane (BM) excitation pattern-predicts that CM amplitude and phase vary little with stimulus frequency. Contrary to expectations, CM amplitude and phase-gradient delay measured in response to low-level tones in chinchillas demonstrate a striking, quasiperiodic pattern of spectral ripples, even at frequencies > 5kHz, where interference with neurophonic potentials is unlikely. The spectral ripples were reduced in the presence of a moderate-level saturating tone at a nearby frequency. When converted to the time domain, only the delayed CM energy was diminished in the presence of the saturator. We hypothesize that the ripples represent an interference pattern produced by CM components with different phase gradients: an early-latency component originating within the tail region of the BM excitation and two delayed components that depend on active cochlear processing near the peak region of the traveling wave. Using time windowing, we show that the early, middle, and late components have delays corresponding to estimated middle-ear transmission, cochlear forward delays, and cochlear round-trip delays, respectively. By extending the classical model of CM generation to include mechanical and electrical irregularities, we propose that middle components are generated through a mechanism of "coherent summation" analogous to the production of reflection-source otoacoustic emissions (OAEs), while the late components arise through a process of internal cochlear reflection related to the generation of stimulus-frequency OAEs. Although early-latency components from the passive tail region typically dominate the round-window CM, at low stimulus levels, substantial contributions from components shaped by active cochlear processing provide a new avenue for improving CM measurements as assays of cochlear health.

Revealing the effects of damping on the flow-induced vibration of flexible cylinders

This study reveals how damping shapes the global vortex-induced vibration (VIV) response of flexible cylinders. Global behavior may vary from full-length standing waves to traveling waves on infinite cylinders. Structural damping rules the standing wave case whereas radiation damping regulates VIV response on very long cylinders. A single scalar equation expresses the balance of power flowing through the structure. In that equation, Arms, which is the root-mean-square response in the VIV excitation region, is shown to be an excellent indicator of global response because of its relation to power flow. Under steady-state conditions, the net power flow must be zero, which directly leads to three independent dimensionless damping parameters, namely α,βR,andc*.βR indicates when radiation damping is important, α reveals the relative importance of structural versus radiation damping, and c∗ locates the global VIV behavior on the spectrum of lightly to strongly damped systems. Structural, hydrodynamic, and wave radiation damping are all taken into account. Plots of Arms∗ versus c∗ show the global effects of damping on response. Uncontrolled factors often reveal themselves as graphical anomalies, leading to new insights on VIV. Data from experiments and numerical simulations are presented to support the conclusions.

Reduction of Ce(IV) to Ce(III) induced by structural characteristics and performance characterization of pyrophosphate MgIn2P4O14-based phosphors

Due to the high sensitivity of the unshielded 5d orbit to the host environment, Ce3+ has a wide-spread emission ranging from near ultraviolet to the red light region, making it a favorable choice for being UV-emitting phosphor activator and sensitizer of other co-doped lanthanide ions in luminescent materials. In present work, Ce4+→Ce3+ self-reduction phenomenon in the layered phosphate MgIn2P4O14 was observed. Noting that self-reduction ability is a valuable characteristic of new host materials, a new viewpoint underlying mechanism was put forward. Structure-, composition-, and temperature-dependence of photoluminescence of the new UV-emitting MgIn2P4O14: Cex phosphors were investigated, with the relative mechanism discussed. Moreover, the sensitization of Dy3+ by Ce3+ was observed in Ce3+/Dy3+ co-doped MgIn2P4O14 phosphors. The excitation region of Dy3+ in MgIn2P4O14: CeDy (220–460 nm) is markedly broadened compared to that of the Dy3+ single-doped phosphor (270–460 nm) on account of the Ce3+→Dy3+ energy transfer process. Present work shows that the MgIn2P4O14: CeDy phosphor can be promising phosphor using for fabrication of both light-emitting diodes (LEDs) and fluorescent lamps (FLs).

Line shape of coherent population trapping resonance in the Λ-scheme under Ramsey-type interrogation in an optically dense medium

We report a study of the Ramsey technique for detecting coherent population trapping resonance in a cold rarified ensemble of atoms with the collective effects related to its finite optical thickness taken into account. It is found that with the growth of the optical thickness of the medium the Ramsey comb becomes shifted with its maxima ‘trimmed’. The possibility of producing a region of maximal atomic excitation near the exit from the medium is found.

Ultrafast dynamics of hard tissue ablation using femtosecond-lasers.

Several studies on hard tissue laser ablation demonstrated that ultrafast lasers enable precise material removal without thermal side effects. Although the principle ablation mechanisms have been thoroughly investigated, there are still open questions regarding the influence of material properties on transient dynamics. In this investigation, we applied pump-probe microscopy to record ablation dynamics of biomaterials with different tensile strengths (dentin, chicken bone, gallstone and kidney stones) at delay times between 1 picosecond and 10 microseconds. Transient reflectivity changes, pressure and shock wave velocities and elastic constants were determined. The result revealed that absorption and excitation show the typical well-known transient behavior of dielectric materials. We observed for all samples a photomechanical laser ablation process, where ultrafast expansion of the excited volume generates pressure waves leading to fragmentation around the excited region. In addition, we identified tensile-strength-related differences in the size of ablated craters and ejected particles. The elastic constants derived were in agreement with literature values. In conclusion, pressure-wave-assisted material removal seems to be a general mechanism for hard tissue ablation with ultrafast lasers. This photomechanical process increases ablation efficiency and removes heated material, thus ultrafast laser ablation is of interest for clinical application where heating of the tissue must be avoided.

Resonant-light diffusion in a disordered atomic layer

Light scattering in dense media is a fundamental problem of many-body physics, which is also relevant for the development of optical devices. In this work we investigate experimentally light propagation in a dense sample of randomly positioned resonant scatterers confined in a layer of sub-wavelength thickness. We locally illuminate the atomic cloud and monitor spatially-resolved fluorescence away from the excitation region. We show that light spreading is well described by a diffusion process, involving many scattering events in the dense regime. For light detuned from resonance we find evidence that the atomic layer behaves as a graded-index planar waveguide. These features are reproduced by a simple geometrical model and numerical simulations of coupled dipoles.

Depth Profiling of Carrier Lifetime in Thick 4H-SiC Epilayers Using Two-Photon Absorption

Depth profiling of the ambipolar carrier lifetime was performed in n-type, 140mm thick silicon carbide (SiC) epilayer using excitation by two-photon absorption (TPA) with a pulsed 586nm laser, and confocal measurement of time resolved photoluminescence (TRPL) decay from the excited region. A depth resolution of ≈10mm was obtained. The PL decay curves were analyzed using a recently developed formalism that takes into account the TPA excitation, carrier diffusion and surface/interface recombination. The carrier lifetime decreases near the top surface of the epitaxial layer as well as near its interface with the substrate.

Interference and phase mismatch effects in coherent triple sum frequency spectroscopy

Triply resonant sum frequency is a fully coherent method for multidimensional vibrational/electronic spectroscopy that creates cross-peaks between fundamental, overtone, and combination band vibrational states and electronic/vibronic states if they are coupled. It is also a methodology that cannot be phase matched. This paper explores the consequences of coherent interference and phase matching on the multidimensional spectra obtained in experiments performed in sample cells. Fringes appear in the multidimensional spectra and simulations show the character of the fringes depend on the refractive index dispersion, the thickness of different parts of the sample cell, the volume of the excitation region, and the relative third order susceptibility of the sample and the windows. It is suggested that the fringes provide a mechanism for determining the third order susceptibility and its multidimensional dispersion.

Internal Quantum Efficiency of Led Structures at Various Charge Carrier Distributions Over InGaN/GaN Quantum Wells

The results of studying the effect of the thickness of GaN barrier layers in the active region of LED structures with InGaN/GaN quantum wells on the internal quantum efficiency (IQE) of photoluminescence are presented. It is shown that a decrease in the thickness of the GaN barrier layers from 15 to 3 nm leads to an increase in the maximum value of IQE and to a shift of the maximum to the region of high excitation powers. The result obtained is explained with consideration for the decrease in the Auger recombination rate due to a more uniform distribution of charge carriers over the active region in structures with a barrier thickness of 3 nm.

T103. The connectivity index: A novel metric to grade epileptogenicity

To investigate the performance of a metric of functional connectivity to classify and grade excitable brain regions based on cortical-evoked potentials to single pulse electrical stimulation (SPES) compared to non-epileptic sites. Patients who received 1-Hz frequency stimulation in 2003–2014 at Yale, were included. The stimulated contacts were classified as seizure onset zone (SOZ), irritative zone (IZ, frequent or periodic spiking, and early ictal propagation within 5 s of seizure onset) or control. Stimulation intensities ranged 9–12 mA (or lower if not tolerated or if auras were triggered) and pulse width of 250–300 μs. Evoked responses were reproduced at least once and averaged from 45–110 single pulse stimulations. Response contacts were classified as seizure onset zone (SOZ), active interictal (IZ), quiet or other when data did not indicate whether a contact was involved in an epileptic activity or not. The normalized number of responses was defined as the number of contacts with any evoked responses divided by the total number of contacts, and the normalized distance is the ratio of the average distances between the site of stimulation and sites of evoked responses to all other recording contacts. A new metric we labeled the connectivity index (Ci) is defined as the product of the two values. Fifty-seven stimulation sessions in 22 patients were analyzed. Stimulation of SOZ contacts evoked cortical potentials at significantly higher rates than control sites (medians 0.72 vs. 0.26, respectively p 0.0003). These differences were even stronger when normalized to the average distance of recorded responses from the stimulation site i.e. metric we labeled connectivity index Ci (median 0.74 vs. 0.16, respectively, p 0.0002). The evoked cortical potentials after stimulation of SOZ were seen at a further distance from the site of stimulation (median of normalized distance 0.96 vs. 0.62, p 0.0005). It was 1.8 times more likely to record an evoked response from the SOZ than non-epileptic contacts after stimulation of a remote-control site. Habitual partial seizures or auras were triggered in 27% of the patients and 35% of the SOZ contacts that were stimulated (median stimulation intensity 4 mA) but in none of the control or IZ contacts. Non-SOZ contacts in multifocal or poor surgical outcome cases had a higher (Ci) than those in localizable onsets (medians Ci of 0.5 vs. 0.12, p 0.04). There was a correlation between the stimulation current intensity and the normalized number of evoked responses (r = + 0.49, p 0.01) but not with distance (r = + 0.1, p 0.64). The SOZ has an enhanced connectivity with more distant sites compared to non-epileptic contacts. The responses evoked by stimulation of the SOZ have longer average latencies and durations. Auras and seizures provoked by SPES are predictive of brain sites involved in seizure generation. There was a correlation between the Ci of non-SOZ contacts and the duration of epilepsy.

Relative Ability of Wedge-Coupled Piezoelectric and Meander Coil EMAT Probes to Generate Single-Mode Lamb Waves.

Ultrasonic guided waves are used extensively when checking for defects in petrochemical and other industries and are mostly generated using piezoelectric transducers on an angled wedge or electromagnetic acoustic transducers (EMATs) in different configurations. Low-frequency inspection allows for long-distance propagation, but it is best suited for detecting relatively large defects, while at higher frequencies, the presence of multiple wave modes limit defect detectability, so achieving practical single Lamb mode excitation via careful transduction is very beneficial. This paper investigates the relative ability of angled piezoelectric and meander coil EMAT probes to produce single-mode transduction in the medium (~1-5 MHz-mm) and high (>5 MHz-mm) frequency-thickness regions of the dispersion curves. The nature of each transducer is studied analytically by simulating the corresponding surface forces, followed by the use of a Fourier transform in time and space (2-D fast Fourier transform) to highlight the excitation region in the wavenumber-frequency space. With angled wedge excitation there is a linear relationship between the excitation frequency and the wavenumber which means that the excitation tends to track typical dispersion curves, allowing for easier pure mode generation. In contrast, the EMAT controls frequency and wavenumber separately which makes it more difficult to generate a pure mode when dispersion curves are close together; however, by narrowing the frequency bandwidth via a large number of cycles in the excitation signal, pure mode generation via an EMAT was shown to be possible even in areas of closely spaced modes. As example cases, analytical results, backed up by experiments, showed that signals dominated by the A0 mode at 1.5 MHz-mm and also the A1 mode at 18 MHz-mm can be generated with both angled piezoelectric and EMAT probes.