Off-resonance Frequency Research Articles

Adaptive control of structural intensity associated with bending waves in a beam

This paper examines a method to control adaptively the structural vibration intensity in a beam. An algorithm is developed to estimate the total, instantaneous structural intensity, using finite-difference techniques. In addition, algorithms based on the filtered-x least-mean-squares algorithm are developed to adaptively control the intensity. To investigate the effectiveness of adaptive control of structural intensity, a number of control actuator/error sensor configurations are used. Adaptive control is implemented at resonance and off-resonance frequencies, and the performance is evaluated by means of a separate accelerometer located in the structural far field. Experimental results demonstrate several trends. First, controlling the acceleration is considerably more effective when the error sensor is located in the far field rather than in the near field. Furthermore, controlling acceleration is more effective than controlling intensity, when the error sensors are in the far field. Conversely, when the error sensors are in the near field, the attenuation achieved by controlling intensity is comparable to or greater than that achieved by controlling acceleration.

Determination of proton magnetization transfer rate constants in heterogeneous biological systems

Two procedures are currently in use for the determination of proton magnetization transfer rate constants between macromolecular tissue components and water. The first method assumes that there are only two spin baths (macromolecular plus solvent) and that during off-resonance irradiation complete saturation of the "immobile" proton spin bath occurs (S. H. Koenig, R. D. Brown, III, R. Ugolini, Magn. Reson. Med. 29, 311 (1993)). This approach neglects the possibility of incomplete saturation and polydispersity, and yields an apparent magnetization transfer rate constant, Kapp. The second approach utilizes a formalism which can account for polydispersity and incomplete saturation of the immobile spin bath (K. Kuwata, D. Brooks, H. Yang, T. Schleich, J. Magn. Reson., in press). In this work magnetization transfer rate constants derived by the use of both methods for two systems, ocular lens tissue and cross-linked bovine serum albumin (BSA) were compared. For both samples Kapp was dependent on B2 off-resonance irradiation frequency and power when the first method was used. The second method provided values of the magnetization transfer rate constant that were similar to the values obtained by the first method, as the limit of complete saturation was approached.

Internal electric energy in a spherical particle illuminated with a plane wave or off-axis Gaussian beam

The electric energy in a lossless or lossy spherical particle that is illuminated with a plane wave or a Gaussian beam is investigated. The analysis uses a combination of the plane-wave spectrum technique and the T-matrix method. Expressions for the electric energy in any mode as well as the total electric energy inside the particle are given. The amount of energy coupling into the particle for different beam illuminations is also investigated. The high-Q (low-order) resonant modes can dominate the electric energy inside a spherical particle many linewidths away from the resonance location, particularly if the beam is focused at the droplet edge or outside the droplet. If the sphere is lossy, low-order modes can still dominate the electric energy if the beam is focused far enough outside the sphere. As the absorption coefficient of the particle increases, the energy in a high-Q mode decreases much faster at the resonance frequency than it does at near or off-resonance frequencies. The effects of the absorption on the dominance of the internal fields by a high-Q mode decreases as the beam is shifted farther away from the particle. As the beam is shifted farther away from the particle the fraction of the incident energy coupled into the sphere at resonance first increases and then decreases. Although the coupled energy decreases as the beam is shifted farther from the sphere, most of that energy is in the lowest-order mode.

A biologically inspired control approach for distributed elastic systems

A multi-degree of freedom control approach, which is largely inspired by biological systems, is presented. Control inputs to a structure are achieved by multiple piezoelectric actuators. One actuator chosen as the ‘‘master’’ actuator is under the direction of the central, sophisticated controller. The other ‘‘slave’’ actuators derive their control inputs by localized, simple learning rules related to the behavior of their neighbor actuators including the ‘‘master.’’ Simulations on the control of the vibrational energy density of a harmonically excited simply supported beam demonstrate a significant improvement in control performance over a single actuator case, particularly for off-resonance frequencies. Thus high attenuations are achieved with a multiple degree of freedom actuator with a single main channel of control.

A three‐dimensional spin‐echo or inversion pulse

In theory, multidimensional pulses can be designed to be selective in any number of dimensions. In practice, available gradient power has enforced a limit to two dimensions. We show here that three-dimensional pi pulses are feasible on commercial imaging machines provided that the range of off-resonance frequencies are limited.

Surface Raman scattering of an azo dye in acidic solutions

The Raman scattering of azo dye adsorbed on silver sols in acidic solution was investigated and its surface-enhanced Raman scattering (SERS) spectra and excitation behavior are reported. It was found that different tautomeric species of the azo dye show different SERS enhancements when excited with laser on- and off-resonance frequencies. These observations can be explained by a charge-transfer model rather than an electromagnetic model

Nonlinear resonance and viscous dissipation in an acoustic chamber

The nonlinear resonance of an air-filled acoustic chamber with rigid endwalls is studied numerically using the fully nonlinear Lagrangian wave equation. The effects of bulk viscous dissipation and external excitation are included. It has been found that at large excitation, the sound amplitude and the quality factor Q of the chamber are independent of viscosity, and are inversely proportional to each other. The highlight of this work is the interpretation of these results in simple physical terms. These results are consistent with those from a previously developed analytical theory, although in the latter they have been overshadowed by algebra and have not been properly interpreted. It has been deduced that the free decay rate of a large-amplitude wave due to bulk dissipation is proportional to its acoustic Mach number, and is also independent of viscosity. Also considered was viscous dissipation on the side walls, but it was found that it is much less important than bulk dissipation at high nonlinearity, considering the typical dimension and boundary layer thickness in this type of system. The wave profiles excited at slightly off-resonance frequencies have also been calculated, and found to be similar to those in a piston-driven resonant tube.

Magnetization transfer contrast imaging of the human leg at 0.1 T: A preliminary study

Magnetization transfer contrast imaging is an MR technique that capitalizes on interactions between the protons of mobile and macromolecularly bound water molecules. Studies to date, conducted primarily on 4.7 T and 1.5 T MR systems, have yielded results unique from conventional T 1- and T 2-weighted imaging studies. In this study, performed on a 0.1 T device, a section of lower leg was imaged in 20 normal human subjects and one patient with muscular dystrophy, using both a standard 500 22 gradient-echo sequence and a 500 22 gradient-echo sequence combined with off-resonance radio frequency irradiation designed to elicit magnetization transfer contrast. Results of the two techniques were compared. Our findings suggest that magnetization transfer contrast imaging is feasible at 0.1 T, and that this technique allows reproducible tissue characterization and improves contrast between certain tissues.

Broadband excitation and detection of cross-relaxation NMR spectra

Nuclear magnetic resonance cross-relaxation spectroscopy maps the proton solidstate NMR spectrum onto the proton water NMR signal of aqueous heterogeneous materials ( Z-3). Application of a relatively long, low-intensity RF pulse, off resonance from the water Larmor frequency but within the linewidth of the solid, partially saturates the nuclear magnetism of the solid protons. Magnetic cross relaxation between the immobile and mobile protons transfers the saturation of the solid protons to the water protons, decreasing the magnitude of the water NMR signal. A cross-relaxation spectrum is obtained by saturating the immobile protons at many discrete frequencies and plotting the ratio of the water signal with and without RF saturation as a function of the saturation frequency. The intensity and the width ofthe cross-relaxation spectrum are a function of the longitudinal, transverse, and cross-relaxation rates of the liquid and solid components, the mole fraction of liquid to solid protons, and the magnitude and duration of the saturating RF pulse. This Communication describes a method for obtaining the entire cross-relaxation spectrum of a spatially homogeneous material with two acquisitions: RF saturation on and RF saturation off. The pulse sequence is demomtrated schematically in Fig. I. A magnetic field gradient and RF pulse are applied to the sample concurrently. The gradient creates a continuum of frequencies across the !;ample and establishes at once all conditions of off-resonance saturation for the RF pulse. The decrease in water magnetization by cross relaxation as a function of off-resonance frequency is stored in the sample as a function of the position in the sample and detected by recording a onedimensional image with the readout gradient applied along the same axis as the frequency-dispersal gradient. The cross-relaxation spectrum is created by dividing the image obtained with RF saturation turned on by one obtained with RF saturation turned off. The pulse sequence shown in Fig. 1 represents the simplest implementation of broadband cross-relaxation spectroscopy. The sequence may be modified to include slice selection, spin-echo detection, and imaging by phase encoding in either or both of the two spatial dimensions which are not needed to record the cross-relaxation spectrum. In addition the magnitude of the readout gradient may be decreased to lower the bandwidths of the receiver and increase the signal-to-noise of the crossrelaxation spectrum.

Comparison of agarose and cross-linked protein gels as magnetic resonance imaging phantoms

Measurements of the magnetic field dependence of spin-lattice relaxation rates and the response of the water-proton signal intensity to off-resonance radio frequency fields show that the commonly used agarose phantom provides a less faithful representation for the magnetic response of tissue than does a cross-linked protein system. The origin of these differences lies in the structure and intramolecular dynamics of the macromolecular system used to make the gel. These distinctions will also cause differences in the magnetic response of the water spin system when paramagnetic relaxation agents or contrast agents are incorporated. Use of a thermally cross-linked bovine serum albumin phantom is suggested.

The synthesis of pulse sequences yielding arbitrary magnetization vectors.

A new procedure and algorithm are presented to allow the synthesis of a pulse sequence which will generate an arbitrary frequency-dependent spin excitation. This procedure is a generalization of our previous paper, where this was done subject to the restriction that the spin excitation was symmetric about zero offset frequency, and pulses were restricted to being about a fixed axis. The required final z-magnetization vector (Mz) is expressed as a function of the off-resonance frequency as an Nth order complex Fourier series. We then form a consistent Fourier series for (Mxy). As many as 2(2)N different pulse sequences may be directly generated all of which produce a different Mxy(f), but the same Mz(f). A pulse sequence is then generated which will yield the desired Mz(f) and Mxy(f). This is done by an analytic inversion of the Bloch equation, not by the classical Fourier approximation. This technique enables us to generate any Mz which is potentially realizable by a pulse sequence.

Spectral and temporal behavior of resonant secondary emission in NaNO 2

Spectral and temporal behavior of resonant secondary emission in NaNO 2 has been studied with exciting light v i around the zero-phonon absorption line at v 00. In the region v i≲ v 00, the emission spectrum consists of multiple-order scattering lines. However, it is confirmed by time-resolved spectroscopy that the emission is transformed from luminescence into Raman scattering with increase in the off-resonance frequency. The result shows the memory effect of the reservoir in the second-order optical process. For v i> v 00, the emission line splits into “Raman-like” and “luminescence-like” lines in the spectrum. The former disappears rapidly, while the latter becomes broader and stronger, as v i exceeds v 00. When v i moves into valleys of the phonon sideband o f the v 00 absorption line, the luminescence line splits into two or more components. The results suggests that the radiative annihilation of the v 00 exciton occurs before the thermalization within the exciton band.

Approximate solutions of the bloch equations for selective excitation

Approximate solutions to the Bloch equations for selective excitation are derived using a perturbation method. High-order terms are analyzed, employing some special properties of corresponding multidimensional integrals. It is found that for excitation by a symmetric RF pulse applied in the presence of a field gradient the spin nutation angle as a function of the off-resonance frequency is approximately proportional to the Fourier dine transform of the RF function. This result is more accurate than the common assumption that the transverse magnetization as a function of the off resonance frequency is approximately proportional to the Fourier transform of the RF function. This analysis is useful for understanding the behaviors of both spin-inversion and phase-reversal 180° selective RF pulses as well as selective-excitation pulses, and it also offers insight to the problems of designing better selective RF pulses.

Non-Markovian effect in resonant light scattering: Time-resolved spectrum.

We present a theoretical work on non-Markovian effects in the time-resolved spectrum (TRS) of resonant light scattering caused by a short pulse excitation. It is found that when the off-resonance frequency becomes comparable or greater than the inverse of the reservoir correlation time ${\ensuremath{\tau}}_{c}$, the finiteness of ${\ensuremath{\tau}}_{c}$ reduces the luminescencelike component which is energetically and temporally uncorrelated with incident radiation. It is emphasized that this non-Markovian effect in TRS is essential for obtaining a full understanding of the resonant light scattering phenomena.

Transient behavior of the second-order optical process in CdxZn1-xTe: Transformation from luminescence into Raman scattering.

We report on the transient behavior of the second-order optical process in the picosecond time region in mixed semiconducting crystals. Time-integrated spectra under resonant excitation below the band gap exhibit ``Raman-like'' lines associated with localized excitons. However, time-resolved spectroscopy enabled us to resolve these Raman-like lines into resonant luminescence and Raman scattering lines. We have found the transformation of luminescence into Raman scattering as a function of incident frequencies without changing the spectral shape. Furthermore, the intensity ratio between Raman scattering and luminescence was determined for a wide range of the off-resonance frequency. The result is unambiguously explained by the theory by taking into account the finiteness of the correlation time of the reservoir. This gives the first experimental evidence for the non-motional-narrowing effect in the second-order optical process.

Off resonance heteronuclear spin-decoupling in solids

It is shown that, under what may be called an “intermediate” decoupling condition, off-resonance heteronuclear spin decoupling of an abundant spin causes not only a line broadening, but also a line splitting, and a frequency shift of the NMR spectra of the dilute spin in solids. Typical examples of the fineshape behavior as a function of off-resonance decoupling frequency are presented and analyzed for the CH, CH 2, and CH 3 carbon nuclei in a single crystal of dimedone. The quadratic dependence of the linewidth on the off-resonance decoupling frequency, under the “strong near resonance” decoupling condition, is examined for both the single-crystal case, and for the CP-MAS spectrum of dimedone.

Magnetostatic wave resonators and oscillators

The purpose of this paper is to describe the current status of magnetostatic wave tunable resonators operating between 2 and 12GHz and their applications to microwave oscillators, the wave propagating in a pure yttrium-iron-garnet epi-layer. The first section deals with magnetostatic surface wave (MSSW) resonators consisting of a pair of reflective grooved gratings and two microstrip-transducers set between the gratings. Results on insertion loss, off-resonance frequency rejection andQ value are presented. However MSSW cavities suffer several disadvantages such as the small saturation power level at low frequencies and the difficulty to temperature compensate MSSW devices. These problems are overcome with magnetostatic forward volume waves (MSFVW). Then the second section is devoted to MSFVW resonators. A peculiar folded geometry made of five grooved gratings and two microstrip-transducers has been designed to provide a sufficient out-of-band rejection. The experimental characteristics of these resonators are given. In the last section of this paper are reported the features of tunable oscillators implemented with MSSW and MSFVW resonators in a hybrid configuration. Results about tunability, output power,FM phase noise are reported. MSFVW resonator stabilized oscillators are very promising and magnetostatic wave resonators seem a challenge to YIG spheres.

Transformation of resonant Raman scattering into luminescence in Cd xZn 1−xTe mixed crystals: Time-resolved spectroscopy

We report an experimental study of time characteristics of secondary emission in Cd xZn 1-xTe mixed crystals ( x = 0.32) under resonant excitation with a picosecond dye laser. When the incident laser frequency is tuned on to the luminescence maximum of localized excitons, the decay curve of the intensity of “Raman-like” lines exhibits a single exponential decay. Off resonance, however, a short-lived component corresponding to Raman scattering appears in addition to the long-lived component. The intensity of the Raman component relative to that of the luminescence component increases with increase of the off-resonance frequency. From these temporal behaviors, we have found, for the first time, the transformation of resonant Raman scattering into luminescence in mixed crystals as a function of incident frequencies.

Theory for selective-vibrational-dephasing experiments with the use of transient stimulated Raman scattering in high laser depletion

A theory for selective-vibrational-dephasing experiments using transient stimulated Raman scattering in high laser depletion is presented. The transient stimulated Raman scattering equations are numerically solved including laser depletion for homogeneously and inhomogeneously broadened vibrational line shapes. A detailed account of the effects of low and high laser depletion on the vibrational amplitudes is given. An interpretation for excite-and-probe vibrational-dephasing experiments undertaken in the low- and high-laser-depletion regimes is developed. For a collinear laser probe pulse, high depletion of the excitation laser pulse leads to enhanced coherent Stokes probe scattering from one vibrational subgroup in the inhomogeneously broadened line shape. This selective scattering is greatly dependent upon the excitation laser pulse width and the off-resonance frequency of the vibrational subgroup. The enhanced selectivity in high laser depletion allows the homogeneous dephasing time ${T}_{2}$ to be measured to a high level of accuracy.

Abnormal intensity effects in heteronuclear NMR double-resonance experiments of A nX spin systems. Theoretical description and experimental possibilities of its suppression

Under certain experimental conditions, anomalous intensity asymmetries within the multiplet of the X lines of an A n X spin system in the X-{A}-off-resonance decoupled NMR spectrum can occur. It is shown, by a simple approximation, that these asymmetries depend on both the experimental parameters such as the flip angle and the repetition time between the irradiated measuring pulses and the off-resonance parameters and on the system parameters such as relaxation times and NOE factor. The asymmetries can be excluded by a new measuring technique (ORAND) which involves the alternating radiation of the single off-resonance decoupling frequency during data acquisition and noise decoupling in the interval between acquisition. The advantage of off-resonance spectra (NOE, simplification of the spectra) is maintained by this technique.