Showing papers on "Resonance published in 1992"

New principle for optical filters

Abstract: A theoretical characterization of the guided‐mode resonance properties of planar dielectric waveguide gratings is presented. Efficient exchange of energy between forward and backward propagating diffracted waves is shown to be possible with smooth line shapes. The linewidths of the resonances can be controlled by the grating modulation amplitude. Due to the inherent separation between the TE and TM modes associated with the waveguide grating, these elements can provide polarization separation. Applications to polarization sensitive filtering and to electro‐optic switching are suggested. The guided‐mode resonance filter represents a basic new optical element.

Dependence of the quality factor of micromachined silicon beam resonators on pressure and geometry

Abstract: An experimental study of damping and frequency of vibrating small cantilever beams in their lowest eigenstate is presented. The cantilever beams are fabricated from monocrystalline silicon by means of micromachining methods. Their size is a few millimeters in length, a few 100 µm in width, and a few 10 µm in thickness. Damping and resonance frequency are studied as a function of the ambient pressure p (1–105 Pa) and the geometry of the beam. The purpose of this research was to obtain design rules for sensors employing vibrating beams. The analysis of the experimental results in terms of a semiqualitative model reveals that one can distinguish three mechanisms for the pressure dependence of the damping: viscous, molecular, and intrinsic. For viscous damping a turbulent boundary layer dominates the damping at high pressures (105 Pa), while at smaller pressure laminar flow dominates. In the latter region, this leads to a plateau for the quality factor Q and in the former to Q p. The pressure pc at which the transition from laminar flow dominated damping to turbulent flow dominated damping occurs depends on the geometry of the beams. pc is independent on the length and decreases with both, the width and the thickness of the beams.

High speed quantum-well lasers and carrier transport effects

Abstract: Carrier transport can significantly affect the high-speed properties of quantum-well lasers. The authors have developed a model and derived analytical expressions for the modulation response, resonance frequency, damping rate, and K factor to include these effects. They show theoretically and experimentally that carrier transport can lead to significant low-frequency parasitic-like rolloff that reduces the modulation response by as much as a factor of six in quantum-well lasers. They also show that, in addition, it leads to a reduction in the effective differential gain and thus the resonance frequency, while the nonlinear gain compression factor remains largely unaffected by it. The authors present the temperature dependence data for the K factor as further evidence for the effects of carrier transport. >

Formulas for the analysis of surface sum‐frequency generation spectrum by CH stretching modes of methyl and methylene groups

Abstract: Derivation has been made of the expressions of surface vibrational sum‐frequency generation(SVSFG). The results include (a) the vibrational selection rule, (b) the expressions of the susceptibility tensors at the surfaces composed of functional groups with C3v and C2v local symmetry, (c) the formulas of the SFG tensor components which apply specifically to the vibrational resonance with the CH stretching modes of methyl and methylene groups, and (d) the interference effect by the nearby Fermi resonance band. Changes introduced by the slight distortion of C3v symmetry into Cs symmetry have also been discussed. Sample calculations have been made of the SVSFG of the CH stretch modes of oriented CH3 groups.

Resonances in electron scattering by molecules on surfaces

Abstract: This article reviews the discovery, exploration, and application of negative-ion resonances in inelastic electron scattering by molecules adsorbed on surfaces. A major theme of the review is the degree to which the properties of resonances in free molecules are perturbed by adsorption. The influence of the surface upon the energy, lifetime (width), symmetry, and decay channels of molecular resonances is discussed, in the light of both experimental and theoretical studies of a wide range (from diatomic molecules to polymers) of both weakly bound (physisorbed) and strongly bound (chemisorbed) molecules. The metallic image potential, electron scattering by the atoms of the surface, and chemical bonding in chemisorption systems are found to be key factors in determining the energy, width, and symmetry of resonances in molecular adsorbates. In the case of oriented adsorbed molecules, the angular distribution of scattered electrons is found to reflect not only the symmetry of the resonant state (as in the gas phase), but also the orientation of the molecular axis. Coherent elastic electron scattering by the surface can modulate the angular distributions, as well as the shape of the resonance profile. Selection rules that govern the observed resonance behavior are discussed. A further consequence of adsorption ismore » the enrichment of the range of channels into which resonances may decay, and the excitation of both molecule-surface and intermolecular vibrational modes has been established. The article concludes with an evaluation of future prospects for the investigation and application of resonances in adsorbed molecules.« less

Macroscopic quantum tunneling in magnetic proteins.

Abstract: We report low-temperature measurements of the frequency-dependent magnetic noise and magnetic susceptibility of nanometer-scale antiferromagnetic horse-spleen ferritin particles, using an integrated dc SQUID microsusceptometer. A sharply defined resonance near 1 MHz develops below T\ensuremath{\sim}0.2 K. The behavior of this resonance as a function of temperature, applied magnetic field, and particle concentration indicates that it results from macroscopic quantum tunneling of the N\'eel vector of the antiferromagnets.

The Physics of non-crystalline solids

Abstract: Plenary lecture the structure of glass novel glasses and processes relaxation phenomena high temperature studies and crystallization ionic-transport high silica glasses/sol-gel synthesis optical and electrical phenomena mechanical properties chemical interactions resonance phenomena.

Dynamics and spatial order of cold cesium atoms in a periodic optical potential.

Abstract: The spatial distribution and the dynamics of Cs atoms in a 1D optical molasses are probed by measuring the absorption (or amplification) spectrum of a weak laser beam. Narrow (35--50 kHz) Raman lines give access for the first time to the frequency and to the damping of the atom's oscillation in the potential wells associated with light shifts. A narrower (8 kHz) Rayleigh resonance demonstrates the existence of a large-scale spatial order of the atoms, presenting some analogy with an antiferromagnetic medium.

Determination of Young's moduli of micromechanical thin films using the resonance method

Abstract: The resonance method has been used to determine the Young's modulus of 0.54 μm low-stress LPCVD Si x N y films. The films are prepared as thin single-layer cantilevers using standard silicon micromachining techniques. By using different excitation methods, including photothermal, acoustic and mechanical, the thin-beam resonating structures can be forced to vibrate. The influences of air damping and cantilever deflection, produced during preparation, on the cantilever resonance behavior are discussed.

Fullerene structure and dynamics: a magnetic resonance potpourri

Abstract: Magnetic resonance spectroscopy has been a particularly powerful tool for probing the structure and dynamical behavior of fullerenes. NMR and EPR studies of native and modified fullerenes are reported here.

Theory of high-resolution mass spectrometry achieved via resonance ejection in the quadrupole ion trap

Abstract: Beginning with the pseudopotential well approximation for ion motion in the quadrupole ion trap, analytical expressions are derived for the ion resonance ejection line shape. Incorporation of terms for ion-molecule collisions and linear variation with time of the resonance excitation frequency enables line shapes to be calculated and displayed as a function of scan rate and relaxation time. Derivation of the relationship between mass and frequency line widths allows the mass resolution to be evaluated as a function of fundamental ion trap operating parameters. The results account for the synergistic effect on resolution when resonance ejection is combined with retarded scan rates and indicate that for any non-zero scan rate there is an optimum collisional damping value corresponding to a resolution maximum. The effects of the experimental variables mass/charge, neutral pressure, and scan rates on mass resolution and accuracy are also evaluated in terms of the model and illustrated in convenient plots. 36 refs., 4 figs.

Variable-angle correlation spectroscopy in solid-state nuclear magnetic resonance

Abstract: We describe here a new solid‐state nuclear‐magnetic‐resonance (NMR) experiment for correlating anisotropic and isotropic chemical shifts of inequivalent nuclei in powdered samples. Spectra are obtained by processing signals arising from a spinning sample, acquired in independent experiments as a function of the angle between the axis of macroscopic rotation and the external magnetic field. This is in contrast to previously proposed techniques, which were based on sudden mechanical flippings or multiple‐pulse sequences. We show that the time evolution of variable‐angle‐spinning signals is determined by a distribution relating the isotropic frequencies of the spins with their corresponding chemical shift anisotropies. Fourier transformation of these data therefore affords a two‐dimensional NMR spectrum, in which line shapes of isotropic and anisotropic interactions are correlated. Theoretical and experimental considerations involved in the extraction of this spectral information are discussed, and the techn...

Raman‐vibronic double‐resonance spectroscopy of benzene dimer isotopomers

Abstract: The results of Raman‐vibronic double resonance experiments on benzene dimer are reported. The results were obtained by mass‐selective, ionization‐detected stimulated Raman spectroscopies. The data pertain to the ν1, ν2, and ν6 fundamentals of numerous dimer isotopomers. The results are discussed in terms of the geometry of the dimer. They show that the species is characterized by two inequivalent benzene sites with one of the sites of low and the other of higher symmetry. These two sites give rise to different Raman resonance frequencies, different vibrational dynamics, and markedly different S1↔S0 vibronic spectra. It is argued that all of the experimental results are consistent with a T‐shaped equilibrium geometry in which the benzene moiety at the top of the T is freely rotating about its C6 axis.

Electroluminescence-detected magnetic-resonance study of polyparaphenylenevinylene (PPV)-based light-emitting diodes

Abstract: The strong electroluminescence (EL)-detected magnetic resonance of PPV-based light emitting diodes is compared to the conductivity ([sigma])- and photoluminescence (PL)-detected resonances. It provides direct evidence that polaron-to-singlet exciton conversion is responsible for the EL. In contrast to the narrow PL-enhancing resonance assigned to polaron recombination, strong EL- and [sigma]-quenching resonances are attributed to the spin-dependent polaron-to-bipolaron decay. The half-field EL- and [sigma]-detected resonances of two distinct triplet excitons is believed to result from triplet-triplet fusion to singlets.

A new infrared electronic transition of the oxidized primary electron donor in bacterial reaction centers: a way to assess resonance interactions between the bacteriochlorophylls.

Abstract: The primary electron donor in the reaction center of purple photosynthetic bacteria consists of a pair of bacteriochlorophylls (PL and PM). The oxidized dimer (P+) is expected to have an absorption band in the mid-IR, whose energy and dipole strength depend in part on the resonance interactions between the two bacteriochlorophylls. A broad absorption band with the predicted properties was found in a previously unexplored region of the spectrum, centered near 2600 cm-1 in reaction centers of Rhodobacter sphaeroides and several other species of bacteria that contain bacteriochlorophyll a, and near 2750 cm-1 in Rhodopseudomonas viridis. The band is not seen in the absorption spectrum of the monomeric bacteriochlorophyll cation in solution, and it is missing or much diminished in the reaction centers of bacterial mutants that have a bacteriopheophytin in place of either PL or PM. With the aid of a relatively simple quantum mechanical model, the measured transition energy and dipole strength of the band can be used to solve for the resonance interaction matrix element that causes an electron to move back and forth between PL and PM, and also for the energy difference between states in which a positive charge is localized on either PL or PM. (The absorption band can be viewed as representing a transition between supermolecular eigenstates that are obtained by mixing these basis states.) The values of the matrix element obtained in this way agree reasonably well with values calculated by using semiempirical atomic resonance integrals and the reaction center crystal structures.(ABSTRACT TRUNCATED AT 250 WORDS)

Resonance-enhanced x-rays in thin films: a structure probe for membranes and surface layers

Abstract: An x-ray resonance effect in an organic thin film on an x-ray reflecting mirror is reported. The resonance effect is the result of interference between reflected and refracted x-rays at the air-organic thin film interface and occurs at incident angles slightly above the critical angle of the film. In excellent agreement with theory, the primary resonant x-ray electric field that is confined in the organic thin film is approximately 20 times as intense as the electric field of the incident beam when measured at a position close to the center of the film. Resonance-enhanced x-rays can be used to characterize the internal structure of Langmuir-Blodgett thin film membranes. This effect may also find use in x-ray-based thin film devices and in the structural analysis of adlayers and surfaces that have thus far proved difficult, if not impossible, to study because of sensitivity limitations.

Resonance frequency, damping, and differential gain in 1.5 mu m multiple quantum-well lasers

Abstract: A systematic investigation is presented into the intrinsic frequency response of quantum-well lasers, using parasitic-free relative intensity noise (RIN) measurements. There is shown to be a strong dependence of the resonance frequency on the number of quantum wells in the active region, originating from variations both in internal losses and in differential gain. The differential gain is found to have values higher than in corresponding bulk lasers, but only in devices with a large number of wells. The damping is also found to vary in a manner consonant with the changes in differential gain; however, comparison with bulk lasers indicates substantially stronger gain suppression in the quantum-well lasers studied. >

Choice of metal and wavelength for surface-plasmon resonance sensors: some considerations.

Abstract: An optimal metal–waυelength combination depends on the measuring principle of the surface-plasmon resonance (SPR) sensor. Silυer at 800 nm is the best for fixed-angle measurements.

Application of Floquet theory to the nuclear magnetic resonance spectra of homonuclear two-spin systems in rotating solids

Abstract: The theory for the nuclear magnetic resonance (NMR) spectra of homonuclear two‐spin systems under sample spinning has been developed. The propagator describing the time‐evolution of the systems, which is driven by the homogeneous Hamiltonian composed of the chemical‐shift difference and the flip–flop parts of the dipolar and J couplings, is treated using Floquet theory; the Floquet eigenvalues and eigenvector components determine the resonance frequencies and intensities, respectively. Nondegenerate Rayleigh–Schrodinger perturbation theory is used to solve the Floquet secular equation. Recurrence equations for the perturbation corrections have been derived, allowing us to evaluate efficiently very high‐order terms, such as 20th‐order terms. Analytical expressions for the resonance frequency obtained in the low‐order approximations provide an intuitive understanding of the main spectral features; the second‐order equations can describe the conspicuous solid‐state effects on the magic‐angle spinning (MAS) s...

Rotational resonance NMR study of the active site structure in bacteriorhodopsin: conformation of the Schiff base linkage

Abstract: Rotational resonance, a new solid-state NMR technique for determining internuclear distances, is used to measure a distance in the active site of bacteriorhodopsin (bR) that changes in different states of the protein. The experiments are targeted to the active site of bR through 13C labeling of both the retinal chromophore and the Lys side chains of the protein. The time course of the rotor-driven magnetization exchange between a pair of 13C nuclei is then observed to determine the dipolar coupling and therefore the internuclear distance. Using this approach, we have measured the distance from [14-13C]retinal to [epsilon-13C]Lys216 in dark-adapted bR in order to examine the structure of the retinal-protein linkage and its role in coupling the isomerizations of retinal to unidirectional proton transfer. This distance depends on the configuration of the intervening C=N bond. The 3.0 +/- 0.2 A distance observed in bR555 demonstrates that the C=N bond is syn, and the 4.1 +/- 0.3 A distance observed in bR568 demonstrates that the C=N bond is anti. These direct distance determinations independently confirm the configurations previously deduced from solid-state NMR chemical shift and resonance Raman vibrational spectra. The spectral selectivity of rotational resonance allows these two distances to be measured independently in a sample containing both bR555 and bR568; the presence of both states and of 25% lipid in the sample demonstrates the use of rotational resonance to measure an active site distance in a membrane protein with an effective molecular mass of about 85 kDa.(ABSTRACT TRUNCATED AT 250 WORDS)

Zero-voltage transition PWM converters

Abstract: A pulse-width-modulated, quasi-resonant, d.c. to d.c. converter that has an auxiliary switch that periodically kills the resonance.

On the Calculation of Maxwellian-averaged Capture Cross Sections

Abstract: A recipe for the calculation of Maxwellian-averaged capture cross sections as a function of temperature kT from the existing excitation function is discussed. Detailed formulae are given to handle the various portions of this excitation function which in general contains both resolved resonance and an unresolved resonance parts. For the temperature region considered, the details of the thermal (0.0253 eV) capture cross section and a possible direct capture compoment are of importance. The treatment of resolved resonances depending on the characteristics of the resonance parameters is discussed

Surface plasmon resonance on gratings as a novel means for gas sensing

Abstract: A novel technique for sensing small changes in dielectric constant using the phenomenon of surface plasmon resonance has been developed. The principle of this technique is based on the measurement of a resonance maximum on a background of weak signal, giving improved signal-to-noise ratios over previously reported surface plasmon techniques. The sensing of the condensation of several organic vapours onto a silver surface is used to demonstrate the effectiveness of the technique. Its potential as a hand-held in-field gas sensor is discussed.