Showing papers on "Resonance published in 2002"

Phonon-enhanced light–matter interaction at the nanometre scale

Abstract: Optical near fields exist close to any illuminated object They account for interesting effects such as enhanced pinhole transmission or enhanced Raman scattering enabling single-molecule spectroscopy Also, they enable high-resolution (below 10 nm) optical microscopy The plasmon-enhanced near-field coupling between metallic nanostructures opens new ways of designing optical properties and of controlling light on the nanometre scale Here we study the strong enhancement of optical near-field coupling in the infrared by lattice vibrations (phonons) of polar dielectrics We combine infrared spectroscopy with a near-field microscope that provides a confined field to probe the local interaction with a SiC sample The phonon resonance occurs at 920 cm(-1) Within 20 cm(-1) of the resonance, the near-field signal increases 200-fold; on resonance, the signal exceeds by 20 times the value obtained with a gold sample We find that phonon-enhanced near-field coupling is extremely sensitive to chemical and structural composition of polar samples, permitting nanometre-scale analysis of semiconductors and minerals The excellent physical and chemical stability of SiC in particular may allow the design of nanometre-scale optical circuits for high-temperature and high-power operation

G-band resonant Raman study of 62 isolated single-wall carbon nanotubes

Abstract: We report G-band resonance Raman spectra of single-wall carbon nanotubes ~SWNTs! at the singlenanotube level. By measuring 62 different isolated SWNTs resonant with the incident laser, and having diameters dt ranging between 0.95 nm and 2.62 nm, we have conclusively determined the dependence of the two most intense G-band features on the nanotube structure. The higher-frequency peak is not diameter dependent (v G 51591 cm 21 ), while the lower-frequency peak is given by v G 5v G 2C/dt , with C being different for metallic and semiconducting SWNTs (CM.CS). The peak frequencies do not depend on nanotube chiral angle. The intensity ratio between the two most intense features is in the range 0.1 ,I v G /I v G,0.3 for most of the isolated SWNTs (;90%). Unusually high or low Iv G /I v G ratios are observed for a few spectra coming from SWNTs under special resonance conditions, i.e., SWNTs for which the incident photons are in resonance with the E44 interband transition and scattered photons are in resonance with E33 . Since the Eii values depend sensitively on both nanotube diameter and chirality, the ( n,m) SWNTs that should exhibit such a special G-band spectra can be predicted by resonance Raman theory. The agreement between theoretical predictions and experimental observations about these special G-band phenomena gives additional support for the (n,m) assignment from resonance Raman spectroscopy.

Sharp asymmetric line shapes in side-coupled waveguide-cavity systems

Abstract: We show that, for an optical microcavity side coupled with a waveguide, sharp, and asymmetric line shapes can be created in the response function by placing two partially reflecting elements into the waveguides. In such a system, the transmission coefficient varies from 0% to 100% in a frequency range narrower than the full width of the resonance itself. We numerically demonstrate this effect by simulating the propagation of electromagnetic waves in a photonic crystal.

Collective cooling and self-organization of atoms in a cavity.

Abstract: We theoretically investigate the correlated dynamics of N coherently driven atoms coupled to a standing-wave cavity mode. For red detuning between the driving field and the cavity as well as the atomic resonance frequencies, we predict a light force induced self-organization of the atoms into one of two possible regular patterns, which maximize the cooperative scattering of light into the cavity field. Kinetic energy is extracted from the atoms by superradiant light scattering to reach a final kinetic energy related to the cavity linewidth. The self-organization starts only above a threshold of the pump strength and atom number. We find a quadratic dependence of the cavity mode intensity on the atom number, which demonstrates the cooperative effect.

The Ionized Gas and Nuclear Environment in NGC 3783. I. Time-Averaged 900 ks Chandra Grating Spectroscopy

Abstract: We present results from a 900 ks exposure of NGC 3783 with the High-Energy Transmission Grating Spectrometer on board the Chandra X-ray Observatory. The resulting X-ray spectrum has the best combination of signal-to-noise and resolution ever obtained for an AGN. This spectrum reveals absorption lines from H-like and He-like ions of N, O, Ne, Mg, Al, Si, and S. There are also possible absorption lines from H-like and He-like Ar and Ca. We also identify inner-shell absorption from lower-ionization ions such as Si_VII-Si_XII and S_XII-S_XIV. The iron absorption spectrum is very rich; L-shell lines of Fe_XVII-Fe_XXIV are detected, strong complex of M-shell lines, and probable resonance lines from Fe_XXV. The absorption lines are blueshifted relative to the systemic velocity by a mean velocity of -590+-150 km/s. We resolve many of the absorption lines, and their mean FWHM is 820+-280 km/s. We do not find correlations between the velocity shifts or the FWHMs with the ionization potentials of the ions. Most absorption lines show asymmetry, having more extended blue wings than red wings. In O_VII we have resolved this asymmetry to be from an additional absorption system at ~ -1300 km/s. The two X-ray absorption systems are consistent in velocity shift and FWHM with the ones identified in the UV lines of C IV, N V, and H I. Equivalent width measurements for all lines are given and column densities are calculated for several ions. We resolve the narrow Fe_K\alpha line at 6398.2+-3.3 eV to have a FWHM of 1720+-360 km/s, which suggests that this narrow line may be emitted from the outer part of the broad line region or the inner part of the torus. We also detect a `Compton shoulder' redward of the narrow Fe_K\alpha line which indicates that it arises in cold, Compton-thick gas.

Intrinsic dissipation in high-frequency micromechanical resonators

Abstract: We report measurements of intrinsic dissipation in micron-sized suspended resonators machined from single crystals of galium arsenide and silicon. In these experiments on high-frequency micromechanical resonators, designed to understand intrinsic mechanisms of dissipation, we explore dependence of dissipation on temperature, magnetic field, frequency, and size. In contrast to most of the previous measurements of acoustic attenuation in crystalline and amorphous structures in this frequency range, ours is a resonant measurement; dissipation is measured at the natural frequencies of structural resonance, or modes of the structure associated with flexural and torsional motion. In all our samples we find a weakly temperature dependent dissipation at low temperatures. We compare and contrast our data to various probable mechanisms, including thermoelasticity, clamping, anharmonic mode-coupling, surface anisotropy and defect motion, both in bulk and on surface. The observed parametric dependencies indicate that the internal defect motion is the dominant mechanism of intrinsic dissipation in our samples.

Resonant light scattering from individual Ag nanoparticles and particle pairs

Abstract: Light scattering by individual Ag nanoparticles and structures have been studied spectroscopically. Individual particles were selected and manipulated with a micromanipulator installed inside a scanning electron microscope (SEM). With typical particle dimensions of some 100 nm, the plasma resonances of particles and the coupled modes of particle pairs were observed in the visible region. The polarization dependence of the resonance frequencies strongly reflects the shape anisotropy; the effect that would be averaged out for experiments on ensembles. With a simple approximation to take the glass substrate into account, the results are in good agreement with the analytical calculations by Mie scattering, and with numerical calculations by the finite-difference time-domain method, both of which are performed with the morphological parameters obtained from the SEM observation for the corresponding particle or particle pair.

Kondo effect of single Co adatoms on Cu surfaces.

Abstract: The Kondo resonance of Co adatoms on the Cu(100) and Cu(111) surfaces has been studied by scanning tunneling spectroscopy. We demonstrate the scaling of the Kondo temperature ${T}_{\mathrm{K}}$ with the host electron density at the magnetic impurity. The quantitative analysis of the tunneling spectra reveals that the Kondo resonance is dominated by the Cu bulk electrons. While at the Cu(100) surface both tunneling into the hybridized localized state and into the substrate conduction band contribute to the Kondo resonance, the latter channel is found to be dominant for Cu(111).

Muscle activity damps the soft tissue resonance that occurs in response to pulsed and continuous vibrations

Abstract: This study tested the hypotheses that when the excitation frequency of mechanical stimuli to the foot was close to the natural frequency of the soft tissues of the lower extremity, the muscle activity increases 1) the natural frequency and 2) the damping to minimize resonance. Soft tissue vibrations were measured with triaxial accelerometers, and muscle activity was measured by using surface electromyography from the quadriceps, hamstrings, tibialis anterior, and triceps surae groups from 20 subjects. Subjects were presented vibrations while standing on a vibrating platform. Both continuous vibrations and pulsed bursts of vibrations were presented, across the frequency range of 10-65 Hz. Elevated muscle activity and increased damping of vibration power occurred when the frequency of the input was close to the natural frequency of each soft tissue. However, the natural frequency of the soft tissues did not change in a manner that correlated with the frequency of the input. It is suggested that soft tissue damping may be the mechanism by which resonance is minimized at heel strike during running.

Bimetallic Layers Increase Sensitivity of Affinity Sensors Based on Surface Plasmon Resonance

Abstract: Two metals are used in resonant layers for chemical sensors based on surface plasmon resonance (SPR) - gold and silver. Gold displays higher shift of the resonance angle to changes of ambient refraction index and is chemically stable. Silver posses narrower resonance curve thus providing a higher signal/noise ratio of SPR chemical sensors, but has a poor chemical stability. A new structure of resonant metallic film based on bimetallic silver/gold layers (gold as an outer layer) is suggested. It combines advantages of both gold and silver resonant layers. Bimetallic resonant films display so high shift of resonance angle on changes of ambient refraction index as gold films, but show narrower resonance curve, thus providing a higher signal / noise ratio. Additionally, the outer gold layer protects silver against oxidation.

Dielectric properties of Mn-substituted Ni–Zn ferrites

Abstract: The dielectric properties of MnxNi0.5−xZn0.5Fe2O4 ferrites with x varying from 0.05 to 0.4 synthesized by the citrate precursor method have been investigated as a function of frequency, temperature, composition, and sintering temperature. An increase in the dielectric constant is observed with the increase in Mn concentration except for x=0.3. Dispersion in the dielectric constant with frequency in the range of 100 Hz–1 MHz is observed. Resonance peaks were observed in tan δ∈ versus frequency curves for all samples. A shift in the resonance frequency towards higher frequency is observed with increase in temperature. The peak height also increases with increase in temperature. Possible mechanisms contributing to these processes have been discussed. From the temperature variation of the dielectric relaxation, activation energies for various samples have been calculated and compared with those obtained from dc resistivity.

Resonant Control of Elastic Collisions in an Optically Trapped Fermi Gas of Atoms

Abstract: We have loaded an ultracold gas of fermionic atoms into a far-off resonance optical dipole trap and precisely controlled the spin composition of the trapped gas. We have measured a magnetic-field Fesh-bach resonance between atoms in the two lowest energy spin states, ‖9/2, -9/2) and ‖9/2, -7/2). The resonance peaks at a magnetic field of 201.5 ′ 1.4 G and has a width of 8.0 ′ 1.1 G. Using this resonance, we have changed the elastic collision cross section in the gas by nearly 3 orders of magnitude.

Electronic spin detection in molecules using scanning-tunneling- microscopy-assisted electron-spin resonance

Abstract: By combining the spatial resolution of a scanning-tunneling microscope (STM) with the electronic spin sensitivity of electron-spin resonance, we show that it is possible to detect the presence of localized spins on surfaces The principle is that a STM is operated in a magnetic field, and the resulting component of the tunnel current at the Larmor (precession) frequency is measured This component is nonzero whenever there is tunneling into or out of a paramagnetic entity We have succeeded in obtaining spectra from free radical molecules from which the g factor of a spin entity may be inferred For the molecules studied here, α,γ-bisdiphenylene-β-phenylallyl, g was found to be 2±01

A 12 MHz micromechanical bulk acoustic mode oscillator

Abstract: We demonstrate a bulk acoustic mode silicon micromechanical resonator with the first eigen frequency at 12 MHz and the quality factor 180 000. Electrostatic coupling to the mechanical motion is shown to be feasible using a high bias voltage across a narrow gap. By using a low-noise preamplifier to detect the resonance, a high spectral purity oscillator is demonstrated (phase noise less than −115 dBc/Hz at 1 kHz offset from the carrier). By analyzing the constructed prototype oscillator, we discuss in detail the central performance limitations of using silicon micromechanics in oscillator applications.

Resonance forever: existence of a critical mass and an infinite regime of resonance in vortex-induced vibration

Abstract: In this paper, we study the transverse vortex-induced vibrations of a cylinder with no structural restoring force (k = 0). In terms of the conventionally used normalized flow velocity, U*, the present experiments correspond to an infinite value (where U* = U/f N D, f N = natural frequency, D = diameter). A reduction of mass ratios m* (mass/displaced mass) from the classically studied values of order m* = 100, down to m* = 1, yields negligible oscillations. However, a further reduction in mass exhibits a surprising result: large-amplitude vigorous vibrations suddenly appear for values of mass less than a critical mass ratio, m* crit = 0.54. The classical assumption, since the work of den Hartog (1934), has been that resonant large-amplitude oscillations exist only over a narrow range of velocities, around U* ∼ 5, where the vortex shedding frequency is comparable with the natural frequency. However, in the present study, we demonstrate that, so long as the body's mass is below this critical value, the regime of normalized velocities (U*) for resonant oscillations is infinitely wide, beginning at around U* ∼ 5 and extending to U* → ∞. This result is in precise accordance with the predictions put forward by Govardhan & Williamson (2000), based on elastically mounted vibration studies (where k > 0). We deduce a condition under which this unusual concept of an infinitely wide regime of resonance will occur in any generic vortex-induced vibration system.

Coherent population trapping resonances in thermal (85)Rb vapor: D(1) versus D(2) line excitation.

Abstract: We have compared coherent population trapping (CPT) resonances, both experimentally and theoretically, for excitation of the D1 and D2 transitions of thermal Rb85 vapor. Excitation of the D1 line results in greater resonance contrast than excitation of the D2 line and in a reduction in the resonance width, in agreement with theoretical expectations. These results translate into a nearly tenfold improvement in performance for the application of CPT resonances to a frequency standard or a sensitive magnetometer when the D1 line, rather than the D2 line, is used.

Resonance superfluidity: Renormalization of resonance scattering theory

Abstract: We derive a theory of superfluidity for a dilute Fermi gas that is valid when scattering resonances are present. The treatment of a resonance in many-body atomic physics requires a novel mean-field approach starting from an unconventional microscopic Hamiltonian. The mean-field equations incorporate the microscopic scattering physics, and the solutions to these equations reproduce the energy-dependent scattering properties. This theory describes the high-${T}_{c}$ behavior of the system, and predicts a value of ${T}_{c}$ that is a significant fraction of the Fermi temperature. It is shown that this mean-field approach does not break down for typical experimental circumstances, even at detunings close to resonance. As an example of the application of our theory, we investigate the feasibility for achieving superfluidity in an ultracold gas of fermionic ${}^{6}\mathrm{Li}.$

Magnetic and magnetoelectric susceptibilities of a ferroelectric/ferromagnetic composite at microwave frequencies

Abstract: A phenomenological theory of magnetic and magnetoelectric (ME) susceptibilities of ferroelectric/ferromagnetic composites is presented and applied to the special case of layered structures. Expressions have been obtained relating the magnetic and ME susceptibility tensor components of the composite (symmetry point group $3m$ and $4mm)$ to ME coupling constants. The theory predicts a unique resonance in the electric-field dependence of the magnetic susceptibility. It is shown that the ME susceptibility is the product of magnetic susceptibility, composite magnetization, and ME coupling constants. The model is used to obtain the magnetic and ME susceptibilities versus electric-field profiles for three bilayer composites of importance: lithium ferrite (LFO)--lead zirconate titanate (PZT), nickel ferrite (NFO)-PZT, and yttrium iron garnet (YIG)-PZT. Our calculations reveal the largest electric-field effects for NFO-PZT and the weakest effect for YIG-PZT. Three measurement methods for ME susceptibility, resonant ME coupling, electrical dipole transitions, and ME effect at ferromagnetic resonance (FMR) are proposed. As an example, we consider multilayers of LFO-PZT and determine the ME constants from data on electric-field influence on FMR. The ME parameters are then used to calculate the susceptibilities. The results indicate strong high-frequency ME effects in the composite. The theory is useful for measurements of ME susceptibility and for the design and analysis of electrically controlled high-frequency magnetic devices.

Laser frequency stabilization by Doppler-free magnetic dichroism

Abstract: We report a method of laser stabilization to an atomic transition frequency, which is based on a magnetically induced nonlinear circular dichroism in the counter-propagating pump-probe configuration. The proposed method does not require any modulation of the laser frequency. In comparison to the linear, Doppler-broadened dichroic lock, the proposed version provides a precise locking to a well-defined atomic resonance at a cost of a smaller capture range. A simple model description of the locking signal is presented and two experimental realizations are shown: for a dye laser at λ=589.6 nm (Na D1 line) and for a diode laser at 795 nm (Rb D1).

The effectiveness of space-filling fractal geometry in lowering resonant frequency

Abstract: The space-filling fractal geometry of the Hilbert curve is examined in terms of its effectiveness in lowering resonant frequency. It is demonstrated that the complex geometry associated with the space-filling Hilbert fractal curve is inherently ineffective in lowering resonant frequency compared to other less complex geometries of the same size and total wire length. The effectiveness of the geometry in lowering resonant frequency is a direct function of the current vector alignment established by the wire layout.

A detailed Raman study on thin single-wall carbon nanotubes prepared by the HiPCO process

Abstract: The Raman spectrum of single wall carbon nanotubes (SWNTs) prepared by high pressure CO decomposition (HiPCO process) has been recorded at nine excitation laser energies ranging from 1.83 eV to 2.71 eV. The characteristic nanotubes features: G band, D band and radial breathing mode (RBM) have been analyzed and compared to those of an arc discharge SWNT material of similar diameter. A strong Breit-Wigner-Fano type (metallic) contribution to the G band was found in the spectra measured with green lasers, while spectra measured with red lasers indicate resonances of semiconducting SWNTs. Analysis of the energy dependence of the position of the D band revealed sinusoid oscillations superimposed on a linear trend. The validity of full DOS calculations for HiPCO materials has been confirmed by a match found between the estimated spectral contribution of metallic SWNTs as calculated from the components of the measured G band and as predicted by the (n, m) indexes of the major scatterers of DOS simulations. The RBM region of the HiPCO spectrum is more complex than usually observed for SWNTs. The analysis performed with a Gaussian distribution and improved fitting parameters leads to a mean diameter and variance of 1.05 nm and 0.15 nm, respectively. A bimodal Gaussian distribution had little influence on the error sum but reduced the standard error slightly. The major spectral features of the RBM could be interpreted using available resonance Raman theory.

Probing the phonon dispersion relations of graphite from the double-resonance process of Stokes and anti-Stokes Raman scatterings in multiwalled carbon nanotubes

Abstract: The Stokes and anti-Stokes Raman spectra of a multiwalled carbon nanotube (MWNT) sample are studied here by four excitation energies and the observed Raman modes are assigned based on the double resonance Raman effect andthe previous results in graphite whiskers. There exists frequency discrepancy between Stokes and anti-Stokes lines (FDSA) of many Raman modes in MWNT's and the discrepancy values are strongly dependent on the excitation energy, in which the FDSA value of the D' mode even changes from a positive value (9 cm - 1 , 1.58 eV) to a negative value (-11 cm - 1 , 2.54 eV). The laser-energy dependence of the FDSA values of some modes in MWNT's is attributed to the nonlinear frequency dependence of Stokes and anti-Stokes Raman lines of these modes on the excitation energy. Raman results and the theoretical analysis of the intravalley and intervalley double resonance processes of Stokes and anti-Stokes Raman scatterings both show that the frequency of an anti-Stokes peak excited by e L is equal to that of the corresponding Stokes peak excited by a laser excitation of e L +∞ω S where ∞ω S is the phonon energy of the Raman mode. Stokes and anti-Stokes double-resonance Raman scatterings have been used to probe the phonon dispersion relations of graphite. The Raman data of the well-known disorder-induced D mode are in good agreement with the theoretical results.

Channel-closing effects in high-order above-threshold ionization and high-order harmonic generation

Abstract: The strong-field approximation, amended so as to allow for rescattering, is used to calculate high-order above-threshold ionization (ATI) spectra. The single-active-electron binding potential is modelled by a zero-range potential. The emphasis is on enhancements of groups of ATI peaks that occur for sharply defined laser intensities. The enhancements are traced to multiphoton resonance with the ponderomotively upshifted continuum threshold. Good agreement both with experimental data and with numerical simulations using the three-dimensional time-dependent Schrodinger equation for an optimized one-electron binding potential is observed. The physical reason for the close agreement of the results of the two, apparently so different, models is discussed. For quantitative agreement with the experimentally observed positions of the resonances, an `effective' continuum threshold has to be introduced. The effects of focal averaging are evaluated and discussed. Resonant enhancement of high-order harmonic generation is also considered.

Electromagnetically induced transparency in ensembles of classical oscillators.

Abstract: We develop a classical model of the parametric effect of electromagnetically induced transparency (EIT) within the line of resonance absorption of an electromagnetic wave in the medium---an effect initially discovered for a quantum three-level system. On the basis of this model, the EIT effect for electromagnetic waves at frequencies of the electron-cyclotron resonance in a cold plasma is considered. Similar to the analogous quantum scheme, the EIT window in the classical model is characterized by group deceleration of the reference electron-cyclotron wave.

Multimode Interactions in Suspended Cables

Abstract: We investigate the nonlinear nonplanar responses of suspended cables to external excitations. The equations of motion governing such systems contain quadratic and cubic nonlinearities, which may result in two-to-one and one-to-one internal resonances. The sag-to-span ratio of the cable considered is such that the natural frequency of the first symmetric in-plane mode is at first crossover. Hence, the first symmetric in-plane mode is involved in a one-to-one internal resonance with the first antisymmetric in-plane and out-of-plane modes and, simultaneously, in a two-to-one internal resonance with the first symmetric out-of-plane mode. Under these resonance conditions, we analyze the response when the first symmetric in-plane mode is harmonically excited at primary resonance. First, we express the two governing equations of motion as four first-order (i.e., state-space formulation) partial-differential equations. Then, we directly apply the methods of multiple scales and reconstitution to determine a second...