1H spectroscopy without solvent suppression: characterization of signal modulations at short echo times.

While most proton ((1)H) spectra acquired in vivo utilize selective suppression of the solvent signal for more sensitive detection of signals from the dilute metabolites, recent reports have demonstrated the feasibility and advantages of collecting in vivo data without solvent attenuation. When these acquisitions are performed at short echo times, the presence of frequency modulations of the water resonance may become an obstacle to the identification and quantitation of metabolite resonances. The present report addresses the characteristics, origin, and elimination of these sidebands. Sideband amplitudes were measured as a function of delay time between gradient pulse and data collection, as a function of gradient pulse amplitude, and as a function of spatial location of the sample for each of the three orthogonal gradient sets. Acoustic acquisitions were performed to demonstrate the correlation between mechanical vibration resonances and the frequencies of MR sidebands. A mathematical framework is developed and compared with the experimental results. This derivation is based on the theory that these frequency modulations are induced by magnetic field fluctuations generated by the transient oscillations of gradient coils.     

Localized proton spectroscopy without water suppression: removal of gradient induced frequency modulations by modulus signal selection

Most Magnetic Resonance Spectroscopy (MRS) localization methods can generate gradient vibrations at acoustic frequencies and/or magnetic field oscillation, which can cause a time-varying magnetic field superimposed onto the static one. This effect can produce frequency modulations of the spectral resonances. When localized MRS data are acquired without water suppression, the associated frequency modulations are manifested as a manifold of spurious peaks, called sidebands, which occur symmetrically around the water resonance. These sidebands can be larger than the small metabolite resonances and can present a problem for the quantitation of the spectra, especially at short echo times. Furthermore, the resonance lineshapes may be distorted if any low frequency modulations are present. A simple solution is presented which consists of selecting the modulus of the acquired Free Induction Decay (FID) signal. Since the frequency modulations affect only the phase of the FID signal, the obtained real spectrum of the modulus is free from the spurious peaks where quantitative results may be directly obtained. Using this method, the distortions caused by the sidebands are removed. This is demonstrated by processing proton MRS spectra acquired without water suppression collected from a phantom containing metabolites at concentrations comparable to those in human brain and from a human subject using two different localization methods (PRESS and Chemical Shift Imaging PRESS-(CSI)). The results obtained illustrate the ability of this approach to remove the spurious peaks. The corrected spectra can then be fit accurately. This is confirmed by the results obtained from both the relative and the absolute metabolites concentrations in phantoms and in vivo.     

H Spectroscopy without Solvent Suppression: Characterization of Signal Modulations at Short Echo Times

While most proton (¹H) spectra acquired in vivo utilize selective suppression of the solvent signal for more sensitive detection of signals from the dilute metabolites, recent reports have demonstrated the feasibility and advantages of collecting in vivo data without solvent attenuation. When these acquisitions are performed at short echo times, the presence of frequency modulations of the water resonance may become an obstacle to the identification and quantitation of metabolite resonances. The present report addresses the characteristics, origin, and elimination of these sidebands. Sideband amplitudes were measured as a function of delay time between gradient pulse and data collection, as a function of gradient pulse amplitude, and as a function of spatial location of the sample for each of the three orthogonal gradient sets. Acoustic acquisitions were performed to demonstrate the correlation between mechanical vibration resonances and the frequencies of MR sidebands. A mathematical framework is developed and compared with the experimental results. This derivation is based on the theory that these frequency modulations are induced by magnetic field fluctuations generated by the transient oscillations of gradient coils.     

Lineshape asymmetry for joint coherent population trapping and three-photon N resonances

We show that a characteristic two-photon lineshape asymmetry arises in coherent population trapping (CPT) and three-photon (N) resonances, because both resonances are simultaneously induced by modulation sidebands in the interrogating laser light. The N resonance is a three-photon resonance in which a two-photon Raman excitation is combined with a resonant optical pumping field. This joint CPT and N resonance can be the dominant source of lineshape distortion, with direct relevance for the operation of miniaturized atomic frequency standards. We present the results of both an experimental study and theoretical treatment of the asymmetry of the joint CPT and N resonance under conditions typical to the operation of an N resonance clock.     

Determination of the structure of hyperfine sublevels of Rb in strong magnetic fields by means of the coherent population trapping technique

The splitting of hyperfine sublevels of the ⁸⁵Rb atom in strong magnetic fields has been studied by means of the coherent population trapping technique. Narrow resonances with a high signal-to-noise ratio have been detected in a 30-μm-thick spectroscopic cell. The magnetic field in the direction transverse to the laser beams has been created by permanent magnets and has reached 1600 G. Owing to the exclusive narrowness of the cell, the field in it is almost uniform. The break of the coupling between the electronic and nuclear moments, as well as the transition to the Paschen-Back regime in magnetic fields above 600 G, has been observed. The derivatives of the frequency shifts of the observed resonances and their asymptotic values in strong magnetic fields have been determined in terms of the magnetic field strength. The experimental results have been interpreted within a theoretical model based on the known constants of the hyperfine structure of the Rb atom.     

Theory of ac quantum conduction through an ideal constriction

A scattering theory is formulated for time-dependent (ac) transport through quantum constrictions or quantum point contacts. This is an extension of the standard scattering treatment for the time-independent (dc) case where quantized conductance steps and resonances occur. For an ideal constriction, the first-order transmission sidebands are derived when a time-dependent sinusoidal potential is applied. The frequency dependence of the conductance is discussed, and possible experiments are suggested.     

Spin-reorientation phase transition: Magnetic resonance, acoustic and optic properties

The problem dealt within this paper is the study of soft modes near the spin-reorientation phase transition (SRPT) in ferromagnets (and partly in antiferromagnets) when the magnetoelastic interaction is taken into account. A detailed discussion is given for the long wavelength magnetoacoustic modes (MAM) and their influence on various physical properties. The results are: the conditions for linear and nonlinear (parametric) excitations of MAM by an ac magnetic field; MAM contributions to the local magnetic susceptibility, nuclear magnetic and magnetoacoustic resonances; the modulation of the sound velocity by an ac magnetic field; and the magnetic birefrigence of light by sound waves. All these phenomena manifest a sharp increase near the SRPT.     

Spin dynamics of magnetic resonance with parametric modulation in a potassium vapor cell

A typical magnetic-resonance scheme employs a static bias magnetic field and an orthogonal driving magnetic field oscillating at the Larmor frequency, at which the atomic polarization precesses around the static magnetic field. Here we demonstrate both theoretically and experimentally the variations of the resonance condition and the spin precession dynamics resulting from the parametric modulation of the bias field. We show that the driving magnetic field with the frequency detuned by different harmonics of the parametric modulation frequency can lead to resonance as well. Also, a series of frequency sidebands centered at the driving frequency and spaced by the parametric modulation frequency can be observed in the precession of the atomic polarization. We further show that the resonant amplitudes of the sidebands can be controlled by varying the ratio between the amplitude and the frequency of the parametric modulation. These effects could be used in different atomic magnetometry applications.     

Spin-wave modes and their intense excitation effects in Skyrmion crystals

We theoretically study spin-wave modes and their intense excitations activated by microwave magnetic fields in the Skyrmion-crystal phase of insulating magnets by numerically analyzing a two-dimensional spin model using the Landau-Lifshitz-Gilbert equation. Two peaks of spin-wave resonances with frequencies of ～1 GHz are found for in-plane ac magnetic field where distribution of the out-of-plane spin components circulates around each Skyrmion core. Directions of the circulations are opposite between these two modes, and hence the spectra exhibit a salient dependence on the circular polarization of irradiating microwave. A breathing-type mode is also found for an out-of-plane ac magnetic field. By intensively exciting these collective modes, melting of the Skyrmion crystal accompanied by a redshift of the resonant frequency is achieved within nanoseconds.     

Control of Spinning Sidebands in High Resolution NMR Spectroscopy

The presence of spinning sidebands can severely compromise the detection of low molarity analytes. Spinning sidebands have traditionally been minimized by improving the magnetic field homogeneity and by varying the spinning of the sample in a linear fashion during data acquisition. The effect of the latter is to spread the spinning sideband intensity over a range of frequencies so that the final result is a spinning sideband whose shape reflects the distribution of spinning speeds. We have designed a customized profile of spinner speed variation that optimizes the reduction of spinning sidebands. The customized profile is based on theoretical considerations of how the intensity of sidebands vary with the rate of sample rotation and also compensates for the mechanical design of the spinner mechanism. The result is a unique combination of an exponential increase in gas flow rate to balance the theoretical considerations coupled with a strategically placed rapid change in air flow to annul the sluggish response of the spinning mechanism to acceleration. The resulting sideband shape is a broad, flat, square step in the baseline that is least likely to interfere with low molarity analyte peaks.     

Diode laser noise-spectroscopy of low-frequency atomic fluctuations in rubidium vapor

We have observed resonant features in the spectrum of the fluctuations of a linearly polarized diode laser beam transmitted through a rubidium vapor cell, corresponding to the evolution of the atomic spin in the presence of a constant magnetic field. The observed resonances occur at a noise frequency corresponding to twice the Larmor frequency of ground state rubidium atoms and are due to two-photon Raman processes involving the carrier frequency and the noise sideband. We observed noise resonances for frequencies of the order of one MHz via heterodyne detection. Due to nonlinear Faraday rotation, we detected emitted light with polarization orthogonal to the incident field. The influence of the laser light fluctuations on the transmitted light noise spectrum was investigated by using two diode laser sources with different spectral bandwidths. The observed features are in qualitative agreement with a semiclassical theoretical model that treats laser fluctuations up to first order.     

Laboratory observation of a nonlinear interaction between shear Alfvén waves

An experimental investigation of nonlinear interactions between shear Alfvén waves in a laboratory plasma is presented. Two Alfvén waves, generated by a resonant cavity, are observed to beat together, driving a pseudomode at the beat frequency. The pseudomode then scatters the Alfvén waves, generating a series of sidebands. The observed interaction is very strong, with the normalized amplitude of the driven pseudomode comparable to the normalized magnetic field amplitude (deltaB/B) of the interacting Alfvén waves.     

Frequency modulation of the 6.2 keV Mössbauer states of181Ta

Mössbauer sidebands up to the first order from a single parent line have been produced by subjecting a non magnetic W(¹⁸¹W) Mössbauer source to a strong oscillating magnetic field of up to 230 Oe amplitude and a frequency of about one megahertz. The sidebands positions and intensities agree very well with theory, which is based on a periodic time-dependent interaction of the magnetic field with the nuclear magnetic moments of ground and excited states, respectively. From the sideband intensity ag-factor ratio ofg _e /g _g =1.75(6) was derived.     

Inspection of a magneto-optical trap via electromagnetically induced absorption

Electromagnetically induced absorption (EIA) was observed for the first time on a sample of ⁸⁵Rb in a magneto-optical trap using low intensity cw copropagating pump and probe optical fields. Narrow resonances revealing the dependence of the ground-state Zeeman sublevels energy structure on the quadrupolar magnetic field and the trapping optical field intensity at different trap positions, were observed. Coherence resonances as narrow as 30 kHz were obtained under low trapping field intensities. The use of EIA spectroscopy for the magnetic field mapping of cold atomic samples is illustrated.     

Electric and magnetic excitations in anisotropic broadside-coupled triangular-split-ring resonators

The electric and magnetic resonances of anisotropic broadside-coupled triangular-split-ring resonators are studied for different incident wave excitations. It is shown that the higher order modes exist in both electric and magnetic resonances. It is observed that the incident electric field couples to the magnetic resonance of the designed structure under different excitations. Multiple resonance features due to the anisotropy of the structure are found in the case of different excitations and the nature of these resonances can be regulated as either an electric or a magnetic mode for different frequencies. In this way, a resonant effective permittivity or permeability can be obtained. Hence, controllable properties of the constitutive material parameters (i.e. electric or magnetic resonances, negative values, etc.) can be determined by changing the incident wave excitation.     

Sideband evolution from spontaneous into amplified emission in a free electron laser

A theoretical study of the evolution of sidebands, originating from large-scale (nonlinear) oscillations in the electron dynamics, is reported for a combined wiggler and guide field system operating near magnetoresonance. These sidebands, already existent in the spectrum of spontaneous emission, are followed up through small signal gain computations, and are finally recovered in the simulations of the nonlinear saturation process. A comparison with a radiation spectrum of an experiment done at the Naval Research Laboratory (NRL) shows good agreement. This indicates that the sidebands, observed in the experiment at saturation power level, are essentially due to the specific structure of the spectrum of spontaneous emission.     

Giant backscattering resonances in the magneto-resistance of GaAs/AlGaAs quantum dots

We discuss the observation of large resonant features, superimposed upon the quantum Hall plateaux of gated GaAs/AlGaAs quantum dots. The resonances correspond to a magnetically induced increase in the edge state backscattering, and under certain conditions can imply a complete reflection of the applied current. We demonstrate that the resonances are correlated to the depopulation of bulk Landau levels, and suggest they result from an increase in backscatterlng via confined Landau levels, as the latter depopulate in a magnetic field. The resonances are therefore analogous to the Shubnikov-de Haas oscillations, observed in two dimensional electron gas systems, and their temperature dependence is found to take the same functional form. We argue that the resonances are an intrinsic feature of edge state transport in quantum dots, since they result from scattering via Landau levels, controllably confined within the dot, and discuss our results in relation to recent theoretical and experimental studies, of edge state transport in small wires and dots.     

Influence of the surface anisotropy on the r.f. collapse effect

The influence of magnetic surface anisotropy on the fast relaxation of the hyperfine field forced by an r.f. field in invar (r.f. collapse effect) has been studied using the Mössbauer technique. The Mössbauer measurements were performed as a function of sample thickness (2.5–12 μm) and intensity (1–9 Oe) of the 50 MHz r.f. field applied. Due to the very high sensitivity of the r.f. collapse effect to the anisotropy field it was possible to detect the influence of “spin pinning” on the r.f. collapse effect. It is shown that a decrease of the sample thickness causes a decrease of the r.f. collapse effect at a given r.f. field frequency and intensity which is connected with the increase of the anisotropy field due to surface anisotropy. The dependence of the r.f. sidebands effect, which accompanies the r.f. collapse effect, on the sample thickness is discussed. The r.f. sidebands effect increases with decreasing sample thickness, which is in good agreement with the magnetostriction model of sidebands formation.     

The continuous wave electron paramagnetic resonance experiment revisited

When the modulation frequency used in continuous wave electron paramagnetic resonance (cw EPR) spectroscopy exceeds the linewidth, modulation sidebands appear in the spectrum. It is shown theoretically and experimentally that these sidebands are actually multiple photon transitions, sigma(+)+kxpi, where one microwave (mw) sigma(+) photon is absorbed from the mw radiation field and an arbitrary number k of radio frequency (rf) pi photons are absorbed from or emitted to the modulation rf field. Furthermore, it is demonstrated that both the derivative shape of the lines in standard cw EPR spectra and the distortions due to overmodulation are caused by the unresolved sideband pattern of these lines. The single-photon transition does not even give a contribution to the first-harmonic cw EPR signal. Multiple photon transitions are described semiclassically in a toggling frame and their existence is proven using second quantization. With the toggling frame approach and perturbation theory an effective Hamiltonian for an arbitrary sideband transition is derived. Based on the effective Hamiltonians an expression for the steady-state density operator in the singly rotating frame is derived, completely describing all sidebands in all modulation frequency harmonics of the cw EPR signal. The relative intensities of the sidebands are found to depend in a very sensitive way on the actual rf amplitude and the saturation of single sidebands is shown to depend strongly on the effective field amplitude of the multiple photon transitions. By comparison with the analogous solutions for frequency-modulation EPR it is shown that the field-modulation and the frequency-modulation technique are not equivalent. The experimental data fully verify the theoretical predictions with respect to intensities and lineshapes.