Off-resonance Conditions Research Articles

Formation and optical control of dissipative vortex solitons in hollow-core optical fibres filled with a cold atomic gas

We consider the problem of formation of optical vortex solitons in a medium of three-level atoms, taking into account the local field effects. The principal possibility of formation of optical solitons controlled by an optical pump wave under the off-resonance Raman conditions of the lambda-type interaction scheme is predicted in an optically dense medium, i.e., in a hollow-core photonic-crystal optical fibre filled with a gas of cold atoms 87Rb.

Inner‐volume imaging in vivo using three‐dimensional parallel spatially selective excitation

This work describes the first experimental realization of three-dimensional spatially selective excitation using parallel transmission in vivo. For the design of three-dimensional parallel excitation pulses with short durations and high excitation accuracy, the choice of a suitable transmit k-space trajectory is crucial. For this reason, the characteristics of a stack-of-spirals trajectory and of a concentric-shells trajectory were examined in an initial simulation study. It showed that, especially when undersampling the trajectories in combination with parallel transmission, experimental parameters such as transmit-coil geometry and off-resonance conditions have an essential impact on the suitability of the selected trajectory and undersampling scheme. Both trajectories were applied in MR inner-volume imaging experiments which demonstrate that acceptably short and robust three-dimensional selective pulses can be achieved if the trajectory is temporally optimized and its actual path is measured and considered during pulse calculation. Pulse durations as short as 3.2 ms were realized and such pulses were appropriate to accurately excite arbitrarily shaped volumes in a corn cob and in a rat in vivo. Reduced field-of-view imaging of these selectively excited targets allowed high spatial resolution and significantly reduced measurement times and furthermore demonstrates the feasibility of three-dimensional parallel excitation in realistic MRI applications in vivo.

Experiments on non-resonant sloshing in a rectangular tank with large amplitude lateral oscillation

Flow of two dimensional nonlinear sloshing in a rigid rectangular tank with free surface is considered. The flow is generated by oscillating the container in a lateral harmonic motion, i.e., d⁎=A⁎sin(2πf⁎t⁎) where d⁎ denotes the displacement of the container externally forced, A* the amplitude of displacement and f* the frequency. Thus, the maximum stroke, S*, of the container for a cycle, measured by a distance from the leftmost to the rightmost location on the container movement, is defined as S*=2A*. It has performed a sequence of experiments on a variety of S*- and f*-values to investigate large amplitude sloshing flows at off-resonant condition far from the system natural frequency, where large amplitude means that the stroke, S*, of the container movement is comparable with the breadth, L*, of the container, i.e., S*/L*∼O(1) and the excitation acceleration is also comparable with the gravity, i.e., π2(S*f*)2/g*∼O(1). Through PIV experiment, it shows that the flow physics on nonlinear off-resonant sloshing problem can be characterized into a combination of three peculiar sloshing motions: (1) standing wave motions which is similar with those of linear sloshing during run-down process, (2) run-up phenomenon like hydraulic jump along the vertical sidewall at the moment of turn-around of the container and (3) gradually propagating bore motion from one sidewall to the opposite wall which is similar with dam breaking problem.

Superenhanced photonic nanojet by core-shell microcylinders

The super-enhancement of photonic nanojets generated at the shadow side surfaces of core-shell microcylinders illuminated by a plane wave is reported. Using high resolution finite-difference time-domain simulation, the enhancements of photonic nanojet at resonance and off-resonance conditions of microcylinders are investigated. The intensity enhancement of photonic nanojet depends strongly on the thickness of metal shell. Under proper resonance condition, the photonic nanojet super-enhancement can be excited in the core-shell microcylinder. Even under off-resonance condition, the photonic nanojet from microcylinder is still strong enough. The results may provide a new technique to detect and image nanoscale objects below the diffraction limit. This could yield a new ultra-microscopy technique for using visible light to detecting nanoparticles, optical gratings, and single molecules.

Homonuclear Dipolar Coupling and CPMG Spin-Echoes in NQR

The nuclear quadrupole resonance signals from both a single crystal and a powder sample of spin-1 nuclei under a Carr–Purcell–Meiboom–Gill are modeled numerically. While the single crystal clearly shows the effects of dipolar coupling for on-resonant pulses, the powder does not. However, for certain off-resonant conditions, the powder sample exhibits the same response as the single crystal. Experimentally, this corresponds to the observation of a rapid decay at these conditions. Using a powder sample of NaNO2, the functional form of the echo train, when the dipolar coupling is not refocused, is clearly different than the decay for an on-resonance sequence, and can be used to characterize the dipolar coupling.

Left-handedness in K-type multilevel system in the presence of spontaneously generated coherence

The left-handedness in atomic vapor medium consisting of five energy levels in K-type configuration is studied in this work. The theoretical modeling has been done using density matrix approach in which the spontaneously generated coherence (SGC) is also included. The system shows negative electrical permittivity (ϵ) as well as negative magnetic permeability (μ) and thus displays the negative refractive index (n) for certain values of system parameters in the presence of SGC. The parameters (ϵ, μ, n) quantifying left-handedness in the system become more negative if the SGC is strong. Under the off-resonant conditions these parameters become less negative and the left-handedness of system degrades.

Theory of quantum energy transfer in spin chains: Superexchange and ballistic motion

Quantum energy transfer in a chain of two-level (spin) units, connected at its ends to two thermal reservoirs, is analyzed in two limits: (i) in the off-resonance regime, when the characteristic subsystem excitation energy gaps are larger than the reservoirs frequencies, or the baths temperatures are low and (ii) in the resonance regime, when the chain excitation gaps match populated bath modes. In the latter case, the model is studied using a master equation approach, showing that the dynamics is ballistic for the particular chain model explored. In the former case, we analytically study the system dynamics utilizing the recently developed Energy-Transfer Born-Oppenheimer formalism [L.-A. Wu and D. Segal, Phys. Rev. E 83, 051114 (2011)], demonstrating that energy transfers across the chain in a superexchange (bridge assisted tunneling) mechanism, with the energy current decreasing exponentially with distance. This behavior is insensitive to the chain details. Since at low temperatures the excitation spectrum of molecular systems can be truncated to resemble a spin chain model, we argue that the superexchange behavior obtained here should be observed in widespread systems satisfying the off-resonance condition.

Response Measurement Accuracy for Off-Resonance Excitation in Atomic Force Microscopy

Displacement measurement in atomic force microscopy (AFM) is most commonly obtained indirectly by measuring the slope of the AFM probe and applying a calibration factor. Static calibration techniques operate on the assumption that the probe response approximates single mode behavior. For off-resonance excitation or different operating conditions the contribution of higher modes may become significant. In this paper, changes to the calibrated slope-displacement relationship and the corresponding implications on measurement accuracy are investigated. A model is developed and numerical simulations are performed to examine the effect of laser spot position, tip mass, quality factor and excitation frequency on measurement accuracy. Free response conditions and operation under nonlinear tip-sample forces are considered. Results are verified experimentally using a representative macroscale system. A laser spot positioned at a nominal position between x = 0.5 and 0.6 is determined to minimize optical lever measurement error under conditions where the response is dominated by contributions from the first two modes, due to excitation as well as other factors.

Self-biased converse magnetoelectric effect

In this letter, we investigate the direct magnetoelectric (DME) and converse magnetoelectric (CME) effects in three-phase metal–ceramic laminate composites. Longitudinally poled and transversely magnetized (L-T) laminate was fabricated by bonding nickel plates between the two particulate magnetoelectric (ME) composite layers of composition 0.8 (0.948 K0.5Na0.5NbO3 – 0.052 LiSbO3) – 0.2 (Ni0.8Zn0.2Fe2O4) (KNNLS-NZF). Under off-resonance condition, the laminates exhibited hysteretic DME and CME responses as a function of applied bias field (Hbias). Self-biased effect characterized by non-zero ME response at zero Hbias was observed. The self-biased DME and CME properties were found to be enhanced under resonance conditions. Without external Hbias, magnetic induction switching was possible by applying AC voltage. These results provide the possibility of using self-biased CME effect in electrically controlled memory devices and magnetic flux control devices.

Quantum communication through open-ended quantum networks

We study a one-dimensional fibre-coupled cavity array with an open-end boundary condition, where each cavity is doped with a two-level atom, and obtain explicit expressions for the dynamics evolution of the array with arbitrary number of nodes in the far off-resonant condition. It is shown that distant state transfer and entanglement generation schemes may be implemented through this quantum network and an arbitrary state is transferred with high fidelity for the array with specific number of nodes. The entanglement transfer through both single array and double independent arrays is also examined.

Optical force on two-level atoms by few-cycle-pulse Gaussian laser fields beyond the rotating-wave approximation

We report a study on light force on a beam of neutral two-level atoms superimposed upon a few-cycle-pulse Gaussian laser field under both resonant and off-resonant conditions. The phenomena of focusing, defocusing, and steering of the neutral atoms in the laser field are analyzed by solving the optical Bloch equation beyond the rotating-wave approximation and the force equation self-consistently. We find that two-level atoms in an atomic beam can be focused and defocused for large, positively and negatively detuned interactions even in the regime of extreme nonlinear optics. The so-called optical potential may be used for stable trapping of the neutral two-level atoms for large positively detuned interaction. This work successfully reproduces some of the features reported in recent experimental and theoretical works.

Morphology-dependent resonances in an eccentrically layered sphere illuminated by a tightly focused off-axis Gaussian beam: parallel and perpendicular beam incidence

Following the recent results in generalized Lorenz–Mie theory concerning the description of an arbitrary shaped electromagnetic beam propagating in an arbitrary orientation, a theoretical investigation of morphology-dependent resonances (MDRs) excited in a sphere with an eccentrically located spherical inclusion illuminated by a tightly focused Gaussian beam is presented. Calculations of extinction efficiency spectra and backward-scattering intensity spectra are made for different locations and radii of the inclusion with respect to the host sphere. Exemplifying field distributions inside of the scatterer under both off-resonance and on-resonance conditions are exhibited. The influences of the relative size of the inclusion with respect to the host sphere and of the separation distance between the two sphere centers on the positions and on the amplitudes of the MDRs peaks are studied. As are the cases for spheres and concentrically multilayered spheres, the resonance positions of MDRs in an eccentrically layered sphere are located at the same size parameter for Gaussian beam illumination and for plane-wave illumination. In contrast with the lift of azimuthal modes m degeneracy in MDR peaks for an eccentric sphere illuminated obliquely by a plane wave, we display a kind of lift that cannot be observed in extinction efficiency spectra with an oblique illumination of a tightly focused Gaussian beam. Nevertheless, asymmetric distributions of the internal field inside of the eccentric sphere at resonance conditions are observed both with an oblique illumination of a tightly focused beam and with an oblique illumination of a plane-wave illumination. Interpretation from a perspective of the localization principle is applied to the simulation results.

Vortex shedding and acoustic resonance of single and tandem finned cylinders

The effect of fins on vortex shedding and acoustic resonance is investigated for isolated and two tandem cylinders exposed to cross-flow in a rectangular duct. Three spacing ratios between the tandem cylinders ( S/D e=1.5, 2 and 3) are tested for a Reynolds number range from 1.6×10 4 to 1.1×10 5. Measurements of sound pressure as well as mean and fluctuating velocities are performed for bare and finned cylinders with three different fin densities. The effect of fins on the sound pressure generated before the onset of acoustic resonance as well as during the pre-coincidence and coincidence resonance is found to be rather complex and depends on the spacing ratio between cylinders, the fin density and the nature of the flow–sound interaction mechanism. For isolated cylinders, the fins reduce the strength of vortex shedding only slightly, but strongly attenuate the radiated sound before and during the occurrence of acoustic resonance. This suggests that the influence of the fins on correlation length is stronger than on velocity fluctuations. In contrast to isolated cylinders, the fins in the tandem cylinder case enhance the vortex shedding process at off-resonant conditions, except for the large spacing case which exhibits a reversed effect at high Reynolds numbers. Regarding the acoustic resonance of the tandem cylinders, the fins promote the onset of the coincidence resonance, but increasing the fin density drastically weakens the intensity of this resonance. The fins are also found to suppress the pre-coincidence resonance for the tandem cylinders with small spacing ratios ( S/D e=1.5, 2 and 2), but for the largest spacing case ( S/D e=3), they are found to have minor effects on the sound pressure and the lock-in range of the pre-coincidence resonance.

사각용기의 슬로싱 유동에 관한 연구

The two-dimensional sloshing problem in a rigid rectangular tank with a free surface is considered. The flow is generated by a container in harmonic motion in time along the horizontal axis, i.e., a container excited by u=Asin() where u denotes the container velocity imposed externally, A is the amplitude of the oscillation velocity, and f is the frequency of oscillation. Experimental apparatus is arranged to investigate the large-amplitude sloshing flows in off-resonant conditions, where the large amplitude means that A~O(1), and the distance, S, is comparable to the breadth, L, of the container, i.e., L/S~O(1). Comprehensive particle image velocimetry (PIV) data are obtained, which show that the flow physics of the nonlinear off-resonant sloshing problem can be characterized into three peculiar free surface motions: standing-wave motions similar to those of linear sloshing, a run-up phenomenon along the vertical sidewall at the moment of turn-over of the container, and gradually propagating bore motion from the sidewall to the interior fluid region, like a hydraulic jump.

Quantum Master Equation Method Based on the Broken-Symmetry Time-Dependent Density Functional Theory: Application to Dynamic Polarizability of Open-Shell Molecular Systems

A novel method for the calculation of the dynamic polarizability (α) of open-shell molecular systems is developed based on the quantum master equation combined with the broken-symmetry (BS) time-dependent density functional theory within the Tamm-Dancoff approximation, referred to as the BS-DFTQME method. We investigate the dynamic α density distribution obtained from BS-DFTQME calculations in order to analyze the spatial contributions of electrons to the field-induced polarization and clarify the contributions of the frontier orbital pair to α and its density. To demonstrate the performance of this method, we examine the real part of dynamic α of singlet 1,3-dipole systems having a variety of diradical characters (y). The frequency dispersion of α, in particular in the resonant region, is shown to strongly depend on the exchange-correlation functional as well as on the diradical character. Under sufficiently off-resonant condition, the dynamic α is found to decrease with increasing y and/or the fraction of Hartree-Fock exchange in the exchange-correlation functional, which enhances the spin polarization, due to the decrease in the delocalization effects of π-diradical electrons in the frontier orbital pair. The BS-DFTQME method with the BHandHLYP exchange-correlation functional also turns out to semiquantitatively reproduce the α spectra calculated by a strongly correlated ab initio molecular orbital method, i.e., the spin-unrestricted coupled-cluster singles and doubles.

Off-resonance effects in surface nuclear magnetic resonance

Surface nuclear magnetic resonance (NMR) is a noninvasive geophysical tool used to investigate groundwater reservoirs. The relevant physical process in surface NMR is the nuclear spin of hydrogen protons in liquid water. Standard single-pulse surface NMR experiments provide estimates of water content in the shallow subsurface. Under favorable conditions, pore-structure and even hydraulic-conductivity information can be extracted from double-pulse surface NMR data. One crucial issue in surface NMR experiments is the resonance condition: the frequency of the excitation field should closely match the Larmor frequency of the protons, which is controlled by the local magnitude of the earth’s magnetic field. Although the earth’s field can be measured accurately by an on-site magnetometer, several effects impede perfect matching of the frequencies. These include temporal variations of the earth’s field, instrumental imperfections, and the magnetic susceptibility of the underlying rocks. We assess the impact of violating the resonance condition on surface NMR experiments. Our investigation involves numerical simulations and measurements using a sample-scale earth-field NMR device and a surface NMR acquisition system. For frequency offsets up to 5 Hz, we find that relatively standard single-pulse surface NMR recording procedures are likely to produce reliable water-content estimates as long as the pulse moments are small to moderate or the aquifer is relatively deep. If strong pulse moments are required or shallow aquifers are probed, off-resonance conditions can lead to anomalous increases in recorded amplitudes that can be mistakenly interpreted in terms of deepwater occurrences. Double-pulse surface NMR experiments are particularly sensitive to off-resonance effects, such that the results may be highly biased even for the small-frequency offsets commonly encountered in field situations.

Effect of Single Femtosecond Pulses on Gold Nanoparticles

Gold nanoparticles find a wide range of applications in optics and photonics; however, their detailed interaction with intense laser light is only partially understood. Previous works have studied the effect of intense pulse trains on gold nanoparticles at a wide range of illumination parameters and observed diverse optical and morphological changes. In this work we study, for the first time, the interaction between single femtosecond pulses and gold nanoparticles. Using transmission electron microcopy and optical spectroscopy, we have found that nanoparticles illuminated by 50 fs pulses with fluence of less than 0.15 J/cm2 per pulse (3 TW/cm2) undergo morphological changes that affect their extinction spectra. Experimentation with particles of different diameters show similar qualitative effects, which are more pronounced for larger particles. Pulses at different excitation wavelengths were found to induce different effects for resonance and off-resonance conditions. The presented results provide valuable experimental data on the complex pulse−particle interaction and would be helpful for better understanding of the physical processes that are involved.

Structure and vibrational modes of AgI-doped AsSe glasses: Raman scattering and ab initio calculations

We report an investigation of the structure and vibrational modes of (AgI) x (AsSe) 100− x , bulk glasses using Raman spectroscopy and first principles calculations. The short- and medium-range structural order of the glasses was elucidated by analyzing the reduced Raman spectra, recorded at off-resonance conditions. Three distinct local environments were revealed for the AsSe glass including stoichiometric-like and As-rich network sub-structures, and cage-like molecules (As 4Se n , n=3, 4) decoupled from the network. To facilitate the interpretation of the Raman spectra ab initio calculations are employed to study the geometric and vibrational properties of As 4Se n molecular units that are parts of the glass structure. The incorporation of AgI causes appreciable structural changes into the glass structure. AgI is responsible for the population reduction of molecular units and for the degradation of the As-rich network-like sub-structure via the introduction of As–I terminal bonds. Ab initio calculations of mixed chalcohalide pyramids AsSe m I 3− m provided useful information augmenting the interpretation of the Raman spectra.

Measurement of a free spectral range of a Fabry–Perot cavity using frequency modulation and null method under off-resonance conditions

In this paper, we discuss a method to measure the free spectral range (FSR) of a Fabry–Perot cavity (FP-cavity) using frequency modulation with one electric optical modulator (EOM) and the null method. A laser beam modulated by the EOM, to which a sine wave signal is supplied from a radio frequency (RF) oscillator, is incident on the FP-cavity. The transmitted or reflected light from the FP-cavity is observed and converted to an RF signal by a high-speed photodetector, and the RF signal is synchronously demodulated with a lock-in amplifier by referring to a cosine wave signal from the oscillator. We theoretically and experimentally demonstrate that the lock-in amplifier signal for the transmitted or reflected light becomes null with a steep slope when the modulation frequency is equal to the FSR under the condition that the carrier frequency of the laser is slightly detuned from the resonance of the FP-cavity. To reduce the measurement uncertainty for the FSR, we also discuss a selection method for laser power, a modulation index and the detuning shift of the carrier frequency, respectively.

Second-order nonlinear optical response of zigzag BN single-walled nanotubes

A theory based on the two-band tight-binding approximation for $\ensuremath{\pi}$ electrons is developed to describe the second-order nonlinear optical (NLO) properties of arrays of uniformly sized and well-aligned boron-nitride single-walled nanotubes (BN-SWNTs) with a zigzag achiral structure. It is assumed that the coherent light beam at frequency $\ensuremath{\omega}$, incident upon the nanotube sample, is linearly polarized along the symmetry axis of the nanotubes. The long-axis NLO susceptibility ${\ensuremath{\chi}}^{(2)}(\ensuremath{\omega})$ of those nanotubes is calculated within the independent nanotube approximation and in neglecting local-field effects. Using the perturbation-theory formalism in the crystal-momentum representation, we derive an explicit analytic expression for the ${\ensuremath{\chi}}^{(2)}(\ensuremath{\omega})$ and apply it to study three distinct second-order NLO effects possible in the BN-SWNTs due to their noncentrosymmetric structure---namely, second-harmonic generation (SHG), linear electro-optical (LEO) effect, and nonlinear optical rectification (NOR). The theory is illustrated by numerical model calculations of the SHG, LEO, and NOR susceptibility spectra for several representative BN-SWNT ensembles consisting of large-diameter nanotubes. The calculated SHG spectra are found to be dominated by the highly peaked $2\ensuremath{\omega}$ resonance at half the band-gap energy of the BN-SWNTs, where the absorption of light is negligible. Distinct features are also found in the LEO and NOR susceptibility spectra, e.g., a sudden switching of the susceptibility from a positive peak value to a negative peak one in the near vicinity of the fundamental absorption edge. A fairly large magnitude of those susceptibilities, reaching the order of ${10}^{\ensuremath{-}7}\text{ }\text{esu}$ under off-resonant conditions and up to ${10}^{\ensuremath{-}6}\text{ }\text{esu}$ in the resonant case, suggests that BN-SWNTs are a promising material for various electro-optical device applications.