Multilevel Excitation Research Articles

Interaction-free bidirectional multi-channel all-optical switching in a multi-level coupling atom-cavity system.

We propose a scheme of interaction-free bidirectional multi-channel all-optical switching in a multi-level coupling system consisting of four-level atoms confined in a cavity and coupled by a free-space control laser. A signal laser field is coupled into the cavity and excites two separate transitions of atoms simultaneously under the collective strong coupling condition. The transmission and reflection of signal fields form bidirectional output channels. A free-space control laser induces destructive quantum interference in the multi-level excitation of an atom-cavity system, which can be used to switch on/off the output signal lights of transmission/reflection channels. There is no direct coupling of control light and signal light through the cavity-confined atoms because the output of signal light is nearly totally suppressed to the opposite direction when control light is present. The proposed all-optical switching scheme can be realized with high switching efficiency, broad bandwidth, and weak light intensity. It may be useful for future devices of optical routing, optical communications, and various quantum logic elements.

Seismic Performance Evaluation of Two-story Dhajji-dewari Traditional Structure

ABSTRACT Experimental and numerical studies performed on traditional timber-frame masonry buildings have confirmed excellent deformability of timber structure; however, observations from dynamic shake table tests and earthquakes have revealed the potential hazard of stone falls poses a risk for occupants to get injured. This research presents a seismic performance evaluation study carried out on a traditional two-story Dhajji-Dewari structure having reduced diagonal braces for economizing construction. Moreover, one in-plane wall and one out-of-plane wall were retrofitted with typical steel wire mesh to control dislodging and fall of infill stones. Shake table tests were performed on a representative 1:3 reduced scale test model. The testing was performed using natural acceleration time history with multi-level excitations. The structure damage mechanism was observed with increasing intensity of base motion. The fundamental dynamic properties like frequency, damping, and acceleration amplification factors were obtained. The structure lateral force-deformation capacity curve was derived and bi-linearized to compute the structure ductility, overstrength, response modification factors (approximated to 3.0), and to develop structural global performance levels (i.e. roof drift limits, the corresponding base shear coefficients, and the wall damage states). A simplified procedure was also presented for the global seismic performance assessment of the Dhajji-Dewari structure in various seismic zones. The overall behavior of the model was observed to be good; however, the Immediate Occupancy and Life Safety limit states were primarily governed by the falling of infill rubbles from walls without steel wire mesh. The proposed steel wire mesh negligibly contributes to the lateral stiffness and strength of the structure and primarily prevented the dislodging and fall of stones. The proposed response modification factor can be used for the seismic analysis of structures using an elastic model to compute design forces for timber members and connection fasteners.

Volterra series identification and its applications in structural identification of nonlinear block-oriented systems

This paper considers the identification of a Volterra system and its applications in structural identification of nonlinear block-oriented models. Any order of the Volterra output is estimated separately via multilevel excitations and optimizations. Then, each order of the Volterra kernels is estimated independently with improved accuracy. Finally, relationships between the first and the second order Volterra kernel functions of block-oriented models are exploited to determine the structures of nonlinear block-oriented systems. The simulation studies verify the effectiveness of the proposed Volterra series identification method and the structure identification method for nonlinear block-oriented systems.

Best Linear Approximation of Wiener Systems Using Multilevel Signals: Theory and Experiments

The problem of measuring the best linear approximation (BLA) of a nonlinear system by means of multilevel excitation sequences is analyzed. A comparison between different types of sequences applied at the input of Wiener systems is provided by numerical simulations and by experiments on a practical circuit, including an analog filter and a clipping nonlinearity. The performance of the sequences is compared with a white Gaussian noise signal for reference purposes. The theoretical characterization of the BLA when using randomized constrained sequences is derived analytically for the cubic nonlinearity case. Numerical and experimental results show that the randomized constrained approach for designing ternary sequences has a low sensitivity to both even- and odd-order nonlinearities, resulting in a response close to the actual response of the underlying linear system.

Emerging multi‐double‐scroll attractor from variable‐boostable chaotic system excited by multi‐level pulse

This study reports a simple approach to emerging multi-double-scroll attractor from a variable-boostable chaotic system via multi-level pulse excitation, which is achieved by replacing a constant controller with a multi-level pulse function. To intuitively show the simple and feasible approach, a four-double-scroll chaotic attractor coined in such a system with four-level pulse excitation is taken as a paradigm.

A novel approach for identification of cascade of Hammerstein model

In this paper, a novel approach is proposed to identify the cascade of Hammerstein model by using Volterra series analytical method. The cascade of Hammerstein model consists of power series associated with linear subsystems. The relationship between the cascade of Hammerstein model and its associated Volterra model is firstly presented in this paper. The basic routine of the identification approach is that, from the system outputs under multilevel excitations, the Volterra series outputs of different order are first estimated by using the wavelet balance method. Then, through each order Volterra outputs and input, the impulse response functions of each order linear subsystems can be estimated, respectively. The simulation studies verify the effectiveness of the proposed identification method for the cascade of Hammerstein model.

Aseismic smart building isolation systems under multi-level earthquake excitations: Part I, conceptual design and nonlinear analysis

As a novel structural control strategy, tuned mass damper (TMD) inspired passive and semi-active smart building isolation systems are suggested to reduce structural response and thus mitigate structural damage due to earthquake excitations. The isolated structure’s upper stories can be utilized as a large scaled TMD, and the isolation layer, as a core design point, between the separated upper and lower stories entails the insertion of rubber bearings and (i) viscous dampers (passive) or (ii) resettable devices (semi-active). The seismic performance of the suggested isolation systems are investigated for 12-story reinforced concrete moment resisting frames modeled as “10 + 2” stories and “8 + 4” stories. Passive viscous damper or semi-active resettable devices are parametrically evaluated through the optimal design principle of a large mass ratio TMD. Statistical performance metrics are presented for 30 earthquake records from the three suites of the SAC project. Based on nonlinear structural models, including P-delta effects and modified Takeda hysteresis, the inelastic time history analyses are conducted to compute the seismic performances across a wide range of seismic hazard intensities. Results show that semi-active smart building isolation systems can effectively manage seismic response for multi-degree-of freedom (MDOF) systems across a broader range of ground motions in comparison to uncontrolled case and passive solution.

Aseismic smart building isolation systems under multi-level earthquake excitations: Part II, energy-dissipation and damage reduction

Based on the performance results of the previously suggested smart building isolation systems (1st companion paper), this following study verifies the control effects of the systems from the view point of energy dissipation and damage level metrics. Several different model cases of the strategically isolated multi-story building structures utilizing passive dampers and semi-active resettable devices are analyzed and the energy-based target indices are compared. Performance comparisons are conducted on statistically calculated story/structural hysteretic energy and story/structural damage demands over realistic suites of earthquake ground motion records, representing seismic excitations of specific return period probability. Again, the semi-active solutions show significant promise for applications of resettable device, offering advantages over passive systems in the consistent damage reductions. The specific results of this study include the identification of differences in the mechanisms by which smart building isolation systems remove energy, based on the differences in the devices used. Less variability is also seen for the semi-active isolation systems, indicating an increased robustness.

Wavelet basis expansion-based spatio-temporal Volterra kernels identification for nonlinear distributed parameter systems

In practical industries, there are many systems belong to nonlinear distributed parameter systems (DPS); unfortunately, modeling of nonlinear DPS is a challenging task because of the infinite-dimensional and nonlinear properties. To model the nonlinear DPS, a spatio-temporal Volterra model is presented with a series of spatio-temporal kernels. It can be considered as a spatial extension of the traditional Volterra model. One question involved in modeling a spatio-temporal functional relationship between the input and output of nonlinear distributed parameter systems using spatio-temporal Volterra series is to identify the spatio-temporal Volterra kernel functions. In addition, in order to derive a low-order model, the Karhunen–Loeve (KL) decomposition is used for the time/space separation. The basic routine of the approach is that, first, from the system outputs, KL decomposition is used for the time/space separation, where the spatio-temporal output is decomposed into a few dominant spatial basis functions with temporal coefficients. Second, according to temporal coefficients of outputs under multilevel excitations, the Volterra series outputs of different orders are estimated with the wavelet balance method. Third, the Volterra kernel functions of different orders are separately estimated through their corresponding Volterra series outputs by expanding them with four-order B-spline wavelet on the interval (BSWI). Finally, the spatio-temporal Volterra model can be reconstructed using the time/space synthesis. The simulation studies verify the effectiveness of the presented identification method.

Wavelet basis expansion-based Volterra kernel function identification through multilevel excitations

Volterra series is a powerful mathematical tool for nonlinear system analysis, which extends the convolution integral for linear system to nonlinear system. There is a wide range of nonlinear engineering systems and structures which can be modeled as Volterra series. One question involved in modeling a functional relationship between the input and output of a system using Volterra series is to identify the Volterra kernel functions. In this article, a wavelet balance method-based approach is proposed to identify the Volterra kernel functions from observations of the in- and outgoing signals. The basic routine of the approach is that, from the system outputs under multilevel excitations, the Volterra series outputs of different orders are first estimated with the wavelet balance method, and then the Volterra kernel functions of different orders are separately estimated through their corresponding Volterra series outputs by expanding them with four-order B-spline wavelet on the interval. The simulation studies verify the effectiveness of the proposed Volterra kernel identification method.

Truncation order and its effect in a class of nonlinear systems

Fundamental results are established for truncation order and its effect in the Volterra class of nonlinear systems. These results provide a significant insight into the analytical relationships among truncation order, truncation error, and estimation error for the output spectra of nonlinear systems, and also lead to the development of novel methods for choosing the truncation order and excitation magnitudes for accurate determination of nonlinear frequency response components. These results should be important in the kernel estimation and frequency domain analysis of nonlinear systems on the basis of the Volterra series, structure and order selection of block-oriented nonlinear systems with polynomial nonlinearity. The results also provide a much improved version of the multi-level excitation method for system analysis and identification.

Detection of interstellar hydrogen peroxide

The molecular species hydrogen peroxide, HOOH, is likely to be a key ingredient in the oxygen and water chemistry in the interstellar medium. Our aim with this investigation is to determine how abundant HOOH is in the cloud core {\rho} Oph A. By observing several transitions of HOOH in the (sub)millimeter regime we seek to identify the molecule and also to determine the excitation conditions through a multilevel excitation analysis. We have detected three spectral lines toward the SM1 position of {\rho} Oph A at velocity-corrected frequencies that coincide very closely with those measured from laboratory spectroscopy of HOOH. A fourth line was detected at the 4{\sigma} level. We also found through mapping observations that the HOOH emission extends (about 0.05 pc) over the densest part of the {\rho} Oph A cloud core. We derive an abundance of HOOH relative to that of H_2 in the SM1 core of about 1\times10^(-10). To our knowledge, this is the first reported detection of HOOH in the interstellar medium.

Light storage in a doped solid enhanced by feedback-controlled pulse shaping

We report on experiments dealing with feedback-controlled pulse shaping to optimize the efficiency of light storage by electromagnetically induced transparency (EIT) in a Pr${}^{3+}$:Y${}_{2}$SiO${}_{5}$ crystal. A learning loop in combination with an evolutionary algorithm permits the automatic determination of optimal temporal profiles of intensities and frequencies in the driving laser pulses (i.e., the probe and coupling pulses). As a main advantage, the technique finds optimal solutions even in the complicated multilevel excitation scheme of a doped solid, involving large inhomogeneous broadening. The learning loop experimentally determines optimal temporal intensity profiles of the coupling pulses for a given probe pulse. The optimized intensity pulse shapes enhance the light-storage efficiency in the doped solid by a factor of $2$. The learning loop also determines a fast and efficient preparation pulse sequence, which serves to optically prepare the crystal prior to light-storage experiments. The optimized preparation sequence is 5 times faster than standard preparation sequences. Moreover, the optimized preparation sequence enhances the optical depth in the medium by a factor of $5$. As a consequence, the efficiency of light storage also increases by another factor of $3$. Our experimental data clearly demonstrate the considerable potential of feedback-controlled pulse shaping, applied to EIT-driven light storage in solid media.

Ionization front in a high-current gas discharge

Spectroscopic measurements of ion/neutral density ratio profiles are made inside the high-current, low-pressure discharge of a coaxial magnetoplasmadynamic thruster and show the existence of a thin ionization front, upstream in the discharge, that effectively ionizes the incoming gas to ionization levels above 50%. The measurements allow an estimate of the width of this ionization front to be on the order of a few millimeters. Due to the known existence of microturbulence in the plasma, which can produce suprathermal electrons, an explanation of the measurements based on the existence of a suprathermal tail in the electron energy distribution function is sought. A theoretical model for the width of the ionization front is combined with a multilevel excitation model for argon and shows that a Maxwellian electron distribution function cannot account for the small length scale of the ionization front, and that the latter is more consistent with an electron distribution function having a suprathermal population, the magnitude of which is estimated by comparing the model to the experiments.

Efficient population transfer by delayed pulses despite coupling ambiguity

In traditional schemes of multilevel multilaser excitation, each laser pulse interacts with only one pair of states, and the rotating wave approximation (RWA) is applicable. Here we study the population transfer process in a three-state system when each of the two lasers interacts with each of the pair of states and when the Rabi frequencies characterizing the interaction strengths of the system are comparable to or larger than the difference of the transition frequencies. We show that complete and robust population transfer is possible under conditions more general than those hitherto considered necessary for stimulated Raman adiabatic passage (STIRAP) or for successive π pulses. Using adiabatic Floquet theory we show that successful population transfer can be interpreted as adiabatic passage by means of a transfer state which connects the initial and final states. The Floquet picture offers a convenient interpretation of the population transfer as accompanied by multiple absorption of photons from or emission into the laser fields.

Multilevel adiabatic population transfer.

We present a theoretical description of a scheme in which coordinated laser pulses transfer population efficiently from any initially populated state (e.g., the ground state) to a multiply excited state of an atom or molecule without producing appreciable population in intermediate states. Our analytic results for multilevel excitation transfer provide a simple yet instructive extension to the counterintuitive pulse sequence studied previously for a three-state system.

Analytic solutions of density-matrix evolutions with the use of Racah tensorial decompositions

The problem of the dynamical evolution of an atomic system excited from state 1 to state $N$ in $N\ensuremath{-}1$ steps by using one or more lasers is studied. The ${N}^{2}$-dimensional dynamical space is shown to factorize into $N$ independent subspaces under certain experimental conditions by using Racah tensorial representation. Different classes of multiple optical soliton and multilevel excitation solutions obtained previously are shown to arise from a unified scheme. A new class of multiphoton excitation solutions is also given. General equations of motion in the higher-dimensional subspaces are given but have not been solved.

Multiple-laser excitation of multilevel atoms

The probability of preparing multilevel atoms in high-lying levels by multiphoton excitation using one or more laser pulses can be increased by suitably tailoring the pulse amplitudes and frequencies. Few, if any, systematic methods designed specifically for multilevel excitation have been proposed. We present in this paper specific pulses and phase modulations which give specific degrees of inversion of multilevel atoms in analytic form.

Effects of magnetic sublevel degeneracy on Rabi oscillations

This paper describes the dynamics of multilevel excitation by coherent radiatin, in the $N$-level rotating-wave approximation, when the levels have nonzero angular momentum $J$ and consequent spatial-orientation degeneracy $2J+1$. Observable populations average over $2J+1$ independent sublevel excitation sequences. The phenomenon of inaccessible sublevels is noted, as is the effect of polarization. The paper illustrates how averaging over orientation can, in some cases, render illegible the Rabi oscillations and the ac Stark effect, whereas in other cases ($J\ensuremath{\rightarrow}J$, circular polarization on $J\ensuremath{\rightarrow}J\ifmmode\pm\else\textpm\fi{}1$ linear polarization) appreciable structure remains.

Introduction to an analysis of kinematical exciton levels

The possibility of the existence of kinematical levels for a multi-level exciton scheme is analysed and it is shown that such levels exist for all the values of the wave vector. This is the consequence of the fact that the exciton gap is much larger than the energy bandwidth. The existence of kinematical levels is proved using a more correct method of decoupling of higher order Green's functions in which pairing is performed not only for the same moments of time but for different moments as well. A matrix formulation of the system of equations for Green's function is proposed and the analysis is performed in this condensed form representing a sensible simplification which could be helpful for the analysis of other multilevel excitation systems, too.