Time Domain Investigation Research Articles

Time domain investigations of acoustical scattering from schools of swim bladder fish

Recent studies of time independent scattering from schools of swim bladder fish reveal important differences between the predictions of a mathematical model, which fully incorporates multiple scattering processes between the fish, and a second approach which treats the school as an effective medium with complex sound speed determined by the swim bladder resonance function. In back scattering, both modeling and data comparisons show that the effective medium approach underestimates the scattering amplitude when the fish separation is greater than about a quarter of the incident wavelength. In contrast, comparisons in the forward scattering direction show good agreement. These results are critically significant for time domain investigations of fish school scattering, aimed at using spatial and temporal variations in the acoustic field to study the stochastic behavior of the distribution and motion of the fish ensembles. A simple approach, using the inverse FFT of the school model and effective medium harmonic solutions, again reveals the limitations of the effective medium approach in back scattering. However, to obtain high resolution time sampling, more sophisticated time domain solution techniques based upon numerical integration and perturbation theory approaches are necessary. Both computational studies and data comparisons will be presented. [Research supported by ONR.]

Time-domain investigation of the acoustic vibrations of metal nanoparticles: Size and encapsulation effects

The acoustic vibrations of single-metal and multi-material nanoparticles are studied by ultrafast pump–probe optical spectroscopy and described in the context of the continuous elastic model. The applicability of this model to the small size range, down to one nanometer, is discussed in the light of recent experimental data and ab initio calculations. Investigations of multi-material nano-objects stress the impact of the intra-particle interface on the characteristics of their vibrational modes, also yielding information on the composition and spatial distribution of the constituting materials.

Selective Excitation of Atomic-Scale Dynamics by Coherent Exciton Motion in the Non-Born-Oppenheimer Regime.

Time-domain investigations of the nonadiabatic coupling between electronic and vibrational degrees of freedom have focused primarily on the formation of electronic superpositions induced by atomic motion. The effect of electronic nonstationary-state dynamics on atomic motion remains unexplored. Here, phase-coherent excitation of the two lowest electronic transitions in semiconducting single-walled carbon nanotubes by broadband <5-fs pulses directly triggers coherent exciton motion along the axis of the nanotubes. Optical pump-probe spectroscopy with sub-10-fs time resolution reveals that exciton motion selectively excites the high-frequency G mode coherent phonon, in good agreement with results obtained from time-domain ab initio simulations. This observed phenomenon arises from the direct modulation of the C-C interatomic potential by coherent exciton motion on a time scale that is commensurate with atomic motion. Our results suggest the possibility of employing light-field manipulation of electron densities in the non-Born-Oppenheimer regime to initiate selective atomic motion.

Ultrafast laser pump X-ray probe experiments by means of asynchronous sampling

A high time resolution in the picosecond range is required for the time-domain investigation of phonon dynamics in crystalline systems. Following a recently developed scheme in the visible spectrum, this resolution can be achieved by a method called asynchronous optical x-ray sampling (ASOXS). A pulsed femtosecond laser with high repetition rate is synchronized to the electron bunches in a storage ring. A slight frequency detuning changes the mutual delay continuously, resulting in a time-domain x-ray sampling of the laser-excited system. At the synchrotron radiation source ANKA a machine mode with low momentum compaction factor αc is available, which delivers ultra-short x-ray pulses in the picosecond range.

Characterization of [formula omitted] index for linear time-varying systems

The increasing interest in ℋ− index in fault detection has led to some recent development in time domain investigation. This paper at first presents a necessary and sufficient condition of the ℋ− index in finite time horizon for linear time-varying systems. It is shown that the feasibility of the ℋ− index, which is given when the ℋ− index is greater than a predefined value, is equivalent to the existence of a certain backward differential Riccati equation under a constraint. The square system as a special case is discussed. The corresponding results are extended to systems with unknown initial condition with a modified definition of the ℋ− index. The condition for weighted index to emphasize the signal effect at different time instant is also derived by transformation. The result is also compared with the famous bounded real lemma. The connection with the linear time-invariant (LTI) case is also discussed. Finally, examples are given to illustrate the main results.

Filter design for GOCE gravity gradients

The GOCE satellite observes gravity gradients with unprecedented accuracy and resolution. The GOCE observations are reliable within a well-defined measurement bandwidth. In this study, different finite and infinite impulse response filters have been designed to obtain the demanded pass. Exhaustive time and frequency domain investigations prove that the proposed infinite impulse response filter can be a real competitor of the existing solution of the filtering problem.

Time and frequency domain investigation of the heat transfer to a synthetic air jet

Heat transfer to a synthetic air jets is investigated experimentally. The influence of peaks in heat transfer outwith the stagnation region of the jet are of particular interest. Heat transfer to the jets is reported for experimental parameters, jet exit to impingement surface spacings, H/D = 1, Reynolds number of 3000, non-dimensional Stroke length, L0/D of 14.and an excitation frequency of 70 Hz Peaks in heat transfer outwith the stagnation region of the jet are investigated in both the time and frequency domain and a connection between the driving frequency of the jet and changes in the rate of heat transfer is outlined. It is shown that two type's changes in the rate of heat transfer outwith the stagnation region are present in synthetic jet impingement heat transfer; those associated with the jet excitation frequency and therefore attributed to interactions between the two jet flow regimes and those associated with the breakdown of coherent structures in the jet flow.

Time-domain investigation of the tunneling modes in photonic heterostructure containing single-negative materials

We present a theoretical investigation into the energy transport and transient wave propagation in metamaterial tunneling structures consisting of e-negative (ENG) and μ-negative (MNG) materials. It is proved that a conjugated matched ENG/MNG bilayer and a (zero-index-material doped) photonic crystal heterostructure can work as a sub-wavelength resonator at tunneling frequency. The tunneling modes need a certain time to achieve the steady state and the charge up characteristic time increases (nearly) exponentially with the thickness of the structures. Under the steady state, the wave in the single-negative-material structures is not evanescent, but a hybrid of a traveling wave and a reactive standing wave. The phase difference between the electric field and the magnetic field varies with the position and time. The investigation of transient wave propagation in the metamaterial tunneling structures will help us to understand the interaction process between wave and metamaterial and to design special functional apparatus.

Current relaxation due to hot carrier scattering in graphene

In this paper, we present direct time-domain investigations of the relaxation of electric currents in graphene due to hot carrier scattering. We use coherent control with ultrashort optical pulses to photoinject a current and detect the terahertz (THz) radiation emitted by the resulting current surge. We pre-inject a background of hot carriers using a separate pump pulse, with a variable delay between the pump and current-injection pulses. We find the effect of the hot carrier background is to reduce the current and hence the emitted THz radiation. The current damping is determined simply by the density (or temperature) of the thermal carriers. The experimental behavior is accurately reproduced in a microscopic theory, which correctly incorporates the nonconservation of velocity in scattering between Dirac fermions. The results indicate that hot carriers are effective in damping the current, and are expected to be important for understanding the operation of high-speed graphene electronic devices.

Comprehensive Study of Solar Cell Structure Defects by Means of Noise and Light Emission Analysis

This paper discusses the issue of silicon solar cells localized defects from metrological and physical points of view. Structure imperfections represent the real problem because of solar cells long-term degradation and conversion efficiency decreasing. To this aim we pay our attention to research relating to the defect light emission and correlation with rectangular microplasma fluctuation. A sensitive CCD camera has been used for mapping of surface photon emission. The operation point of the samples has been set to reverse bias mode, and different electric field intensity was applied. We managed to get interesting information using a combination of optical investigation and electrical noise measurement in time and spectral domain. It will be revealed that a direct correlation between noise and photon emission exists and the results related to several defect spots are presented in detail in this paper.

Subcell dispersive finite-difference time-domain schemes for infinite graphene-based structures

A systematic three-dimensional (3D) frequency-dependent finite-difference time-domain technique for the consistent and rigorous analysis of infinite graphene layers is developed in this study. The generalised formulation divides the overall geometry into unit cells and applies the appropriate Floquet periodic boundary conditions to their lateral surfaces. In this framework, the infinite graphene sheet is carefully manipulated by means of an efficient subcell discretisation concept along with a complex surface conductivity representation defined by quantum mechanical equations. This conductivity model is, subsequently, converted to its volume counterpart to allow realistic time-domain investigations, while the dispersive nature of graphene is described via an auxiliary differential equation algorithm. Furthermore, a set of linearly polarised normally incident wideband pulses or obliquely incident monochromatic waves excite the computational domain based on a properly modified total-field/scattered-field scheme. Numerical simulations, involving an assortment of multilayer arrangements, reveal the promising accuracy and stability of the proposed method through detailed comparisons with data from analytical closed-form expressions.

Time-Domain Large Signal Investigation on Dynamic Responses of the GDCC Quarterly Wavelength Shifted Distributed Feedback Semiconductor Laser

A numerical investigation on the dynamic large-signal analysis using a time-domain traveling wave model of quarter wave-shifted distributed feedback semiconductor lasers diode with a Gaussian distribution of the coupling coefficient (GDCC) is presented. It is found that the single-mode behavior and the more hole-burning effect corrections of quarter wave-shifted distributed feedback laser with large coupling coefficient can be improved significantly by this new proposed light source.

Investigation on Microwave Negative Group Delay Circuit

In this article, a fundamental analytical approach and experimental verification of a radio frequency active circuit capable of exhibiting negative group delay at microwave wavelengths are presented. By using the S-parameter theory, basic properties and characteristics of the negative group delay circuit under study are established. To highlight the functionality principle of this innovative circuit, time-domain investigations based on ultra-wide-band pulse signal tests are performed. Then, the mechanism of propagating output signal envelopes in time advance compared to the input is demonstrated. To validate this concept, a prototype of a negative group delay device was designed, fabricated, and tested. Experimental results in good agreement with theory and simulation were obtained. In the frequency domain, a negative group delay of about −2.5 ns with amplification of 2 dB was measured at around 622 MHz. Due to this negative group delay effect, envelope advance of about −1.5 ns was observed by considering a pulse signal with a half-height full-width of 10 ns, which modulates a sine carrier with a frequency of 622 MHz. The negative group delay topology presented is potentially useful for the compensation of radio frequency/microwave signal delays.

Dc current transport behavior in amorphous GeSe films

The dc electric conduction behavior of amorphous GeSe films sandwiched between the Pt bottom electrode and various counter electrodes (Pt, Cr, and Ti) was examined in a voltage as well as a time domain. The time domain investigation identified time-dependent resistance change and relaxation. The voltage domain analysis revealed that the current transport in these metal-insulator-metal junctions is probably attributed to bulk-limited band-conduction due to delocalized charge carriers.

Dehydroindigo, the Forgotten Indigo and Its Contribution to the Color of Maya Blue

A comprehensive investigation of the electronic spectral and photophysical properties of the oxidized form of indigo, dehydroindigo (DHI), has been carried out in solution at 293 K. It is shown that dehydroindigo readily converts into its neutral keto form, the blue indigo, in a process which depends on the solvent and water content of the medium. DHI was investigated in toluene, in benzene, and in methanol and it was found that both the oxidized and the keto indigo forms are present in solution. In marked contrast to what has been found for keto-indigo, where the internal conversion channel dominates >99% of the excited state deactivation, or with the fully reduced leuco-indigo, where fluorescence, internal conversion, and singlet-to-triplet intersystem crossing coexist, in the case of DHI in toluene and benzene, the dominant excited state deactivation channel involves the triplet state. Triplet state yields (phi(T)) of 70-80%, with negligible fluorescence (< or = 0.01%) are observed in these solvents. In methanol the phi(T) value decreases to approximately 15%, with an increase of the fluorescence quantum yield to 2%, which makes these processes competitive with the S(1) --> S(0) internal conversion deactivation process. The data are experimentally compatible with the existence of a lowest lying singlet excited state of n,pi* origin in toluene and of pi,pi* origin in methanol. A time-resolved investigation in the picosecond time domain suggests that the emission of DHI involves three interconnected species (involving rotational isomerism), with relative contributions depending on the emission wavelength. DFT calculations (B3LYP 6-31G** level) were performed in order to characterize the electronic ground (S(0)) and excited singlet (S(1)) and triplet (T(1)) states of DHI. The HOMO-LUMO transition was found to accompany an n --> pi* transition of the oxygen nonbonding orbitals to the central CC and adjacent C-N bonds. Calculations also revealed that in S(0) the two indole-like moieties deviate from planarity from ca. 20 degrees, whereas in S(1) and T(1) the predicted structure is basically planar; a gradual decrease of the carbon-carbon central bond distance is seen in the order S(0), S(1), T(1). An additional study on the blue pigment Maya Blue was made, and the comparison between the solid-state spectra of indigo, DHI, and Maya Blue suggests that, in line with recent investigations, DHI is present together with indigo in Maya Blue. These results are relevant to the discussion of the involvement of dehydroindigo in the palette of colors of the ancient Maya Blue pigment.

Time-Domain Investigation on Cable-Induced Transient Coupling Into Metallic Enclosures

A hybrid time-domain method is proposed for characterizing electromagnetic interference (EMI) signals coupled into some composite structures with metallic enclosures, braided shielded cable, printed circuit boards, and even lumped active devices. In order to rapidly capture the induced interior EMI, the finite-difference time-domain, modified node analysis, and multiconductor transmission lines methods are combined together and implemented successfully. Numerical investigation is carried out to demonstrate the frequency-dependent transfer impedance of the coaxial cable, the induced voltage at the place of active loaded element in the transmission line network, and the enclosure shielding effectiveness of these composite enclosures. The captured transient response information is useful for further designing electromagnetic protection of the inner circuits against the impact of voltage or current surge caused by nonintentional as well as intentional electromagnetic interference.

Realization of time-resolved two-vacuum-ultraviolet-photon ionization

Ultrafast dynamics of excited molecules is studied through time-resolved two-vacuum-ultraviolet (vuv)-photon ionization using a nonlinear volume autocorrelator unit. The two-vuv-photon process is induced by the intense fifth harmonic radiation of a femtosecond Ti:sapphire laser. In a proof-of-principle experiment, ultrafast dynamics of excited ethylene and oxygen molecules are investigated. Molecular decay times are deduced by comparing the experimental data with the results of a numerical model that accounts for the spatial and temporal characteristics of the harmonic field. The present experiments pave a convenient way for time domain investigations in the vuv-xuv spectral region in all states of matter at few femtosecond to subfemtosecond temporal scales.

Optical activity in an artificial chiral media: a terahertz time-domain investigation of Karl F Lindman’s 1920 pioneering experiment

Chiral media interact preferentially with either left- or right-circularly polarized electromagnetic waves, leading to effects including circular dichroism, optical rotation and circular preferential scattering. In this experiment, we revisit Lindman's famous 1920 experiment linking artificial chiral materials to optical activity and we record the first time-domain measurements of a single-cycle THz pulse transmitted through randomly oriented metallic helices. Time-resolved measurements of co- and cross-polarized components of the transmitted electric field allow the electric field trajectory to be reconstructed and time dynamics of the two circular components to be investigated. For the first time, we show that time dynamics reveal two distinct effects that are separated in time: local preferential circular scattering and collective coupling. These findings are important on furthering our understanding on the analogy between optical activity arising from light interaction with large chiral molecules and that from macroscopic artificial chiral media.

Vibration and Position Tracking Control of a Flexible Beam Using SMA Wire Actuators

This paper presents vibration and position control of a flexible beam structure by adopting shape memory alloy (SMA) wire actuators. The governing equation of motion of the proposed flexible structure is obtained via Hamilton's principle. The dynamic characteristics of the SMA wire actuator are experimentally identified and incorporated with the governing equation to furnish a control system model in the state space. Subsequently, a sliding mode controller which has inherent robustness to external disturbances and parameter uncertainties is formulated. The controller is then empirically realized for vibration control with relatively large tip displacement. In addition, tip position tracking control to follow desired trajectories with low-frequency sine and square waves is undertaken. Control performances such as tracking error are evaluated through both computer simulation and experimental investigation in time domain.

A Time Domain Investigation on the Hydrodynamic Resonance Phenomena of 3-D Multiple Floating Structures

A 3-D time domain method is developed to investigate the gap influence on the wave forces for 3-D multiple floating structures. Special hydrodynamic resonance due to small gaps between multiple floating structures on wave forces is examined. Strong and complicate hydrodynamic interactions between the floating bodies are observed and the numerical computations have proved the existence of the sharp peak force response on each floating body at some special resonant wave numbers. By comparison with the results from the frequency domain technique, the results obtained from the time domain method reveal the similar resonant phenomena and hydrodynamic interaction. The resonant wave numbers are also proved around kL= nπ ( n = 1,2,…,∞) with a corresponding frequency shift. The strong hydrodynamic interaction feature is practically significant for the design of module structures and the links (connection) in whole the floating body system.