Step-like Pulse Research Articles

Filament Formation Mechanism for a Nanosecond Surface Barrier Discharge. Part 2. The Local-Energy Approximation

The development of a surface barrier discharge driven by a negative steplike voltage pulse with an amplitude of V = –8 kV in air at the atmospheric pressure and a pulse with an amplitude of V = –15 kV in nitrogen at a pressure of 6 atm is simulated numerically. Calculations for V = –8 kV were carried out using the local-electric-field and the local-electron-energy approximations. It is demonstrated that both approximations yield similar results on the dynamics of discharge development as a whole, the cathode-layer structure, and the field distribution at the front of the discharge. Substantial differences are observed in parameters of the discharge layer adjacent to the dielectric surface, which allowed to simulate an effect similar to filamentation of the discharge in nitrogen at a pressure of 6 atm and voltage of V = –15 kV in the local-energy approximation.

Study of step-like pulses from a mode-locked erbium-doped fiber laser in L-band

This paper presents a numerical simulation and an experiment on the generation of step-like pulse in an ultra-long cavity passively mode-locked erbium-doped fiber laser based on nonlinear optical loop mirror (NOLM). The gain competition phenomenon and the peak power clamping (PPC) effect of the step-like pulse are investigated with reasonable transmittance equation. To the best of our knowledge, this paper is the first interpretation and simulation of step-like pulse. And we believe that the findings contribute to the enrichment of nonlinear dynamics understanding and are anticipated to enhance laser performance optimization.

Machine Learning for Accurate Energy Analysis of Double Beta Decay Events

Point-contact germanium detectors are used to detect and analyze particles and their decay processes. This research focuses on the search for a phenomenon known as neutrinoless double beta decay, where two neutrons within a nucleus transform into two protons, emitting electrons in the process. Unlike standard double beta decay, no neutrinos are emitted, suggesting that the neutrino may act as its own antiparticle, effectively canceling itself out. Detecting this process would provide crucial insights into the nature of neutrino mass. During such an event, the detector registers a spike in energy due to the emitted electrons, producing a step-like pulse in the output channels. The energy of the event is determined by the difference in charge between the ‘top’ and ‘bottom’ steps of the pulse. However, accurately determining the energy is complicated by the presence of a resistor in the detector, which causes the top step to decay exponentially back to a baseline in preparation for the next event. This decay makes it challenging to determine the true energy of the pulse. My work this summer focused on developing machine learning models to reconstruct the step-like nature of pulses from their decayed versions recorded by the detector. This involved constructing deep neural networks trained on inputs (decayed pulses) and corresponding targets (the original step pulses) to learn to accurately reconstruct the target from the input. Initially, the networks were trained on simulated detector data. Once proficient in reconstruction, electronic noise was added to the input pulses to simulate real-world conditions, under which the models were trained to remove both electronic noise and decay. The models were trained to handle pulses with varying decay constants to improve their generalization ability. These neural networks can now be applied to real detector data for accurate energy analysis of double beta decay events.

Dynamic negative capacitance response in GeTe Rashba ferroelectric

We experimentally investigate capacitance response of a thick ferroelectric GeTe single-crystal flake on the Si/SiO2 substrate, where p-doped Si layer serves as a gate electrode. We confirm by resistance measurements, that for three-dimensional flakes, electron concentration is not sensitive to the gate electric field due to the screening by bulk carriers. Unexpectedly, we observe that sample capacitance C is strongly diminishing for both gate field polarities, so C(Vg) is a maximum near the zero gate voltage. Also, we observe well-developed hysteresis with the gate voltage sweep direction for the experimental C(Vg) curves. From our analysis, the capacitance behavior is explained by the known dependence of the Rashba parameter on the electric field for giant Rashba splitting in GeTe. In this case, the hysteresis in capacitance should be ascribed to polarization evolution in GeTe surface layers, which also allows to realize the regime of dynamic negative capacitance. The latter can be directly observed in time-dependent resistive measurements, as non-monotonic evolution of voltage response to the step-like current pulse. Thus, the negative capacitance regime can indeed improve performance and, therefore, the energy efficiency of electronic devices.

Probing electron-hole weights of an Andreev bound state by transient currents

Andreev bound states (ABSs) are localized quantum states that contain both electron and hole components. They ubiquitously reside in inhomogeneous superconducting systems. Following theoretical analysis, we propose to probe the electron-hole weights of an ABS via a local tunneling measurement that detects the transient current under a steplike pulse bias. With our protocol, the ABS energy level can also be obtained from peaks of the Fourier spectrum of the transient current. Our protocol can be applied to detect robust zero-energy Majorana bound states (MBSs), which have equal electron-hole weights, in candidate platforms where local tunneling spectroscopy measurement is possible. In the one-dimensional Majorana nanowire model, we numerically calculate the electron-hole weights for different types of low-energy bound states, including ABSs, quasi-MBSs, and MBSs.

Generation of J-shaped pulses in ultra-long Ytterbium doped mode locked fiber laser

We report, for the first time, generation of J-shaped pulses in figure-eight type ultra-long, all-normal dispersion Ytterbium-doped fiber laser mode locked by the combined effect of nonlinear polarization evolution and nonlinear optical loop mirror. Cavity birefringence based spectral filtering introduced by polarization controllers is used for the formation of J-shaped as well as step-like and flat-top pulses. The shape and duration of generated pulse is found to depend on applied birefringence as well as pump power. J-shaped pulses of 180 ns duration and 332 nJ pulse energy at 201.45 kHz fundamental repetition rate are generated at 200 mW pump power. Experimentally, a strong correlation between spectral and temporal profile of the generated pulse shapes is established where it has been shown that asymmetric spectral filtering is responsible for generation of J-shaped and step-like pulses. Numerically, the spectral transmission of figure-eight type resonator under different cavity birefringence and intra-cavity power is investigated which qualitatively supports the experimental results.

Rectangular Pulse Generation From a Mode Locked Raman Fiber Laser

Rectangular and step-like pulses are generated in a mode-locked Raman fiber laser with a nonlinear optical loop mirror. The Raman laser has a typical figure-8 configuration. Stable rectangular pulses with a fundamental repetition rate of 733 kHz are obtained. As the pump power increases, the Raman pulse evolves to a step-like rectangular pulse. The maximum pulse energy is 64.1 nJ with a pulsewidth of 25.1 ns. After spectrally resolved measurements, the step-like pulse is proved to be induced by the dual wavelength oscillation. It is the first report of rectangular pulsed output in the mode-locked Raman fiber laser to the best of our knowledge.

Precursor propagation in inhomogeneous broadened media: experimental and numerical simulation.

We report experimental results on the propagation temporal characteristics of the precursor in an inhomogeneous sample. The transient behavior of a step-like pulse in an atomic hot medium is two orders of magnitude faster than the radiative broadened case up to now presented in the literature. Moreover, we show the dependence on the resonant or nonresonant condition. Numerical simulations compare favorable to experimental results.

Fast algorithm for transient current through open quantum systems

Transient current calculation is essential to study the response time and capture the peak transient current for preventing meltdown of nanochips in nanoelectronics. Its calculation is known to be extremely time consuming with the best scaling $T{N}^{3}$ where $N$ is the dimension of the device and $T$ is the number of time steps. The dynamical response of the system is usually probed by sending a steplike pulse and monitoring its transient behavior. Here, we provide a fast algorithm to study the transient behavior due to the steplike pulse. This algorithm consists of two parts: algorithm I reduces the computational complexity to ${T}^{0}{N}^{3}$ for large systems as long as $T<N$; algorithm II employs the fast multipole technique and achieves scaling ${T}^{0}{N}^{3}$ whenever $T<{N}^{2}$ beyond which it becomes $T{log}_{2}N$ for even longer time. Hence it is of order $O(1)$ if $T<{N}^{2}$. Benchmark calculation has been done on graphene nanoribbons with $N={10}^{4}$ and $T={10}^{8}$. This algorithm allows us to tackle many large scale transient problems including magnetic tunneling junctions and ferroelectric tunneling junctions.

Topological Insulator-Assisted Dual-Wavelength Fiber Laser Delivering Versatile Pulse Patterns

We report on the generation of dual-wavelength mode-locked pulses in an erbium-doped fiber laser incorporating a topological insulator (TI): Bi2Se3/Polyvinyl Alcohol (PVA) film which could operate as both an excellent saturable absorber (SA) for mode-locking and a high-nonlinear medium for mitigating the mode competition of erbium-doped fiber and stabilizing the dual-wavelength oscillation. The TI: Bi2Se3/PVA film is prepared through a liquid-phase exfoliation/spin-coating method. Its saturating intensity and modulation depth are measured to be about 31.5 MW/cm2 and 3.4%, respectively. Taking advantage of the high nonlinearity and saturable absorption of the TI: Bi2Se3, three dual-wavelength pulse patterns, namely bright pulse, bright-dark soliton pair, and hybrid step-like and dark pulse, could be achieved under different lengths of single mode fiber by properly adjusting the pump power and the polarization state. For the bright pulse operation, we obtain its fundamental repetition rate of 9.75 MHz. For the bright-dark soliton pair operation, we find that it contains two distinct parts in the whole optical spectrum: one is at shorter wavelength corresponds to bright soliton and the other one is at longer wavelength corresponds to dual-wavelength dark soliton. In addition, we also achieved the harmonic mode-locking (HML) of the bright-dark soliton pair. The highest obtained repetition rate is about 433.8 MHz, which corresponds to the 280th harmonic of the fundamental repetition rate. For the hybrid step-like and dark pulse operation, we achieve its fundamental repetition rate of 8.805 MHz. Our findings suggest that TI: Bi2Se3 can operate as both an excellent SA for short pulse generation and a promising highly nonlinear photonic material for the related nonlinear optics applications.

All fiber tunable- or dual-wavelength Yb-doped fiber laser covering from dissipative soliton to dissipative soliton resonance

In this Letter, a simple and passively mode-locking Yb-doped all fiber laser using a nonlinear polarization rotation technique operating under dissipative soliton (DS) or dissipative soliton resonance (DSR) conditions is proposed. Furthermore, using a combination of a bandpass filter and a Loyt filter, tunable single-wavelength or dual-wavelength operation under two different conditions is realized, respectively. The tunable single-wavelength DS laser has a 5 nm tuning range from 1029 to 1034 nm with a pulse width of 110 ps. The tunable single-wavelength DSR operation laser has a range of 4 nm. In-depth research on the mechanism of the conversion between DS and DSR is carried out. Particularly, under dual-wavelength DSR operation, the obtained step-like pulses consist of two rectangular pulses with different energies. This work could help give a deeper insight into normal dispersion pulses.

Intracellular Calcium Alters IKs Amplitude and Kinetics in Rabbit Myocytes

The slowly activating delayed rectifier K+ current (IKs) contributes to repolarization of the cardiac action potential (AP). Intracellular Ca2+ ([Ca2+]i) can modulate IKs, but details of this potentially important regulation are largely unknown. Here, we aimed at assessing [Ca2+]i regulation of IKs and how it governs myocyte AP duration. IKs was recorded from freshly isolated rabbit ventricular myocytes using the patch clamp technique with intracellular pipette solutions that buffered [Ca2+]i to 0, 100, 300, and 500 nM. From a holding potential of −50 mV, IKs was recorded by implementing step-like pulses from −40 to 50 mV in 10 mV increments for 3 s, followed by a tail-pulse to −50 mV for 3 s. When the [Ca2+]i was increased to 300 nM, the maximally activated tail IKs (IMAX) was more than 2-fold greater compared to 0 nM [Ca2+]i (IMAX ∼0.8 pA/pF vs. 0.3 pA/pF). Importantly, when the pipette solution contained 500 nM [Ca2+]i (IMAX ∼1.0 pA/pF), IMAX was 3-fold greater than 0 nM and more than 2-fold greater than 100 nM [Ca2+]i (IMAX ∼0.4 pA/pF). The potential of half-maximal activation (V½) was not different for any situations (∼15 mV). However, deactivation kinetics of tail IKs were slower for cells recorded with 300 and 500 nM [Ca2+]i compared to 0 and 100 nM [Ca2+]i (τdeact ∼1200 ms vs. 800 ms). These results indicate that a rise in [Ca2+]i increases IKs amplitude in rabbits, without altering the voltage dependence of activation, and slows IKs deactivation. Computational modeling suggests these [Ca2+]i-dependent changes might contribute to ventricular AP duration alterations, especially in the presence of adrenergic activation.

Investigation of transient heat current from first principles using complex absorbing potential

We report on a first-principles investigation of transient heat current through molecular devices under steplike pulse of external and gate voltages. Using the nonequilibrium Green's function (NEGF) approach, an exact solution of transient heat current is obtained that goes beyond the wide-band limit. Combining with density-functional theory (DFT), we propose a time-dependent NEGF-DFT formalism to study the transient heat current under a steplike pulse for molecular devices from first principles. Anticipating the huge computational cost in the transient regime, we develop an algorithm to speed up the calculation using the complex absorbing potential (CAP). By adding the CAP to replace the Hamiltonian of leads, the effective self-energy of the Green's function becomes independent of energy, allowing analytic calculation of the triple integrations in the exact solution of transient heat current using the theorem of residue. With this linear scaling algorithm, the computational complexity is greatly reduced, and a first-principles calculation of transient heat current of molecular devices becomes possible. As an example, we apply our NEGF-DFT-CAP formalism for a molecular device, the Di-thiol benzene molecule connected by two semi-infinite aluminum leads, and we calculate the transient heat current under an upward gate voltage pulse. The enhancement of heat current is observed.

Observations of three types of pulses in an erbium-doped fiber laser by incorporating a graphene saturable absorber

We experimentally observed three types of pulses generated in an erbium-doped fiber laser by incorporating a homemade graphene saturable absorber (GSA). The generated pulses from the laser oscillator include dual-wavelength dark pulses, fundamentally step-like pulses, and non-soliton second-harmonic pulses. These operation regimes are first reported by using graphene as the saturable absorber. Our results will further indicate that the GSA can function well for obtaining various ultrafast pulse phenomena, highlighting the practical potential of graphene in ultrafast photonics technologies.

First-principles investigation of transient current in molecular devices by using complex absorbing potentials

Based on the non-equilibrium Green's function (NEGF) coupled with density function theory (DFT), namely, NEGF-DFT quantum transport theory, we propose an efficient formalism to calculate the transient current of molecular devices under a step-like pulse from first principles. By combining NEGF-DFT with the complex absorbing potential (CAP), the computational complexity of our formalism (NEGF-DFT-CAP) is proportional to $\emph{O}(N)$ where $N$ is the number of time steps in the time-dependent transient calculation. Compared with state-of-the-art algorithm of first principles time-dependent calculation that scales with at least $N^2$, this order N technique drastically reduces the computational burden making it possible to tackle realistic molecular devices. To ensure the accuracy of our method, we carry out the benchmark calculation compared with exact NEGF-TDDFT formalism and they agree well with each other. As an illustration, we investigate the transient current of molecular device Al-C$_3$-Al from first principles.

Influence of the Coulomb interaction on the spin-polarized current in the quantum dot system in the presence of the bias voltagepulse

We consider the influence of the intra-dot Coulomb interaction U on the electron transport through the quantum dot coupled with non-magnetic leads. The transient spin-dependent current generated by a pulsed bias voltage applied across the quantum dot system is calculated using the equation of motion method for appropriate correlation functions. We find that the coherent quantum beats in both total current and spin current observed for vanishing U are strongly suppressed by non-zero interaction in the case of a single step-like voltage pulse applied to the system. It is found that for a pulsed bias voltage the Coulomb interaction can lead to much stronger spin polarized current occurring in some time windows. The possibility of the pure spin current together with the vanishing charge current is also discussed for the quantum dot coupled with threeleads.

First-principles investigation of transient dynamics of molecular devices

Based on the nonequilibrium Green's function (NEGF) and time-dependent density-functional theory (TDDFT), we propose a formalism to study the time-dependent transport behavior of molecular devices from first principles. While this approach is equivalent to the time-dependent wave-function approach within TDDFT, it has the advantage that the scattering states and bound states are treated on equal footing. Furthermore, it is much easier to implement our approach numerically. Different from the time-dependent wave-function $[\ensuremath{\psi}(t,E)$] approach, our formalism is in the time space [${\mathbf{G}}^{r}(t,{t}^{\ensuremath{'}})$], making this method superior in the time-dependent transport problem with many subbands in the transverse direction. For the purpose of numerical implementation on molecular devices, a computational tractable numerical scheme is discussed in detail. We have applied our formalism to calculate the transient current of two molecular devices Al-1,4-dimethylbenzene-Al and Al-benzene-Al from first principles. In the calculation, we have gone beyond the wideband limit and used the adiabatic local density approximation that was used within TDDFT. It is known that when the wideband limit is abandoned, the boundary condition of the transport problem is non-Markovian, resulting in a memory term in the effective Hamiltonian of the scattering region. To overcome the computational complexity due to the memory term, we have employed a fast algorithm to speed up the calculation and reduced the CPU time from the scaling ${N}^{3}$ to ${N}^{2}{\mathrm{log}}_{2}^{2}(N)$ for the steplike pulse, where $N$ is the number of time steps in the time evolution of the Green's function. To ensure the accuracy of our method, we have done a benchmark transient calculation on an atomic junction using a time-dependent wave-function approach within TDDFT in momentum space, which agrees very well with the result from our method.

Transient heat generation in a quantum dot under a step-like pulse bias

We study the transient heat generation in a quantum dot system driven by a step-like or a square-shaped pulse bias. We find that a periodically oscillating heat generation arises after adding the sudden bias. One particularly surprising result is that there exists a heat absorption from the zero-temperature phonon subsystem. Thus the phonon population in non-equilibrium can be less than that of the equilibrium electron–phonon system. In addition, we also ascertain the optimal conditions for the operation of a quantum dot with the minimum heat generation.

Temperature Rise of Copper and HTSC Tapes in Liquid Nitrogen by a Step-wise Current Pulse

Abstract Features of a heat transfer to liquid nitrogen from copper or superconducting tapes loaded by a short step-like current pulse have been studied experimentally. The delay of a heat transfer development have been observed and studied. A phenomenological model enabling us to describe qualitatively the non-stationary heat transfer process on the basis of the stationary boiling curve is proposed.

Ultrafast stimulated emission due to quasidirect transitions in silicon nanocrystals

We report on a detailed study of ultrafast stimulated emission arising from efficient quasidirect transitions in silicon nanocrystals under femtosecond pumping. In nanometer-sized silicon nanocrystals, the quasidirect transitions are rapidly cut off (on subpicosecond time scale) owing to surface trapping of carriers. Consequently, also the core-related radiative recombination undergoes ultrafast decay. We propose, theoretically describe, and experimentally demonstrate an extension of the commonly used variable stripe length (VSL) method, which enables us to measure transient stimulated emission even on the subpicosecond time scale. The extension consist in introducing a step-like or gradual pump pulse delay along the stripe length. By applying the extended VSL method in combination with the shifting excitation spot technique, we reveal the presence of room-temperature transient optical gain in silicon nanocrystals (lifetime $l$$1\phantom{\rule{0.222222em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{ps}$) at wavelengths $\ensuremath{\sim}$$590\phantom{\rule{0.222222em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ with peak values of the order of $100\phantom{\rule{0.222222em}{0ex}}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Finally, on the basis of our results we discuss possible ways of obtaining a laser source based on silicon nanocrystals.