Pump Strength Research Articles

Parametric Oscillation, Frequency Mixing, and Injection Locking of Strongly Coupled Nanomechanical Resonator Modes

We study locking phenomena of two strongly coupled, high quality factor nanomechanical resonator modes to a common parametric drive at a single drive frequency in different parametric driving regimes. By controlled dielectric gradient forces we tune the resonance frequencies of the flexural in-plane and out-of-plane oscillation of the high stress silicon nitride string through their mutual avoided crossing. For the case of the strong common parametric drive signal-idler generation via nondegenerate parametric two-mode oscillation is observed. Broadband frequency tuning of the very narrow linewidth signal and idler resonances is demonstrated. When the resonance frequencies of the signal and idler get closer to each other, partial injection locking, injection pulling, and complete injection locking to half of the drive frequency occurs depending on the pump strength. Furthermore, satellite resonances, symmetrically offset from the signal and idler by their beat note, are observed, which can be attributed to degenerate four-wave mixing in the highly nonlinear mechanical oscillations.

Versatile soliton emission from a [formula omitted] mode-locked fiber laser

Recently, few-layer tungsten disulfide (WS2), as a shining 2D material, has been discovered to possess both the saturable absorption ability and large nonlinear refractive index. Here, we demonstrate versatile soliton pulses in a passively mode-locked fiber laser with a WS2-deposited microfiber. The few-layer WS2 is prepared by the liquid-phase exfoliation method and transferred onto a microfiber by the optical deposition method. Study found, the WS2-deposited microfiber can operate simultaneously as a mode-locker and a high-nonlinear device. In experiment, by further inserting the WS2 device into the fiber laser, besides the dual-wavelength soliton, noise-like soliton pulse, conventional soliton and its harmonic form are obtained by properly adjusting the pump strength and the polarization states. For the dual-wavelength soliton pulses and noise-like pulse, the maximum output power of 14.2 mW and pulse energy of 4.74 nJ is obtained, respectively. In addition, we also achieve the maximum harmonic number (135) of conventional soliton, corresponding to a repetition rate of ∼497.5 MHz. Our study shows clearly that WS2-deposited microfiber can be as a high-nonlinear photonic device for studying a plenty of nonlinear soliton phenomena.

Spatiotemporal evolution of continuous-wave field and dark soliton formation in a microcavity with normal dispersion**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFF0200702), the National Natural Science Foundation of China (Grant Nos. 61690222 and 11573058), and the CAS-SAFEA International Partnership Program for Creative Research Teams.

Stable dark soliton and dark pulse formation in normally dispersive and red-detuned microcavities are investigated by numerically solving the normalized Lugiato-Lefever equation. The soliton essence is proved by fitting the calculated field intensity profile with the analytical formula of a dark soliton. Meanwhile, we find that a dark soliton can be generated either from the nonlinear evolution of an optical shock wave or narrowing of a locally broad dark pulse with smoother fronts. Explicit analytical expression is obtained to describe the oscillatory fronts of the optical shock wave. Furthermore, from the calculation results, we show that for smaller frequency detunings, e.g., , in addition to the dark soliton formation, a single dark pulse with an oscillatory dip can also arise and propagate stably in the microcavity under proper pump detuning and pump strength combination. The existence region together with various field intensity profiles and the corresponding spectra of single dark pulse are demonstrated.

Optimum gain for plasmonic distributed feedback lasers

Although nanolasers typically have low $Q$ factors and high lasing thresholds, they have been successfully implemented with various gain media. Intuitively, it seems that an increase in the gain coefficient would improve the characteristics of nanolasers. For a plasmonic nanolaser---in particular, a distributed feedback laser---we propose a self-consistent model that takes into account both spontaneous emission and the multimode character of laser generation to show that for a given pumping strength, the gain coefficient has an optimal value at which the radiation intensity is at a maximum and the radiation linewidth is at a minimum.

Pulse dynamics in all-normal dispersion ultrafast fiber lasers

We present a numerical investigation of pulse shape evolution and pulse regime transformation in an all-normal dispersion mode-locked fiber laser. We found that the pump strength and spectral filtering had great effects on the properties of the generated dissipative solitons. The pulse had a parabolic intensity profile and linear chirp when the pump strength was large enough under certain bandwidth of the spectral filter. Such a parabolic pulse worked in the intermediate regime between the dissipative soliton and similariton; however, the pulse resembled a dissipative soliton. Pulse regime transformation will happen when the pump strength is large enough; the pulse will become multi-pulse, bound-state pulse, or noise-like pulse under different filter bandwidths and pump strengths. The results of our numerical simulations could offer a better understanding of the dynamics of all-normal dispersion mode-locked fiber lasers and also provide insight into the dissipative fiber laser systems.

Quantum charge pumping through resonant crossed Andreev reflection in a superconducting hybrid junction of silicene

We theoretically investigate the phenomena of adiabatic quantum charge pumping through a normal-insulator-superconductor-insulator-normal (NISIN) setup of silicene within the scattering matrix formalism. Assuming thin barrier limit, we consider the strength of the two barriers ($\chi_{1}$ and $\chi_{2}$) as the two pumping parameters in the adiabatic regime. Within this geometry, we obtain crossed Andreev reflection (CAR) with probability unity in the $\chi_{1}$-$\chi_{2}$ plane without concomitant transmission or elastic cotunneling (CT). Tunability of the band gap at the Dirac point by applying an external electric field perpendicular to the silicene sheet and variation of the chemical potential at the normal silicene region, open up the possibility of achieving perfect either CAR or transmission process through our setup. This resonant behavior is periodic with the barrier strengths. We analyze the behavior of the pumped charge through the NISIN structure as a function of the pumping strength and angles of the incident electrons. We show that large ($Q\sim2e$) pumped charge can be obtained through our geometry when the pumping contour encloses either the CAR or transmission resonance in the pumping parameter space. We discuss possible experimental feasibility of our theoretical predictions.

Numerical simulations of fast-axis instability of vector solitons in mode-locked fiber lasers.

We demonstrate the fast-axis instability in mode-locked fiber lasers numerically for the first time. We find that the energy of the fast mode will be transferred to the slow mode when the strong pump strength makes the soliton period short. A nearly linearly polarized vector soliton along the slow-axis could be generated under certain cavity parameters. The final polarization of the vector soliton is related to the initial polarization of the seed pulse. Two regimes of energy exchanging between the slow mode and the fast mode are explored and the direction of the energy flow between two modes depends on the phase difference. The dip-type sidebands are found to be intrinsic characteristics of the mode-locked fiber lasers under high pulse energy.

Bifurcation Analysis of the Yamada Model for a Pulsing Semiconductor Laser with Saturable Absorber and Delayed Optical Feedback

Semiconductor lasers exhibit a wealth of dynamics, from emission of a constant beam of light to periodic oscillations and excitability. Self-pulsing regimes, where the laser periodically releases a short pulse of light, are particularly interesting for many applications, from material science to telecommunications. Self-pulsing regimes need to produce pulses very regularly and, as such, they are also known to be particularly sensitive to perturbations, such as noise or light injection. We investigate the effect of delayed optical feedback on the dynamics of a self-pulsing semiconductor laser with saturable absorber (SLSA). More precisely, we consider the Yamada model with delay---a system of three delay-differential equations (DDEs) for two slow and one fast variables---which has been shown to reproduce accurately self-pulsing features as observed in SLSA experimentally. This model is also of broader interest because it is quite closely related to mathematical models of other self-pulsing systems, such as excitable spiking neurons. We perform a numerical bifurcation analysis of the Yamada model with delay, where we consider the feedback delay, the feedback strength, and the strength of pumping as bifurcation parameters. We find a rapidly increasing complexity of the system dynamics when the feedback delay is increased from zero. In particular, there are new feedback-induced dynamics: stable quasi-periodic oscillations on tori, as well as a large degree of multistability, with up to five pulse-like stable periodic solutions with different amplitudes and repetition rates. An attractor map in the plane of perturbations on the gain and intensity reveals a Cantor-set--like, intermingled structure of the different basins of attraction. This suggests that, in practice, the multistable laser is extremely sensitive to small perturbations.

Optimal pumping strength for BBM92 key distribution protocol

In a typical, entanglement-based quantum key distribution (QKD) protocols the maximally entangled, singlet states are used. The standard way to generate them is to use parametric down conversion with low intensity laser beams. This way the probability to produce two or more entangled pairs is negligible. However, one obtains it at a cost of a large likelihood of producing no particles at all. In this work, we ask the question if this approach is the optimal one from the point of view of key generation rate. For a case study, we take BBM 92 protocol and find the optimal intensity of the beam. We show optimal intensities are much higher than the ones typically used. Our results can be generalized to other protocols based on singlet states.

A Biophysical Model for Cytotoxic Cell Swelling.

Cytotoxic edema most commonly results from energy shortage, such as in cerebral ischemia, and refers to the swelling of brain cells due to the entry of water from the extracellular space. We show that the principle of electroneutrality explains why chloride influx is essential for the development of cytotoxic edema. With the help of a biophysical model of a single neuron, we show that a tipping point of the energy supply exists, below which the cell volume rapidly increases. We simulate realistic time courses to and reveal critical components of neuronal swelling in conditions of low energy supply. Furthermore, we show that, after transient blockade of the energy supply, cytotoxic edema may be reversed by temporary blockade of Na+ channels.

Effects of intense optical phonon pumping on the structure and electronic properties of yttrium barium copper oxide

We investigate the structural modulations induced by optical excitation of a polar phonon mode in YBa$_2$Cu$_3$O$_7$, using first-principles calculations based on density functional theory. We focus on the intense-excitation regime in which we expect that fourth-order phonon-phonon coupling terms dominate, and model the structural modulations induced by pulses of such intensity. Our calculations of the phonon-phonon anharmonicities confirm that the cubic coupling between modes, shown in earlier work to cause a quasi-static change in the apical O - Cu distance and a buckling of the CuO$_2$ planes, is the leading contribution at moderate pump strengths. At higher pump strengths ($\sim$10 MV/cm) the previously neglected quartic couplings become relevant and produce an additional shearing of the CuO$_2$ planes. Finally, we analyze the changes in the electronic and magnetic properties associated with the induced structural changes.

Collectively induced many-vortices topology via rotatory Dicke quantum phase transition

We examine the superradiance of a Bose–Einstein condensate pumped with a Laguerre–Gaussian laser of high winding number, e.g., . The laser beam transfers its orbital angular momentum (OAM) to the condensate at once due to the collectivity of the superradiance. An ℓ-fold rotational symmetric structure emerges with the rotatory superradiance. ℓ number of single-charge vortices appear at the arms of this structure. Even though the pump and the condensate profiles initially have cylindrical symmetry, we observe that it is broken to ℓ-fold rotational symmetry at the superradiance. Breaking of the cylindrical symmetry into the ℓ-fold symmetry and OAM transfer to the condensate become significant after the same critical pump strength. Reorganization of the condensate resembles the ordering in the experiment by Esslinger and colleagues (2010 Nature 264 1301). We numerically verify that the critical point for the onset of the reorganization, as well as the properties of the emitted pulse, conform to the characteristics of superradiant quantum phase transition.

Ultrahigh-brightness, spectrally-flat, short-wave infrared supercontinuum source for long-range atmospheric applications.

Fiber based supercontinuum (SC) sources with output spectra covering the infrared atmospheric window are very useful in long-range atmospheric applications. It is proven that silica fibers can support the generation of broadband SC sources ranging from the visible to the short-wave infrared region. In this paper, we present the generation of an ultrahigh-brightness spectrally-flat 2-2.5 μm SC source in a cladding pumped thulium-doped fiber amplifier (TDFA) numerically and experimentally. The underlying physical mechanisms behind the SC generation process are investigated firstly with a numerical model which includes the fiber gain and loss, the dispersive and nonlinear effects. Simulation results show that abundant soliton pulses are generated in the TDFA, and they are shifted towards the long wavelength side very quickly with the nonlinearity of Raman soliton self-frequency shift (SSFS), and eventually the Raman SSFS process is halted due to the silica fiber's infrared loss. A spectrally-flat 2-2.5 μm SC source could be generated as the result of the spectral superposition of these abundant soliton pulses. These simulation results correspond qualitatively well to the following experimental results. Then, in the experiment, a cladding pumped large-mode-area TDFA is built for pursuing a high-power 2-2.5 μm SC source. By enhancing the pump strength, the output SC spectrum broadens to the long wavelength side gradually. At the highest pump power, the obtained SC source has a maximum average power of 203.4 W with a power conversion efficiency of 38.7%. It has a 3 dB spectral bandwidth of 545 nm ranging from 1990 to 2535 nm, indicating a power spectral density in excess of 370 mW/nm. Meanwhile, the output SC source has a good beam profile. This SC source, to the best of our knowledge, is the brightest spectrally-flat 2-2.5 μm light source ever reported. It will be highly desirable in a lot of long-range atmospheric applications, such as broad-spectrum LIDAR, free space communication and hyper-spectral imaging.

Rate-Equation Analysis for a Coupled-Cavity Laser with MMI Anti-Phase Coupler

Rate equations are derived for the coupled-cavity laser with a multimode-interference coupler. A strategy and scheme is indicated for iterative self-consistent numerical solution of the steady-state equations. The presence of the linewidth-enhancement parameter is explicitly taken into account. Locking in stable single-mode anti-phase operation is numerically demonstrated and locking ranges are given. Numerical results are given for the output power and the operation frequency as functions of the pump strengths of the individual lasers. The shapes of output-intensity curves agree well with measured curves.

Rate-Equation Analysis for a Coupled-Cavity Laser with MMI Anti-Phase Coupler

Rate equations are derived for the coupled-cavity laser with a multimode-interference coupler. A strategy and scheme is indicated for iterative self-consistent numerical solution of the steady-state equations. The presence of the linewidth-enhancement parameter is explicitly taken into account. Locking in stable single-mode anti-phase operation is numerically demonstrated and locking ranges are given. Numerical results are given for the output power and the operation frequency as functions of the pump strengths of the individual lasers. The shapes of output-intensity curves agree well with measured curves.

Direct generation of dip-type sidebands from WS_2 mode-locked fiber laser

In this paper, we demonstrated the direct generation of dip-type sidebands from a passively mode-locked fiber laser based on the evanescent field interaction of two-dimensional material WS2 and microfiber. The WS2 nanosheets in polyvinyl alcohol solution are prepared through the liquid-phase exfoliation method and transferred onto a tapered fiber by the optical deposition method. The WS2-deposited microfiber device could both operate as a mode-locker for soliton generation and induce a strong polarization effect to constitute an artificial polarizer for dip-type sidebands formation. In experiment, six kinds of dip-sidebands, namely, dip-dip type, hybrid peak-dip and dip type (І, ІІ), dual-peak-dip type (І, ІІ), and nearly-flat-top type are achieved by properly adjusting the pump strength and the polarization state in the laser cavity. Study found that the dynamic evolution of the dip-type sidebands along with the pump power is not reversible. In addition, we also obtained the maximum depth of ~16.8 dB and ~22 dB for dual-peak-dip type І and dual-peak-dip type ІІ, respectively, and their second-order forms. Our study shows clearly that WS2-assisted microfiber device can be as a mode-locker for soliton generation and a polarizer for studying nonlinear optics phenomenon in the fiber lasers simultaneously.

Neuroprotective Role of Gap Junctions in a Neuron Astrocyte Network Model

A detailed biophysical model for a neuron/astrocyte network is developed to explore mechanisms responsible for the initiation and propagation of cortical spreading depolarizations and the role of astrocytes in maintaining ion homeostasis, thereby preventing these pathological waves. Simulations of the model illustrate how properties of spreading depolarizations, such as wave speed and duration of depolarization, depend on several factors, including the neuron and astrocyte Na+-K+ ATPase pump strengths. In particular, we consider the neuroprotective role of astrocyte gap junction coupling. The model demonstrates that a syncytium of electrically coupled astrocytes can maintain a physiological membrane potential in the presence of an elevated extracellular K+ concentration and efficiently distribute the excess K+ across the syncytium. This provides an effective neuroprotective mechanism for delaying or preventing the initiation of spreading depolarizations.

Ultra-broadband nonlinear saturable absorption of high-yield MoS2 nanosheets

High-yield MoS2 nanosheets with strong nonlinear optical (NLO) responses in a broad near-infrared range were synthesized by a facile hydrothermal method. The observation of saturable absorption, which was excited by the light with photon energy smaller than the gap energy of MoS2, can be attributed to the enhancement of the hybridization between the Mo d-orbital and S p-orbital by the oxygen incorporation into MoS2. High-yield MoS2 nanosheets with high modulation depth and large saturable intensity generated a stable, passively Q-switched fiber laser pulse at 1.56 μm. The high output power of 1.08 mW can be attained under a very low pump power of 30.87 mW. Compared to recently reported passively Q-switched fiber lasers utilizing exfoliated MoS2 nanosheets, the efficiency of the laser for our passive Q-switching operation is larger and reaches 3.50%. This research may extend the understanding on the NLO properties of MoS2 and indicate the feasibility of the high-yield MoS2 nanosheets to passively Q-switched fiber laser effectively at low pump strengths.

Watt-Level Ultrafast Fiber Laser Based on Weak Evanescent Interaction With Reduced Graphene Oxide

We propose a Watt-level, all-fiber, ultrafast Er/Yb-codoped double-clad fiber laser mode-locked by reduced graphene oxide (rGO) interacting with photonic crystal fiber (PCF) ultraweakly. The rGO solution is filled into the cladding holes of PCF, and after evaporation, the rGO flakes, contributing saturable absorption and high nonlinearity, interact with a light intensity of only 1/ $10^{7}$ of that in the center of PCF. This ultraweakly interacting structure can enhance the thermal damage threshold and decrease the accumulated nonlinearity, which is proved to be an excellent choice for high-power mode-locked laser. By incorporating the saturable absorber into an Er/Yb-codoped fiber ring cavity, stable soliton laser with femtosecond duration is generated, and harmonic mode-locking is formed when increasing pump strength. The maximum average power of 1.14 W is obtained. To the best of our knowledge, this report is the highest power obtained directly from an all-fiber, graphene-mode-locked laser system operating with fundamental transverse mode. In addition, the experimental results demonstrate that our proposed structure is effective in generating high-power pulses when some other materials are used, including but not limited to graphene oxide, carbon nanotubes, topological insulator, or metal nanoparticles.

The influence of potassium concentration on epileptic seizures in a coupled neuronal model in the hippocampus.

Experiments on hippocampal slices have recorded that a novel pattern of epileptic seizures with alternating excitatory and inhibitory activities in the CA1 region can be induced by an elevated potassium ion (K(+)) concentration in the extracellular space between neurons and astrocytes (ECS-NA). To explore the intrinsic effects of the factors (such as glial K(+) uptake, Na(+)-K(+)-ATPase, the K(+) concentration of the bath solution, and K(+) lateral diffusion) influencing K(+) concentration in the ECS-NA on the epileptic seizures recorded in previous experiments, we present a coupled model composed of excitatory and inhibitory neurons and glia in the CA1 region. Bifurcation diagrams showing the glial K(+) uptake strength with either the Na(+)-K(+)-ATPase pump strength or the bath solution K(+) concentration are obtained for neural epileptic seizures. The K(+) lateral diffusion leads to epileptic seizure in neurons only when the synaptic conductance values of the excitatory and inhibitory neurons are within an appropriate range. Finally, we propose an energy factor to measure the metabolic demand during neuron firing, and the results show that different energy demands for the normal discharges and the pathological epileptic seizures of the coupled neurons.