Pump Saturation Research Articles

Quantifying bacterial efflux within subcellular domains of Pseudomonas aeruginosa.

Molecular efflux is a mechanism through which bacteria actively expel undesirable substances. This is a crucial line of defense against toxic chemicals in harsh environments. Understanding how efflux works is critical for designing antimicrobial strategies. Though much is already known about efflux proteins, important details about the mechanisms of efflux (e.g., importance of specific subcellular domains and ejection rates) have yet to be experimentally quantified. Herein, we use the nonlinear optical technique, second harmonic light scattering, to simultaneously measure the efflux rates from the periplasm and cytosol of a Gram-negative bacterium. The influence of efflux on the uptake kinetics of a mild antimicrobial, malachite green (MG), by Pseudomonas aeruginosa was quantified. It is observed that efflux primarily occurs from the periplasm and is two orders of magnitude faster than from the cytosol. Efflux pumps activate to maintain MG concentrations in the periplasm below 1 µM, while efflux from the cytosol maintains MG concentration below 0.1 µM. Efflux pumps are shown to saturate when exogenous MG concentrations are greater than 25 µM, while the cytosol efflux function saturates at >15 µM. Finally, efflux pumps can simultaneously eject different compounds, as proven by experiments with both MG and hexane, a known effluxable compound.IMPORTANCEMolecular efflux pumps are a crucial defense mechanism that protects bacteria from an otherwise unchecked influx of toxic molecules present in the extracellular environment. The efflux functions constitute a significant hindrance to antimicrobial efficacy. While much is now known regarding the structure of these channels, knowledge of the influence of efflux in individual subcellular domains and the associated ejection rates is still lacking. Using the nonlinear optical technique, second-harmonic light scattering, we have measured the threshold concentrations for pump activation, saturation concentrations, and efflux rates from both the periplasm and cytosol in living Gram-negative bacteria. The quantified efflux data in the different subcellular compartments not only provide a clear mechanistic understanding but also are critical for developing antimicrobial strategies.

Optical cooling of a Yb-doped alumino-phosphosilicate fiber in air by -250 mK.

Recent progress in the fabrication of Yb-doped silicate fibers with low concentration quenching and low background absorption loss has led to the demonstration of anti-Stokes-fluorescence cooling in several aluminosilicate compositions. This breakthrough is critical to combat deleterious thermal effects due to the quantum defect in fiber lasers and amplifiers. Since cooling efficiencies remain low (1-2.7%), it is paramount to engineer compositions that improve this metric. We report a silica fiber with a core glass heavily doped with aluminum and phosphorus that sets, to our knowledge, a few new records. This few-mode fiber (16-µm core) was cooled in air by -0.25 K from room temperature with ∼0.5 W of 1040-nm power. The measured cooling efficiency is 3.3% at low pump power and 2.8% at the power that produced maximum cooling. The critical quenching concentration inferred from the measured dependence of cooling on pump power and careful calibration of the pump absorption and saturation is 79 wt.%. The inferred background absorption loss is 15 dB/km. Together with the fiber's average Yb concentration of 4.2 wt.%, these metrics rank among the best reported in a silica glass.

Experimental study on activating bismuth active centers in bismuth/erbium co-doped optical fiber by ionizing radiations

Post-treatments by ionizing radiations have been proposed previously for regulating the properties of bismuth/erbium co-doped optical fiber (BEDF). In this work, the effects of common ionizing radiations (i.e. gamma ray, electron beam and mixed γ/neutron) on Bismuth Active Centers (BACs) in BEDF have systematically been investigated. Our experimental results show obvious BAC activation by ionizing radiation – evident from the increase in saturable absorption at 830 nm that is closely associated with the BAC responsible for the desirable near infrared (NIR) emissions. This radiation induced BAC activation has been confirmed from the emission of BAC associate with silica (BAC-Si). The saturation pump power of the BAC-Si emission in all irradiated samples is found higher than that in pristine (or non-irradiated) samples. In addition, the effect on emission by different radiation is found in the order of gamma ray > mixed gamma/neutron > electron beam. On the other side, we have also observed significant radiation induced unsaturable loss that can affect the pumping efficiency and Figure of Merit (FOM).

In situ effective pumping speed measurements as a tool for non-evaporable getter activation and saturation monitoring

We have developed method of in situ effective pumping speed measurements based on injecting finite gas pulse into a vacuum volume and subsequent recording of pressure response using a Residual Gas Analyzer (RGA). The pressure burst caused by injected gas pulse falls exponentially in time and the exponential coefficient of such pressure decay is proportional to total effective pumping speed of all vacuum pumps acting on vacuum chamber volume. The effective pumping speed can be extracted from the dependence of injected gas species pressure vs time recorded by an RGA. Non-Evaporable Getters (NEGs) are widely used in multiple ultra and extremely high vacuum devices and applications during the last several decades. Areas of NEG applications were recently expanded into vacuum devices with relatively high-pressure levels (up to 10−7 Torr) by the invention of high capacity ZAO NEG which can be operated at elevated temperatures. The intrinsic property of all NEG-based vacuum pumps is the reduction of their pumping speed, called “NEG saturation”, after an extended period of operation. Typical time interval for such process can span from about a month and up to a few years depending on NEG type, vacuum level of NEG pump operation, and residual gas content. After that, the NEG pump will require a re-activation cycle to restore its pumping speed close to the initial value. For many applications it is difficult to realize what is the current pumping speed of NEG pump is and when the pump should be re-activated. A method for in situ effective pumping speed measurements developed in this study can be used as a tool for NEG-based pump activation and saturation monitoring. Application of such an approach to monitor pumping status of NEG pumps was utilized and will be continually used at Extended EBIS which was recently developed and commissioned for Relativistic Heavy Ion Collider (RHIC) and future Electron Ion Collider (EIC). The results obtained during commissioning are presented and discussed.

Maser polarization through anisotropic pumping

Context. Polarized emission from masers is an excellent tool to study magnetic fields in maser sources. The linear polarization of the majority of masers is understood as an interplay of maser saturation and anisotropic pumping. However, for the latter mechanism, no quantitative modeling has been presented yet. Aims. We aim to construct a comprehensive model of maser polarization, including quantitative modeling of both anisotropic pumping and the effects of maser saturation on the polarization of masers. Methods. We extended regular (isotropic) maser excitation modeling with a dimension that describes the molecular population alignments, as well as including the linear polarization dimension to the radiative transfer. The results of the excitation analysis yielded the anisotropic pumping and decay parameters, which were subsequently used in one-dimensional proper maser polarization radiative transfer modeling. Results. We present the anisotropic pumping parameters for a variety of transitions from class I CH3OH masers, H2O masers, and SiO masers. SiO masers are highly anisotropically pumped due to them occurring in the vicinity of a late-type star, which irradiates the maser region with a strong directional radiation field. Class I CH3OH masers and H2O masers occur in association with shocks, and they are modestly anisotropically pumped due to the anisotropy of the excitation region. Conclusions. Our modeling constitutes the first quantitative constraints on the anisotropic pumping of masers. We find that anisotropic pumping can explain the high polarization yields of SiO masers, as well as the modest polarization of unsaturated class I CH3OH masers. The common 22 GHz H2O maser has a relatively weak anisotropic pumping; in contrast, we predict that the 183 GHz H2O maser is strongly anisotropically pumped. Finally, we outline a mechanism through which non-Zeeman circular polarization is produced, when the magnetic field changes direction along the propagation through an anisotropically pumped maser.

Polarization-dependent intensity ratios in double resonance spectroscopy.

Double Resonance is a powerful spectroscopic method that unambiguously assigns the rigorous quantum numbers of one state of a transition. However, there is often ambiguity as to the branch (ΔJ) of that transition. Spectroscopists have resolved this ambiguity by using the dependence of the double resonance intensity on the relative polarization directions of pump and probe radiation. However, published theoretical predictions for this ratio are based upon a weak (i.e., non-saturating) field approximation. This paper presents theoretical predictions for these intensity ratios for cases where the pump field is strongly saturating in the two limits of transitions dominated by homogeneous or of inhomogeneous broadening. Saturation reduces but does not eliminate the magnitude of the polarization effect (driving the intensity ratio closer to unity) even with strong pump saturation. For the case of an inhomogeneously broadened line, such as when Doppler broadened linewidth dominates over the power-broadened homogeneous line width, a large fraction of the low pump power polarization anisotropy remains. This paper reports predicted polarization ratios for both linear and circular pump and probe field polarizations. The present predictions are compared with experimental measurements on CH4 ground state → ν3 → 3ν3 transitions recently reported by de Oliveira et al.63 and these are in better agreement than with the weak field predictions.

Persistent dynamics in coupled non-degenerate parametric oscillators: pump saturation prevents mode competition.

The coherent dynamics in networks of coupled oscillators is of great interest in wave-physics since the coupling produces various dynamical effects, such as coherent energy exchange (beats) between the oscillators. However, it is common wisdom that these coherent dynamics are transients that quickly decay in active oscillators (e.g. lasers) since pump saturation causes mode competition that results, for homogeneous gain, in the prevalence of the single winning mode. We observe that pump saturation in coupled parametric oscillators counter-intuitively encourages the multi-mode dynamics of beating and indefinitely preserves it, despite the existence of mode competition. We explore in detail the coherent dynamics of a pair of coupled parametric oscillators with a shared pump and arbitrary coupling in a radio frequency (RF) experiment, as well as in simulation. Specifically, we realize two parametric oscillators as different frequency-modes of a single RF cavity and couple them arbitrarily using a digital high-bandwidth FPGA. We observe persistent coherent beats that are maintained at any pump level, even high above the threshold. The simulation highlights how the interplay of pump depletion between the two oscillators prevents them from synchronizing, even when the oscillation is deeply saturated.

Triply-resonant sum frequency conversion with gallium phosphide ring resonators.

We demonstrate quasi-phase matched, triply-resonant sum frequency conversion in 10.6-µm-diameter integrated gallium phosphide ring resonators. A small-signal, waveguide-to-waveguide power conversion efficiency of 8 ± 1.1%/mW; is measured for conversion from telecom (1536 nm) and near infrared (1117 nm) to visible (647 nm) wavelengths with an absolute power conversion efficiency of 6.3 ± 0.6%; measured at saturation pump power. For the complementary difference frequency generation process, a single photon conversion efficiency of 7.2%/mW from visible to telecom is projected for resonators with optimized coupling. Efficient conversion from visible to telecom will facilitate long-distance transmission of spin-entangled photons from solid-state emitters such as the diamond NV center, allowing long-distance entanglement for quantum networks.

Coherence Recovery in a Hybrid Partially Mode Locked III-V-on-Silicon Laser

We analyse the spectral properties of an optical frequency comb emitted by a III-V-on-silicon ring laser operated in the hybrid mode-locking regime. We demonstrate that the mode-locked laser has disparate responses to an external RF modulation depending on the saturable absorber bias and pump current. Firstly, we show that, when the mode-locking is fully achieved, the hybrid operation maintains the full mode-locking characteristics across the whole locking region. Secondly, starting from an incomplete passive mode-locking regime, and using the modulation frequency as a control parameter, we demonstrate the gradual improvement of the mode-locking characteristics within the locking tongue. This effect is maximized at the RF locking tongue boundary and manifests in an increase of the spectral width and a diminution of the modal linewidth dispersion.

The influence of geotextile on the characteristics of railway subgrade mud pumping under cyclic loading

The mud pumping beneath railways is both widespread and harmful and seriously affects the stability of railroad tracks and their operational safety. This paper presents a series of mud pumping tests, and analyses the influence of geotextile on the characteristics of saturated subgrade mud pumping. Our results show that the addition of geotextile between subgrade and gravel layers can significantly reduce accumulated axial strain by 40% to 76%, reduce residual excess pore water pressure by 33% to 92%, and reduce soil moisture content at the interface between the subgrade and gravel layers. However, we find that geotextile has no significant effect on soil dynamic stress. This suggests that the geotextile can act as a separator between the subgrade and gravel layers thus significantly reducing the thickness of an interlayer. Additionally, geotextile can also act as a filter and reduce the displacement caused by the migration of fine particles. In general, placing geotextile between the subgrade and gravel layers can mitigate railway subgrade mud pumping under cyclic loading, but with an increase in cyclic loading amplitude, the mitigating effectiveness of geotextile on mud pumping gradually weakens.

Nd:YAG end-pumped zigzag multi-pass slab amplifier optimization: numerical and experimental study regarding the saturation effects.

A numerical modeling of Nd:YAG end-pumped zigzag multi-pass slab amplifier is developed. Spatial dependent rate equations have been solved taking into consideration the pump and laser beam saturation effects with regard to overlap correction of the signal path through gain medium. Based on our results, efficient amplifier structure with optimized output power could be designed with respect to the change in the number of signal beam-pass, doping concentration and slab length. In order to verify our modeling an experimental setup is designed.

Modeling of a high-power continuous wave Yb:YAG planar waveguide amplifier under thermal steady-state.

The performance of a novel, to the best of our knowledge, high-power Yb:YAG planar waveguide amplifier under thermal steady-state is studied numerically. We present a 3D thermal steady-state amplification model, which considers the effects of temperature on the Boltzmann occupation factors, pump absorption, and laser emission cross sections, in addition to reabsorption and pump saturation of Yb:YAG. The performance of a multipass-pumped planar waveguide amplifier under thermal steady-state was simulated considering the loss in the claddings, and a comparison with the simulated results under the ideal state was carried out. Our analysis results show that the core's temperature rise and the claddings' pump absorption lead to decreased pump absorption and optical-optical efficiencies. However, a high pump absorption efficiency (96%) and optical-optical efficiency (61.2%) are achieved when the pump power is 10 kW. Meanwhile, a lower fracture and minor thermal distortion can be expected in reality.

Saturation and optical pumping effects in the fluorescence that follows the excitation of the D2 transition in atomic rubidium

In this article we study saturation effects in the fluorescence spectra at the D2 transition in 87Rb excited by two counter-propagating laser beams of the same frequency. The effects appear as dips in the Doppler broadened fluorescence emission lines at the atomic transitions and in all but one of the cross-over resonances. The 5SF=1→5P3/2F=0, 1 cross-over presents a strong dependence in the laser intensity. For low intensities it appears as a weak dip, but at higher laser intensities an increase in fluorescence is clearly present. A rate equation calculation which includes the velocity profile is used to interpret the experimental data. For the 5SF=2→5P3/2F=1, 2 and 3 manifold, a five level model that takes into account the hyperfine structure of both initial and final states is in good agreement with the experiment. For the 5SF=1→5P3/2F=0, 1 and 2 manifold, it is necessary to include the magnetic sublevel structure of all hyperfine levels involved. The calculation also shows that the onset of this magnetic pumping effect occurs near the 5S→5P3/2 saturation intensity. In both cases the comparison between theory and experiment allowed a direct determination of the ratio of the laser intensity to the atomic saturation intensity.

Spectroscopic properties of Yb3+-doped fluorophosphate glasses with low phosphate content

A series of new fluorophosphate glasses of 5Ba(PO3)2–10BaF2–18.5CaF2–10MgF2–18.5SrF2–(38–x)AlF3–xYbF3 (x = 0.5, 1, 2, 5, 8, 10) composition was synthesized. Experimental characterization, including absorption, luminescence, and luminescence kinetics measurements, was performed. Based on experimental data, the optimal concentration of Yb3+ ions, providing the highest gain, was established. To characterize potential laser performance of the studied glasses several parameters, including pump saturation, minimal pump intensity, and minimal fraction of excited ions needed to balance gain and absorption, were calculated. Dependence of gain cross section on the excited state population was determined to characterize amplifying properties of the glasses.

Experimental investigation of short pulse Raman amplification with backward pumping.

We report an experimental study on the backward-pumped Raman amplification of short pulses into a 20.3 km long optical fiber. We demonstrate that the gain and the pump saturation depend on the pulse duration. We also reveal that for short enough pulses, the amplification process remains linear, and very high peak powers, even larger than the Raman pump, are achievable. Numerical simulations reproduce the experimental results with excellent agreement.

Q-switched erbium-doped fiber laser based on nanodiamond saturable absorber

In this paper, we investigate nanodiamond (ND) particles as saturable absorber (SA) to produce Q-switched pulses in Erbium doped fiber laser (EDFL). The solid nanocomposite film is prepared by the drop-casting method where ND is embedded into polyvinylidene fluoride-trifluoroethylene (PVDF-TRFE) as a host polymer. Both physical and optical properties of the prepared film are experimentally demonstrated. To analyze the structure of fabricated ND-PVDF-TRFE film, various measurement methods are adopted. The ND-SA is positioned in the ring cavity of the EDFL to explore their abilities for generating stable Q-switched pulses. The results show that the fabricated ND-SA can generate stable laser pulses. Furthermore, the ND composite film proves that 10% of ND could be presented as an excellent promising material for various photonic applications. The Q-switched EDFL with 10% ND-SA began to self-start of stable Q-switch pulse at a saturated pump power of 59 mW with a 77.84 kHz repetition rate and pulse width of 4.34 μs. Besides, at a pump power of 187 mW, the Q-switched EDFL generates very stable pulses with a wavelength of 1558 nm, a repetition rate of 127.2 kHz, pulse duration of 1.565 μs, and an output power of 0.447 mW.

Highly-sensitive Refractometer Based on a D-shaped Fiber Bragg Grating Integrated into A Loop-mirror Optical Fiber Laser

In this paper, we report a new type of refractometer based on a D-shaped fiber Bragg grating (FBG) integrated in a loop-mirror optical fiber laser. This proposed sensor is used in wavelength interrogation method, in which the D-shaped FBG is applied as a refractive index (RI) sensing probe and a mirror to select mode of laser. The D-shaped FBG is prepared by the removal of a portion of the fiber cladding covering the FBG by means of side-polishing technique. The D-shaped FBG sensing probe integrated in a loop-mirror optical fiber laser with saturated pump technique, the characteristics of sensing signals have been improved to obtain stable intensity, narrower bandwidth and higher optical signal-to-noise ratio compare to normal reflection configuration. The limit of detection (LOD) of this sensor can be achieved to 2.95 x 10-4 RIU in the refractive index (RI) range of 1.42-1.44. Accordingly, we believe that the proposed refractometer has a huge potential for applications in biochemical-sensing technique.

Maximizing Performance of Quantum Cascade Laser-Pumped Molecular Lasers

Quantum cascade laser (QCL)-pumped molecular lasers (QPMLs) have recently been introduced as a new source of powerful (>1 mW), tunable (>1 THz), narrow-band (<10 kHz), continuous-wave terahertz radiation. The performance of these lasers depends critically on molecular collision physics, pump saturation, and on the design of the laser cavity. Using a validated three-level model that captures the essential collision and saturation behaviors of the QPML gas nitrous oxide (N2O),we explore how threshold pump power and output terahertz power depend on pump power, gas pressure, as well as on the diameter, length, and output-coupler transmissivity of a cylindrical cavity.The analysis indicates that maximum power occurs as pump saturation is minimized in a manner that depends much more sensitively on pressure than on cell diameter, length, or transmissivity. A near-optimal compact laser cavity can produce more than 10 mW of power tunable over frequencies above 1 THz when pumped by a multi-watt QCL.

Parametrically resonant surface plasmon polaritons

We present the theory of parametrically resonant surface plasmon polaritons (SPPs). We show that a temporal modulation of the dielectric properties of the medium adjacent to a metallic surface can lead to efficient energy injection into the SPP modes supported at the interface. When the permittivity modulation is induced by a pump field exceeding a certain threshold intensity, such a field undergoes a reverse saturable absorption process. We introduce a time-domain formalism to account for pump saturation and depletion effects. Finally, we discuss the viability of these effects for optical limiting applications.

Characteristics of Pumping Current in a YBCO Coil by a Pulse-Type Magnetic Flux Pump

2G high-temperature superconducting (HTS) wires and YBCO-coated conductors perform better carrying current capability, which is potentially applied to the manufacture of HTS magnets. This article presents the experimental results of the pumping current for YBCO coils using a pulse-type magnetic flux pump in the conduction-cooling system and liquid nitrogen bath (LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) cryogenic environment. The optimization of the flux pump used in the conduction-cooling system is that a constantan heater was added to keep the temperature of the pumping bridge at a certain value. The excitation effects of the YBCO coil at different temperatures were investigated in the conduction-cooling system. A fast increase of pumping current in the YBCO coil occurs when the temperature of the YBCO sheet (i.e., pumping bridge) is in the range of 50-80 K. The relationships among the saturated pumping current and input voltage, working frequency, numbers of magnetic poles were also studied. Using the seven-pole configuration, the saturated current can reach 155 A when the frequency is 20 Hz and the voltage is 6 V. The excitation characteristics of the flux pump in the LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> cooling system show the possibility of the pulse-type magnetic flux pump for the practical application of HTS magnets.