Carrier Reservoir Research Articles

Mycobacterium tuberculosis Phosphoenolpyruvate Carboxykinase Is Regulated by Redox Mechanisms and Interaction with Thioredoxin

Tuberculosis remains a major health concern worldwide. Eradication of its causative agent, the bacterial pathogen Mycobacterium tuberculosis, is particularly challenging due to a vast reservoir of latent carriers of the disease. Despite the misleading terminology of a so-called dormant state associated with latent infections, the bacteria have to maintain basic metabolic activities. Hypoxic conditions have been widely used as an in vitro system to study this dormancy. Such studies identified a rearrangement of central carbon metabolism to exploit fermentative processes caused by the lack of oxygen. Phosphoenolpyruvate carboxykinase (Pck; EC 4.1.1.32) is the enzyme at the center of these metabolic rearrangements. Although Pck is associated with gluconeogenesis under standard growth conditions, the enzyme can catalyze the reverse reaction, supporting anaplerosis of the tricarboxylic acid cycle, under conditions leading to slowed or stopped bacterial replication. To study the mechanisms that regulate the switch between two Pck functions, we systematically investigated factors influencing the gluconeogenic and anaplerotic reaction kinetics. We demonstrate that a reducing environment, as found under hypoxia-triggered non-replicating conditions, accelerates the reaction in the anaplerotic direction. Furthermore, we identified proteins that interact with Pck. The interaction between Pck and the reduced form of mycobacterial thioredoxin, gene expression of which is increased under hypoxic conditions, also increased the Pck anaplerotic activity. We thus propose that a reducing environment and the protein-protein interaction with thioredoxin in particular enable the Pck anaplerotic function under fermentative growth conditions.

Carrier dynamics in inhomogeneously broadened InAs/AlGaInAs/InP quantum-dot semiconductor optical amplifiers

We report on a characterization of fundamental gain dynamics in recently developed InAs/InP quantum-dot semiconductor optical amplifiers. Multi-wavelength pump-probe measurements were used to determine gain recovery rates, following a powerful optical pump pulse, at various wavelengths for different bias levels and pump excitation powers. The recovery was dominated by coupling between the electronic states in the quantum-dots and the high energy carrier reservoir via capture and escape mechanisms. These processes determine also the wavelength dependencies of gain saturation depth and the asymptotic gain recovery level. Unlike quantum-dash amplifiers, these quantum-dots exhibit no instantaneous gain response, confirming their quasi zero-dimensional nature.

Influence of interactions with noncondensed particles on the coherence of a one-dimensional polariton condensate

One-dimensional polariton condensates (PoCos) in a photonic wire are generated through non-resonant laser excitation, by which also a reservoir of background carriers is created. Interaction with this reservoir may affect the coherence of the PoCo, which is studied here by injecting a condensate locally and monitoring the coherence along the wire. While the incoherent reservoir is mostly present within the excitation laser spot, the condensate can propagate ballistically through the wire. Photon correlation measurements show that far from the laser spot the second order correlation function approaches unity value, as expected for the coherent condensed state. When approaching the spot, however, the correlation function increases up to values of 1.2 showing the addition of noise to the emission due to interaction with the reservoir. This finding is substantiated by measuring the first order coherence by a double slit experiment, which shows a reduced visibility of interference at the excitation laser spot.

Optimizing InAs/InP (113) B quantum dot lasers with considering mutual effects of coverage factor and cavity length on two-state lasing

Optimizing InAs/InP (113) B quantum dot lasers with considering mutual effects of coverage factor and cavity length on two-state lasing

Gate dielectric degradation: Pre-existing vs. generated defects

We consider the possibility that degradation of the electrical characteristics of high-k gate stacks under low voltage stresses of practical interest is caused primarily by activation of pre-existing defects rather than generation of new ones. In nFETs in inversion, in particular, defect activation is suggested to be associated with the capture of an injected electron: in this charged state, defects can participate in a fast exchange of charge carriers with the carrier reservoir (substrate or gate electrode) that constitutes the physical process underlying a variety of electrical measurements. The degradation caused by the activation of pre-existing defects, as opposed to that of new defect generation, is both reversible and exhibits a tendency to saturate through the duration of stress. By using the multi-phonon assisted charge transport description, it is demonstrated that the trap activation concept allows reproducing a variety of experimental results including stress time dependency of the threshold voltage, leakage current, charge pumping current, and low frequency noise. Continuous, long-term degradation described by the power law time dependency is shown to be determined by the activation of defects located in the interfacial SiO2 layer of the high-k gate stacks. The findings of this study can direct process optimization efforts towards reduction of as-grown precursors of the charge trapping defects as the major factor affecting reliability.

Influence of the carrier reservoir dimensionality on electron-electron scattering in quantum dot materials

Influence of the carrier reservoir dimensionality on electron-electron scattering in quantum dot materials

Positron Annihilation Study of Fe- and Zn-Doped SmBa2Cu3O7−δ Systems

Polycrystalline SmBa2Cu3−x(Fe, Zn)xO7−δ (x=0.0–0.4) samples are prepared by the traditional solid-state reaction technique. To make clear how the different substitution sites and magnetism of doped ions influence the physical mechanism of SBCO superconductivity, we investigated systemically polycrystalline samples SmBa2Cu3−x(Fe, Zn)xO7−δ by means of X-ray diffraction, the test of superconductivity and Positron Annihilation Spectroscopy (PAS). The results show that Fe doping induces the localization of carriers (electron and hole) in the Cu–O chains and weakens the function of carrier reservoir, while Zn doping makes the electronic localization in the CuO2 planes and increases the electronic density in SmBa2Cu3O7−δ systems. Compared with the Fe doping, Zn doping suppresses the superconductivity stronger. Combining the results of the electronic density ne and the transition temperature Tc with Fe and Zn doping, we can arrive at the conclusion that the localized carriers in Cu–O chains and CuO2 planes have enormous influence on superconductivity by affecting the paring and the charge transfer between the reservoir layers and the CuO2 planes. In addition, the present results indicate that Tc depression has no direct correlation with the magnetism of Fe and Zn ions.

Four-wave mixing in quantum dot semiconductor optical amplifiers

The theory of four-wave mixing (FWM) in the quantum dot (QD) semiconductor optical amplifiers (SOAs) is discussed by combining the QD rate equations system, the quantum-mechanical density-matrix theory, and the pulse propagation in QD SOAs including the three region of QD structure ground state (GS), excited state (ES), and wetting layer. Also, relations for differential gain, gain integral, and nonlinear susceptibility of both pump, probe, and signal pulses were discussed. Gain and differential gain have been calculated for QD structure. FWM efficiency and its components [spectral hole burning (SHB), carrier heating, and carrier density pulsation] are calculated. It is found that inclusion of ES in the formulas and in the calculations is essential since it works as a carrier reservoir for GS. It is found that QD SOA with enough capture time from ES to GS will reduce the SHB component, and so it is suitable for telecommunication applications that require symmetric conversion and independent detuning.

Collective light emission revisited: reservoir induced coherence.

We study the collective emission of a few emitters (up to three) in a cavity. In addition to the radiation coupling responsible for sub- and superradiance, we investigate emitters additionally coupled through a joint carrier reservoir. For such emitters, typically embedded in a solid state environment, the carrier reservoir provides a continuous pumping mechanism for the steady state emission. We show that the statistical properties of the emitted light depend strongly on the interaction between the emitters and the reservoir. Unexpectedly, the presence of the reservoir enhances the coherence of the emitted light already for a few emitters. This results from the fact that the carrier reservoir introduces new many-body correlations to the electronic transition and in this way suppresses multiphoton processes.

Sub-Nanosecond Phase Recovery in Quantum-Dot Semiconductor Optical Amplifiers

In this paper, optimal design of a pumping scheme for achieving the best gain and phase response in quntum-dot semiconductor optical ampli ers is investigated. For the rst time, the dynamic response of the quntum-dot semiconductor optical ampli er is evaluated under three di erent pumping schemes, known as optical pumping, electrical pumping, and electro-optical pumping. Simulation results show that the shortest gain recovery time in quntum-dot semiconductor optical ampli ers can be achieved under the electrical pumping scheme. However, under the optical pumping and electro-optical pumping schemes, the quntum-dot semiconductor optical ampli er represents a shorter phase recovery compared to the electrical pumping scheme. We found that a sub-nanosecond phase recovery in the quntum-dot semiconductor optical ampli er can be achieved under the OP and electro-optical pumping schemes, which can never be achieved under the electrical pumping, because of the slow carrier dynamics of the carrier reservoir. Also, it was found that the gain recovery process in an optically pumped quntum-dot semiconductor optical ampli er can be signi cantly accelerated at cryogenic temperatures. This result demonstrates the superiorities of the optical pumping scheme over the electrical pumping and electro-optical pumping schemes at low temperatures, where both the gain and phase recovery times of an optically pumped quantum-dot semiconductor optical ampli er are drastically decreased.

Influence of carrier lifetimes on the dynamical behavior of quantum-dot lasers subject to optical feedback

We examine changes in the dynamics of a semiconductor quantum-dot (QD) laser subject to optical feedback that correlate to changes in the QD laser band structure. By employing a microscopic model for the carrier-carrier scattering processes between the QDs and the carrier reservoir we are able to tune the carrier lifetimes in the QDs, e.g., by modifying the QD confinement energies or the pump current. By using numerical continuation methods as well as asymptotic theory we demonstrate that the feedback sensitivity crucially depends on these lifetimes through the damping of the turn-on oscillations, and small lifetimes on the order of this relaxation time scale lead to an increased feedback resistivity. Thus intelligent band structure engineering can lead to stable continuous wave operation of the laser over a large parameter range.

Acceleration of carrier recovery in a quantum well semiconductor optical amplifier due to the tunneling effect

In this paper, we theoretically demonstrate that carrier recovery can be accelerated through the tunneling effect in a novel (to our knowledge) quantum well (QW) semiconductor optical amplifier. In the active region, we design the repeated element, including a shallow QW and a following deep QW. Through numerical calculation, we find this novel structure is helpful for improving the dynamic characteristics. In the single element, the shallow QW acts as a perfect carrier reservoir, while the deep QW acts as a “real” active region. Gain recovery time is shortened significantly.

Theory of direct capture from two‐and three‐dimensional reservoirs to quantum dot states

AbstractWe present a comparison of Coulomb scattering rates from a two‐dimensional carrier reservoir/three‐dimensional carrier reservoir and quantum dot states. The scattering rates are microscopically determined in the limit of Born‐Markov approximation.The calculations highlight that, within the considered parameter range the scattering from a two‐dimensional carrier reservoir is more efficient than from a three‐dimensional carrier reservoir. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical investigation on gain recovery dynamics in step quantum well semiconductor optical amplifiers

In this paper, gain recovery dynamic characteristics in the step quantum well (QW) semiconductor optical amplifier (SOA) are theoretically investigated via a detailed model. We numerically solve the coupled rate equations including microscopically calculated carrier-phonon scattering rates between the carrier reservoir and the ground state on the basis of Fermi’s golden rule. The carrier-phonon scattering rates are given as functions of the width and height of the step in the QW of the SOA. It is demonstrated that the electron scattering rate in the step QW SOA depends on the potential parameters of the carrier reservoir region. Finally, it is shown that the SOA with a larger transition rate has a shorter carrier recovery time than the other SOAs via analyzing and comparing the gain and phase recovery dynamics in different types of SOA samples.

Phase Recovery Acceleration in Quantum-Dot Semiconductor Optical Amplifiers

In this paper, the acceleration of phase recovery in quantum-dot semiconductor optical amplifiers (QD-SOAs) is investigated by employing optical pumping (OP) scheme. For this purpose, the state space theory has been used to derive a dynamic model for the QD-SOA. The derived nonlinear state space model is employed to simulate the gain and phase responses of the device. For the first time, we show that OP can realize a shorter phase recovery time in comparison with electrical pumping (EP) scheme under equal pumping powers. We found that under OP, the contribution of slow phase recovery component induced by the slow carrier dynamics of the carrier reservoir is drastically reduced, and consequently, a fast phase response becomes feasible. Also, we found that under EP, the gain is recovered within a shorter time compared to OP scheme.

Voltage modulated electro-luminescence spectroscopy to understand negative capacitance and the role of sub-bandgap states in light emitting devices

Voltage modulated electroluminescence spectra and low frequency (≤100 kHz) impedance characteristics of red electroluminescent diodes under forward bias are investigated. Light emission under periodic voltage modulation tracks the onset of observed negative capacitance for each modulation frequency. Active participation of sub-bandgap defects including the shallower states in minority carrier recombination dynamics is sought to explain the results. The phenomenon of negative capacitance is understood as a necessary dielectric response to compensate any irreversible transient changes in the injected minority carrier reservoir due to radiative recombinations mediated by slowly responding sub-bandgap defects. Experimentally measured variations of the in-phase component of modulated electroluminescence spectra with forward bias levels and with modulation frequencies support the dynamic influence of these sub-bandgap states in the radiative recombination process. Predominant negative sign of the in-phase component of voltage modulated electroluminescence signal further confirms the bi-molecular nature of light emission. Effect of these states on the net density of minority carriers available for radiative recombination is discussed. These sub-bandgap states can even supress the external quantum efficiency of such devices under high frequency operation commonly used in optical communication.

A PCR screen for malaria carrier infections using human saliva samples

With endemic countries now aiming for elimination, the detection of malaria infections, with or without symptoms, is increasingly important for monitoring and evaluation programmes. Current malaria screening methods necessitate blood withdrawal. This invasive approach is constrained, especially for identifying the asymptomatic carrier reservoir, since segments of communities with blood taboos avoid participating. Proof of concept has previously been shown for molecular detection of malaria infection using human saliva samples. The current study optimized saliva-based malaria detection in an area of southern Zambia. Saliva pellet fractions proved a more reliable source of amplifiable parasite DNA compared to the soluble fraction. After optimizing DNA extraction and amplification, saliva-based PCR showed 94.1% sensitivity and 97% specificity, using nested PCR on blood samples as gold standard. This study demonstrates that saliva samples are a reliable non-invasive alternative to blood for the PCR detection of asymptomatic and submicroscopic malaria reservoirs.

Impact of carrier-carrier scattering and carrier heating on pulse train dynamics of quantum dot semiconductor optical amplifiers

We investigate the impact of carrier-carrier scattering on the gain recovery dynamics of a quantum dot (QD) semiconductor optical amplifier. Simulations, based on semiconductor Bloch equations with microscopically calculated Coulomb scattering rates between the carrier reservoir and the QDs, show a very good agreement with experimentally obtained pump-probe dynamics over a range of injection currents. With the microscopically obtained scattering rates at hand, we can conclude that fast cascading relaxation processes between the two-dimensional carrier reservoir and the QDs in combination with carrier heating enhancing the scattering efficiency drives the ultrafast gain recovery observed in QD based semiconductor devices.

80 Gb/s wavelength conversion using a quantum-dot semiconductor optical amplifier and optical filtering

Wavelength conversion of 40 Gb/s and 80 Gb/s return-to-zero on-off-keying signals using a quantum-dot semiconductor optical amplifier in combination with a delay interferometer as subsequent filter is demonstrated. The performance of the 80 Gb/s wavelength converter measured in terms of the bit-error ratio demonstrated here is the highest reported up to now for quantum-dot semiconductor optical amplifiers. The typical fast gain dynamics manifests itself in open eye diagrams of the converted signal. The slow phase dynamics of the carrier reservoir however induces severe patterning and requires compensation. Adaptation of the free-spectral range of the delay interferometer is necessary in order to mitigate these phase effects and to achieve error-free wavelength conversion.

Large-Signal Response of Semiconductor Quantum-Dot Lasers

In this paper, the large-signal response of a quantum-dot laser is investigated. Based on experimental results, we show that including a dynamic device temperature as well as Auger recombination processes in the carrier reservoir is crucial to model the dynamic response of a quantum-dot laser for large variations of the pump current. A detailed analysis of the influence of temperature effects on the dynamics of the device is performed. Simulated eye diagrams are presented and compared with experimental results at the emission wavelength of 1.3 μm.