Increased Threshold Currents Research Articles

Optothermal control of spin Hall nano-oscillators

We investigate the impact of localized laser heating on the auto-oscillation properties of a 170 nm wide nano-constriction spin Hall nano-oscillators (SHNOs) fabricated from a NiFe/Pt bilayer on a sapphire substrate. A 532 nm continuous wave laser is focused down to a spot size of about 500 nm at a power ranging from 0 to 12 mW. Through a comparison with resistive heating, we estimate a local temperature rise of about 8 K/mW. We demonstrate reversible laser tuning of the threshold current, the frequency, and the peak power and find that the SHNO frequency can be tuned by up to 350 MHz, which is over three times more than the current tuning alone. Increasing the temperature also results in increased signal jitter, an increased threshold current, and a reduced maximum current for auto-oscillations. Our results open up for optical control of single SHNOs in larger SHNO networks without the need for additional voltage gates.

Study on the Increased Threshold Current in the Development of 220-GHz Folded Waveguide Backward-Wave Oscillator

Backward-wave oscillator (BWO) is a promising candidate as a millimeter-wave and submillimeter-wave (MMW/SMMW) power source and has a compact configuration, moderate output power, and frequency-tuning abilities. The physical conditions for increasing the threshold current of a typical MMW/SMMW-BWO are theoretically studied in this paper. We demonstrate the greater effect of axial phase velocity variation of deformed slow-wave structures on the threshold current in a 220-GHz folded waveguide (FW) BWO in comparison with other insensitive factors. For further verification, a prototype tube of FW BWO is developed. This tube exhibited increased threshold current during experiments. This finding is consistent with the error analysis proposed by our theory.

Efficient High-Power Laser Diodes

High-power broad-area diode lasers are the most efficient light sources, with 90-μm stripe GaAs-based 940-980 nm single emitters delivering > 10 W optical output at a power conversion efficiency η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> (10 W) > 65%. A review of efforts to increase η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> is presented here and we show that for well-optimized structures, the residual losses are dominated by the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> -side waveguide and nonideal internal quantum efficiency η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> . The challenge in measuring efficiency to sufficient precision is also discussed. We show that η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> can most directly be improved using low heat sink temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">HS</sub> with η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> (10 W) reaching > 70% at <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">HS</sub> = -50 °C. In contrast, increases in η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> at T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">HS</sub> = 25 °C require improvements in both material quality and design, with growth studies targeting increased η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> and reduced threshold current and design studies seeking to mitigate the impact of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> -side waveguide. “Extreme, double asymmetric” (EDAS) designs are shown to substantially reduce <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> -side losses, at the penalty of increased threshold current. The benefit of EDAS designs is shown here using diode lasers with 30-μm stripes, (in development as high beam quality sources for material processing). Efficiency increases of ~ 10% relative to conventional designs are demonstrated at high powers.

Nociceptin/Orphanin FQ Modulates Motor Behavior and Primary Motor Cortex Output Through Receptors Located in Substantia Nigra Reticulata

This study was set to investigate whether motor effects of nociceptin/orphanin FQ (N/OFQ) can be related to changes in primary motor cortex output. N/OFQ injected i.c.v. biphasically modulated motor performance, low doses being facilitating and higher ones inhibitory. These effects were counteracted by the N/OFQ receptor antagonist [Nphe(1) Arg(14),Lys(15)]N/OFQ-NH(2) (UFP-101) confirming the specificity of N/OFQ action. However, UFP-101 alone facilitated motor performance, suggesting that endogenous N/OFQ inhibits motor function. N/OFQ and UFP-101 injected into the substantia nigra reticulata but not motor cortex replicated these effects, suggesting motor responses were mediated by subcortical circuits involving the basal ganglia. Intracortical microstimulation technique showed that i.c.v. N/OFQ also biphasically modulated motor cortex excitability and movement representation. Low N/OFQ doses caused a leftward shift of threshold distribution curve in the forelimb area without affecting the number of effective sites. Conversely, high N/OFQ doses increased unresponsive and reduced excitable (movement) sites in vibrissa but not forelimb area. However, increased threshold currents and rightward shift of threshold distribution curve were observed in both areas, suggesting an overall inhibitory effect on cortical motor output. UFP-101 alone evoked effects similar to low N/OFQ doses, suggesting tonic inhibitory control over forelimb movement by endogenous N/OFQ. As shown in behavioral experiments, these effects were replicated by intranigral, but not intracortical, N/OFQ or UFP-101 injections. We conclude that N/OFQ receptors located in the substantia nigra reticulata mediate N/OFQ biphasic control over motor behavior, possibly through changes of primary motor cortex output.

Behavioral and Physiological Characterization of Sensorimotor Gating in the Goldfish Startle Response

Prepulse inhibition (PPI) is typically associated with an attenuation of auditory startle behavior in mammals and is presumably mediated within the brainstem startle circuit. However, the inhibitory mechanisms underlying PPI are not yet clear. We addressed this question with complementary behavioral and in vivo electrophysiological experiments in the startle escape circuit of goldfish, the Mauthner cell (M-cell) system. In the behavioral experiments we observed a 77.5% attenuation (PPI) of startle escape probability following auditory prepulse-pulse stimulation. The PPI effect was observed for prepulse-pulse interstimulus intervals (ISIs) ranging from 20 to 600 ms and its magnitude depended linearly on prepulse intensity over a range of 14 dB. Electrophysiological recordings of synaptic responses to a sound pulse in the M-cell, which is the sensorimotor neuron initiating startle escapes, showed a 21% reduction in amplitude of the dendritic postsynaptic potential (PSP) and a 23% reduction of the somatic PSP following a prepulse. In addition, a prepulse evoked a long-lasting (500 ms) decrease in M-cell excitability indicated by 1) an increased threshold current, 2) an inhibitory shunt of the action potential (AP), and 3) by a linearized M-cell membrane, which effectively impedes M-cell AP generation. Comparing the magnitude and kinetics of inhibitory shunts evoked by a prepulse in the M-cell dendrite and soma revealed a disproportionately larger and longer-lasting inhibition in the dendrite. These results suggest that the observed PPI-type attenuation of startle behavior can be correlated to distinct postsynaptic mechanisms mediated primarily at the M-cell lateral dendrite.

Orienting and defensive behaviors elicited by superior colliculus stimulation in rats: effects of 5-HT depletion, uptake inhibition, and direct midbrain or frontal cortex application

Electrical or chemical stimulation of the superior colliculus (SC) in rats produces orienting and defensive responses. Defensive behaviors are modulated by serotonin (5-hydroxytryptamine, 5-HT), and serotonergic fibers provide a dense innervation of the SC. Here, we examined the role of 5-HT in modulating the behavioral responses of rats elicited by electrical SC stimulation. Low-intensity (107±12 μA) stimulation of the SC elicited orienting head movements, while higher intensities (204±20 μA) produced running and jumping responses. Treatment with the 5-HT depletor p-chlorophenylalanine (300 mg/kg/day×3, i.p.) lowered current thresholds to elicit orienting and running by 40 and 21%, respectively. Conversely, concurrent administration of the 5-HT uptake inhibitor fluoxetine (10 mg/kg, i.p.) and the 5-HT 1A receptor antagonist WAY 100635 (0.5 mg/kg, s.c.) increased threshold currents to produce head and running movements by 41 and 18%, respectively. We investigated the anatomical substrate of this inhibitory effect of 5-HT with intracerebral 5-HT application by means of reverse microdialysis. Application of 5-HT (1–50 mM) into the midbrain immediately adjacent to the SC stimulation electrode resulted in a pronounced (approximately four-fold for 50 mM 5-HT) dose- and time-dependent increase in stimulation thresholds to elicit head movements. Application of 5-HT into the frontal cortex (up to 100 mM) had no significant effect on SC-evoked behavioral responses. These results show that 5-HT exerts an inhibitory influence over orienting and defensive behaviors initiated in the mammalian SC. It appears that this inhibitory effect is mediated, to a large extent, by a direct action of 5-HT at the level of the midbrain.

Modelling the effects of electric fields on nerve fibres: Influence of the myelin sheath

The excitation and conduction properties of computer-based cable models of mammalian motor nerve fibres, incorporating three different myelin representations, are compared. The three myelin representations are a perfectly insulating single cable (model A), a finite impedance single cable (model B) and a finite impedance double cable (model C). Extracellular stimulation of the three models is used to study their strength-duration and current-distance (I-X) relationships, conduction velocity (CV) and action potential shape. All three models have a chronaxie time that is within the experimental range. Models B and C have increased threshold currents compared with model A, but each model has slope to the I-X relationship that matches experimental results. Model B has a CV that matches experimental data, whereas the CV of models A and C are above and below the experimental range, respectively. Model C is able to produce a depolarising afterpotential (DAP), whereas models A and B exhibit hyperpolarising afterpotentials. Models A and B are determined to be the preferred models when low-frequency stimulation (< approximately 25 Hz) is used, owing to their efficiency and accurate excitation and conduction properties. For high frequency stimulation (approximately 25 Hz and greater), model C, with its ability to produce a DAP, is necessary accurately to simulate excitation behaviour.

Control of dopant distribution profiles in GaInP laser diodes with Mg- and Se-doped AlInP cladding layers

High doping levels on either side of a double heterostructure laser are required for optimal performance and temperature stability, especially in the AlGaInP material system. However, in most cases these are accompanied by strong diffusion during growth, shifting the p–n junction and damaging the active zone. We investigated the possibiltity of suppressing the diffusion towards the active zone by introducing thin intermediate layers of different materials and doping levels as well as short-period superlattices. Using SIMS measurements we found that the non indium-containing layers GaP and GaAs are suitable to slow down diffusion of the magnesium on the p-side and produce a rather steep dopant front. On the other hand, the presence of the additional interfaces seems to have a negative influence on the electrical properties of the device, resulting in increased threshold currents and reduced quantum efficiencies. To determine the decisive factors for this behavior, additional optical measurements were done on the samples. In another approach we investigated n-doped AlInP intermediate layers and their influence on the magnesium profile.

Beryllium diffusion in short-period Al xGa 1 − xAs/AlAs-superlattices and vertically compact laser structures grown by molecular beam epitaxy

Dopant diffusion is investigated by depth profiling with secondary ion mass spectrometry (SIMS) in heterostructures containing Be-doped short-period superlattice Al x Ga 1 − x As layers (0.3 ⩽x⩽0.8) grown by molecular beam epitaxy. At a Be concentration of 2×10 18 cm − 3 we observe Be outdiffusion into the undoped GaAs layers only at a substrate temperature of 660 °C. At a doping concentration of 2×10 19 cm − 3 a strong increase in diffusion occurs for all growth temperatures. Depth profiles show solubility limits for Be of 1×10 19 cm − 3 at x=0.6 and 2×10 18 cm − 3 at x=0.8 in Al x Ga 1 − x As layers. Lasers were fabricated with different p-cladding thickness of 450 and 600 nm. An increased threshold current is observed for the structure with the thinner cladding. SIMS depth profiles show that the amount of diffused Be into the active region was about two times higher in the sample with the thin cladding layer. Therefore, we attribute the increase of the threshold current to the enhanced Be diffusion from the part of the p-cladding layer with highest doping concentration and from the heavily-doped graded layer since in both layers the solubility limits were exceeded.

Doping-type dependence of turn-on delay time in 1.3-μm InGaAsP-InP modulation-doped strained quantum-well lasers

The dependence of turn-on delay time on doping type in 1.3-/spl mu/m InGaAsP-InP modulation-doped (MD) strained quantum-well (QW) lasers is theoretically investigated, based on the detailed band structure model including the band mixing effects. It is found that the turn-on delay time in n-type MD lasers is reduced to 1/4 that of undoped lasers due to both a lower threshold current and a reduced carrier lifetime. The reduction of the delay time is smaller in p-type MD lasers, however, because of the increased threshold current. These results show that the n-type MD-QW lasers are superior for high-speed modulation under zero-bias conditions.

Quinacainol, a new antiarrhythmic with Class I antiarrhythmic actions in the rat

The antiarrhythmic and electrophysiological actions of quinacainol, a new Class I antiarrhythmic, were assessed in rats. Electrophysiological actions of quinacainol were assessed in vivo in terms of drug-induced changes in ECG, responses to left ventricular electrical stimulation, and changes in epicardial intracellular potentials to precisely characterize the electrophysiological effects of this putative subclass Ic antiarrhythmic compound. Antiarrhythmic actions were assessed in conscious rats subjected to occlusion of the LAD coronary artery. Antiarrhythmic actions occurred with 2.0 and 4.0 mg/kg, whereas 8.0 mg/kg was pro-arrhythmic. At doses of 0.5 mg/kg and above quinacainol increased threshold currents and for ventricular fibrillation. Doses of 2.0 mg/kg and above increased ventricular refractoriness. From 1.0 to 8.0 mg/kg quinacainol reduced dV/dt max of phase 0 of epicardial action potentials but only 8.0 mg/kg increased action potential duration. The Q-T interval was also increased with the highest dose. Quinacainol dose-relatedly increased P-R interval whereas QRS did not change. Thus the Class I electrophysiological properties of quinacainol over the dose range tested did not fit accurately into a single subclass of the various subclasses of Class I. However, the Class Ic actions seen with 2.0 and 4.0 mg/kg were associated with antiarrhythmic actions.

Mexiletine's antifibrillatory actions are limited by the occurrence of convulsions in conscious animals

The antiarrhythmic actions of the racemate and enantiomers of mexiletine were studied in conscious and anaesthetised rats. Racemate or enantiomers, at 20 mg/kg i.v., had little effect on ischaemia-induced ventricular fibrillation in conscious or anaesthetised rats. In conscious rats 20 mg/kg caused convulsions in 78–89% of rats when the plasma concentration of racemate was 20 ± 2 μM. In anaesthetized animals a higher dose (40 mg/kg) of racemate could be given; this completely prevented ischaemia-induced fibrillation when the plasma concentration was 26 ± 2 μM. Racemate and enantiomers accumulated in the heart and brain of conscious animals to give tissue: plasma ratios of 7.5 and 23, respectively. With electrical stimulation, both racemate and enantiomers dose dependently (4–32 mg/kg) increased threshold currents for induction of ventricular fibrillation, increased refractory period and minimally changed the ECG; findings expected with a Class Ib antiarrhythmic. The above studies failed to show major differences between racemate or enantiomers except for consistently lower (20–30%) plasma concentrations of R(−) at all dose levels. In conclusion, mexiletine prevented ischaemia-induced ventricular fibrillation in anaesthetised animals but only when given at dosed producing convulsions in conscious animals.

Optimum cavity length for high conversion efficiency quantum well diode lasers

The cavity length which maximizes the peak power conversion efficiency is determined for quantum well diode lasers. These calculations are based upon simple models of the diode injection laser’s electrical and optical behaviors, including saturation in the quantum well gain-current characteristic. Here the influences of the distributed optical cavity loss, electrical resistivity, and facet reflectivity on the optimum cavity length are described. Although a lower facet reflectivity results in increased threshold current, there are advantages to longer devices, as the peak conversion efficiency is not reduced. Since the optimum cavity length is greater for low reflectivity, the diode series resistance is smaller. Furthermore, when operating at the point where conversion efficiency is a maximum, the power output of the device with low facet reflectivity exceeds that of the device with higher facet reflectivity. Therein lies the principle advantage of reduced front-facet reflectivities in high power, high efficiency quantum well diode lasers. Good agreement results when these predictions are applied to a strained InGaAs/AlGaAs single quantum well laser (λ=0.93 μm).

Effect of active layer placement on the threshold current of 1.3-μm InGaAsP etched mesa buried heterostructure lasers

The threshold current of InGaAsP etched mesa buried heterostructure (EMBH) lasers is strongly influenced by the position of the active layer in the etched mesa. Transmission electron microscopy results are presented which show the presence of a thin planar disorder along the etched 111A interface. Nonradiative recombination of carriers is believed to be responsible for the increased threshold current of EMBH lasers whose active region is bounded by 111A interface. Lasers with threshold current as low as 14 mA at 30 °C have been fabricated by proper placement of the active layer and by optimizing layer thicknesses and doping levels.

Catastrophic and latent damage in GaAlAs lasers caused by electrical transients

A study has been made of the susceptibility of 20-μm oxide stripe GaAlAs lasers to current transients. The current amplitude (as a function of duration) required to cause increased threshold currents or degradation rates has been found. Damage caused by forward bias transients was found to occur at the facets and was probably due to high surface recombination, leading to local absorption and excessive heating. Even when the immediate damage was insufficient to cause a detectable increase in threshold current, increased degradation rates resulted. Damage during reverse bias transients resulted from local avalanche breakdown which, though usually observed at the facets, was occasionally found outside the stripe. Such damage did not always lead to increased degradation rates. The reverse bias breakdown voltage of the lasers was found to be a more sensitive indicator of transient damage than the threshold current. The results provide substantial evidence to confirm that precautions must be taken against electrostatic discharge whenever lasers are handled. In addition, all laser bias circuitry, including that in test and measurement equipment as well as in optical fiber systems, must be checked to ensure that dangerous transients cannot occur.

Calculation of Auger rates in a quantum well structure and its application to InGaAsP quantum well lasers

We discuss the possible intrinsic limitation on the temperature dependence of threshold current of long wavelength InGaAsP quantum well lasers caused by Auger recombination and intervalence band absorption. A model calculation of the various band-to-band Auger rates are presented. We find that the Auger rates increase with decreasing band gap and also with increasing temperature as in the case of bulk semiconductors. The temperature dependence and the band-gap dependence arises from the energy and momentum conservation (in the direction normal to the well) that the four particle states involved must satisfy. The estimated Auger coefficient is large enough to be important in single quantum well lasers. However, the Auger effect should be much smaller in a multiquantum well laser than for a single quantum well laser. This is because the carrier density at threshold is smaller (by a factor of 3 to 4) for multiquantum well lasers than for single quantum well lasers. The increased intervalence band absorption with increasing temperature in the active layer may not be responsible for increased threshold current of single quantum well lasers.

Gallium arsenide-phosphide: Crystal, diffusion and laser properties

Crystal growth, diffusion, and fabrication conditions which yield Ga(AsP) junction lasers with threshold current densities as low as 3000 A/cm 2 are described. A slow vapour transport method of crystal growth results in uniform quality material, while deep junctions diffused under excess As vapour pressure are adequately planar. A study of Zn diffusion into n-type Ga(As 1− x P x ) shows that p- n junction depths depend on the usual diffusion parameters and significantly on the crystal composition of Ga(As 1− x P x ) and the surface preparation of the wafers prior to diffusion. Zinc diffusion occurs via simultaneous interstitial and substitutional mechanisms with the interstitial mechanism predominating in degenerately-doped n-type material. A linearly increasing activation energy with increasing GaP composition is suggested. Radiative recombination in Ga(AsP) junction lasers which is found to occur on the p side of the junction results from electron transitions from smeared conduction band states to Zn acceptor states. Laser threshold current is found to decrease with increasing depletion region width and reverse breakdown voltage. Successful laser operation at temperatures up to 255°K is reported, with threshold currents for Ga(As 0.70P 0.30) following approximately a T 3 temperature dependence in the range between 100 and 200°K. The shift in emission energy in this range is approximately 3.1 × 10 −4 eV/°K. Variation of index of refraction with wavelength is calculated from the spacing of Fabry-Perot modes in laser spectra and yields a value of dn dλ ≈ −1.6 × 10 −4/ A ̊ . For Ga(As 1− x P x ) with x > 0.33, the proximity of the indirect 〈100〉 conduction band minima to the direct [000] minimum causes relatively deep donor states associated with the 〈100〉 minima to lie near or below the direct band edge, which in turn at reduced temperature leads to carrier freeze-out and increased threshold currents. Consistent with the donor ionization energies observed in GaP, the effective direct-indirect transition in Ga(As 1− x P x ) is shown to occur at a value of x less for the deeper donor S than that for the shallower donor Te. The peculiarities of the direct-indirect transition are used to estimate the position of the indirect donor levels of Te, Se, and S in Ga(AsP).