Ns Duration Research Articles

Holmium lasers passively Q-switched with PbS quantum-dot-doped glasses

PbS quantum-dot-doped glasses are demonstrated as saturable absorber Q-switches for 2 microm holmium lasers. Q-switched pulses from Ho3+:Y3Sc2Al3O12 and Ho3+:Y3Al5O12 lasers of 50 and 70 ns in duration, respectively, have been obtained demonstrating up to 13% of energy conversion efficiency from free-running to Q-switched output.

High-resolution THz spectrometer with kHz scan rates

We demonstrate a rapid scanning high-resolution THz spectrometer capable of acquiring THz field transients with 1 ns duration without mechanical delay line. The THz spectrometer is based on two 1-GHz Ti:sapphire femtosecond lasers which are linked with a fixed repetition rate difference in order to perform high-speed asynchronous optical sampling. One laser drives a high-efficiency large-area GaAs based THz emitter, the other laser is used for electro-optic detection of the emitted THz-field. At a scan rate of 9 kHz a time resolution of 230 fs is accomplished. High-resolution spectra from 50 GHz up to 3 THz are obtained and water absorption lines with a width of 11 GHz are observed. The use of femtosecond lasers with 1 GHz repetition rate is essential to obtain rapid scanning and high time-resolution at the same time.

90% pump depletion and good beam quality in a pulse-injection-seeded nanosecond optical parametric oscillator

We measured 90% pump depletion in a singly resonant image-rotating nanosecond optical parametric oscillator that was pulse-injection seeded by a self-generated signal pulse. The oscillator was pumped by an 8 ns duration single-frequency 532 nm pulse from an injection-seeded Q-switched Nd:YAG laser and resonated an 803 nm signal. The pump and pulsed-seed beams had flat-topped spatial fluence profiles with diameters of approximately 6 mm, giving a cavity Fresnel number at 803 nm approaching 400. The beam cleanup effects of the image-rotating cavity produce a far-field signal spatial fluence profile with approximately 60% of its energy falling within the diffraction-limited spot size.

Conformational Dynamics of M2 Helices in KirBac Channels: Helix Flexibility in Relation to Gating via Molecular Dynamics Simulations

KirBac1.1 and 3.1 are bacterial homologues of mammalian inward rectifier K channels. We have performed extended molecular dynamics simulations (five simulations, each of >20 ns duration) of the transmembrane domain of KirBac in two membrane environments, a palmitoyl oleoyl phosphatidylcholine bilayer and an octane slab. Analysis of these simulations has focused on the conformational dynamics of the pore-lining M2 helices, which form the cytoplasmic hydrophobic gate of the channel. Principal components analysis reveals bending of M2, with a molecular hinge at the conserved glycine (Gly134 in KirBac1.1, Gly120 in KirBac3.1). More detailed analysis reveals a dimer-of-dimers type motion. The first two eigenvectors describing the motions of M2 correspond to helix kink and swivel motions. The conformational flexibility of M2 seen in these simulations correlates with differences in M2 conformation between that seen in the X-ray structures of closed channels (KcsA and KirBac) in which the helix is undistorted, and in open channels (e.g. MthK) in which the M2 helix is kinked. Thus, the simulations, albeit on a time scale substantially shorter than that required for channel gating, suggest a gating model in which the intrinsic flexibility of M2 about a molecular hinge is coupled to conformational transitions of an intracellular 'gatekeeper' domain, the latter changing conformation in response to ligand binding.

Laboratory Simulations of Supernova Shockwave Propagation

Supernovae launch spherical shocks into the circumstellar medium (CSM). These shocks have high Mach numbers and may be radiative. We have created similar shocks in the laboratory by focusing laser pulses onto the tip of a solid pin surrounded by ambient gas; ablated material from the pin rapidly expands and launches a shock through the surrounding gas. Laser pulses were typically 5 ns in duration with ablative energies ranging from 1–150 J. Shocks in ambient gas pressures of ~1 kPa were observed at spatial scales of up to 5 cm using optical cameras with schlieren. Emission spectroscopy data were obtained to infer electron temperatures (< 10 eV). In this experiment we have observed a new phenomena; at the edge of the radiatively heated gas ahead of the shock, a second shock forms. The two expanding shocks are simultaneously visible for a time, until the original shock stalls from running into the heated gas. The second shock remains visible and continues to expand. A minimum condition for the formation of the second shock is that the original shock is super-critical, i.e., the temperature distribution ahead of the original shock has an inflexion point. In a non-radiative control experiment the second shock does not form. We hypothesize that a second shock could form in the astrophysical case, possibly in radiative supernova remnants such as SN1993J, or in shock-CSM interaction.

Erbium Excitation in a SiO[sub 2] : Si-nc Matrix under Pulsed Pumping

The photoluminescence of Er3+ ions in a SiO2 matrix containing silicon nanocrystals 3.5 nm in diameter is studied under resonant and nonresonant pulsed pumping with pulses 5 ns in duration. The effective erbium excitation cross section under pulsed pumping, σeff = 8.7 × 10−17 cm2, is close to that for nanocrystals. Comparison of the erbium photoluminescence intensity obtained for a SiO2 matrix with and without nanocrystals made it possible to determine the absolute concentration of optically active nanocrystals capable of exciting erbium ions, the concentration of optically active erbium, and the average number of erbium ions excited by one nanocrystal. The study revealed that excitation transfer from one erbium ion to another is a relatively slow process, which accounts for the low efficiency of erbium ion excitation under pulsed pumping in a SiO2 matrix containing silicon nanocrystals.

Magneto- and Electroresistance of Ultrathin Anisotropically Strained La-Sr-MnO Films

The magnetoresistance anisotropy of ultrathin La0.83Sr0.17MnO3 films deposited on NdGaO3 substrate by metalorganic chemical vapour deposition technique was investigated. The electric-field-induced resistance change was studied up to electric fields of 10 kV/cm using ns duration electrical pulses. It was found that in ultrathin ( 50 nm) films the origin of electric-field-induced resistance change is thermal. However, the films with thicknesses of about 20 nm, exhibit negative electric-field-induced resistance change, having a pure electronic nature. This effect is explained in terms of two-layer systems with imperfections located at the interface between the layers.

Production and propagation of single-event transients in high-speed digital logic ICs

The production and propagation of single-event transients in scaled metal oxide semiconductor (CMOS) digital logic circuits are examined. Scaling trends to the 100-nm technology node are explored using three-dimensional mixed-level simulations, including both bulk CMOS and silicon-on-insulator (SOI) technologies. Significant transients in deep submicron circuits are predicted for particle strikes with linear energy transfer as low as 2 MeV-cm/sup 2//mg, and unattenuated propagation of such transients can occur in bulk CMOS circuits at the 100-nm technology node. Transients approaching 1 ns in duration are predicted in bulk CMOS circuits. Body-tied SOI circuits produce much shorter transients than their bulk counterparts, making them more amenable to transient filtering schemes based on temporal redundancy. Body-tied SOI circuits also maintain a significant advantage in single-event transient immunity with scaling.

Influence of reducing-oxidizing conditions on the optical properties of Co^2+-doped magnesium aluminosilicate glass ceramics and their use as an effective saturable absorber Q switch

Nonlinear optical properties of Co(2+)-doped magnesium aluminosilicate glass ceramics prepared under normal and reducing conditions are studied in the range of the 4A2 --> 4T1(4F) transition of tetrahedral Co2+ ions. The results of passive Q switching of a 1.54-microm Er3+:glass laser with saturable absorbers made of Co(2+)-doped glass ceramics prepared at different conditions are presented. Q-switched pulses of 60 ns in duration and of 4.6 mJ in energy, corresponding to approximately 20% of the Q-switching conversion efficiency, are achieved.

Stimulation of Capacitative Calcium Entry in HL-60 Cells by Nanosecond Pulsed Electric Fields

Nanosecond pulsed electric fields (nsPEFs) are hypothesized to affect intracellular structures in living cells providing a new means to modulate cell signal transduction mechanisms. The effects of nsPEFs on the release of internal calcium and activation of calcium influx in HL-60 cells were investigated by using real time fluorescent microscopy with Fluo-3 and fluorometry with Fura-2. nsPEFs induced an increase in intracellular calcium levels that was seen in all cells. With pulses of 60 ns duration and electric fields between 4 and 15 kV/cm, intracellular calcium increased 200-700 nM, respectively, above basal levels (approximately 100 nM), while the uptake of propidium iodide was absent. This suggests that increases in intracellular calcium were not because of plasma membrane electroporation. nsPEF and the purinergic agonist UTP induced calcium mobilization in the presence and absence of extracellular calcium with similar kinetics and appeared to target the same inositol 1,4,5-trisphosphate- and thapsigargin-sensitive calcium pools in the endoplasmic reticulum. For cells exposed to either nsPEF or UTP in the absence of extracellular calcium, there was an electric field-dependent or UTP dose-dependent increase in capacitative calcium entry when calcium was added to the extracellular media. These findings suggest that nsPEFs, like ligand-mediated responses, release calcium from similar internal calcium pools and thus activate plasma membrane calcium influx channels or capacitative calcium entry.

Effects of submicrosecond, high intensity pulsed electric fields on living cells-intracellular electromanipulation

Development of technology to produce nanosecond duration pulsed electric fields has allowed examination of the effects of ultrashort duration, high intensity electric fields on living cells. Theoretically, high intensity (MV/m) electric field applications with durations of less than one microsecond, when shortened toward nanoseconds, should increasingly affect intracellular rather than surface membranes of living cells. Experimentally, square-wave, 60 ns duration, high energy (36-53 kV/cm) pulses applied in trains of 1-10 pulses result in progressive increases in the numbers of permeabilized intracellular granules in a human eosinophil cell model-without large surface membrane effects. Electron micrographic examination of cells treated in this way demonstrates alteration of intracellular granule morphology consistent with permeabilization of granule membrane, i.e., intracellular electromanipulation. Continuous microscopic examination of individual living cells exposed to long or short duration pulsed electric field applications shows that permeabilization of surface membrane (median 5 minutes) with anodic preference (electroporation) and prompt cellular swelling follow a single, long duration (100 microsecond) pulse. In contrast, after a single short duration (60 ns) pulse, onset of surface membrane permeability is delayed (median 17 minutes), the increased permeability shows no anodic preference, and cellular swelling is absent suggesting that these effects are due to intracellular electromanipulation rather than direct effects on the surface membrane. Submicrosecond, intense pulsed electric fields applied to living cells achieve preferential effects on intracellular rather than surface membranes, potentially providing new approaches for selective/generalized cell or tissue ablation, growth stimulation and tissue remodeling.

A prokaryotic glutamate receptor: homology modelling and molecular dynamics simulations of GluR0

GluR0 is a prokaryotic homologue of mammalian glutamate receptors that forms glutamate-activated, potassium-selective ion channels. The topology of its transmembrane (TM) domain is similar to that of simple potassium channels such as KcsA. Two plausible alignments of the sequence of the TM domain of GluR0 with KcsA are possible, differing in the region of the P helix. We have constructed homology models based on both alignments and evaluated them using 6 ns duration molecular dynamics simulations in a membrane-mimetic environment. One model, in which an insertion in GluR0 relative to KcsA is located in the loop between the M1 and P helices, is preferred on the basis of lower structural drift and maintenance of the P helix conformation during simulation. This model also exhibits inter-subunit salt bridges that help to stabilise the TM domain tetramer. During the simulation, concerted K+ ion–water movement along the selectivity filter is observed, as is the case in simulations of KcsA. K+ ion exit from the central cavity is associated with opening of the hydrophobic gate formed by the C-termini of the M2 helices. In the intact receptor the opening of this gate will be controlled by interactions with the extramembranous ligand-binding domains.

Shock wave experiments using a pulsed electron beam to study copper and molybdenum thermoelastic responses

This paper presents the experimental and numerical studies of copper and molybdenum thermomechanical responses using a pulsed source of electrons (3 Mev - 3kA - 60 ns duration). These experiments permit to characterise the dynamic behaviour of materials under high pressure, high temperature and high deformation rate. The rapid deposition of energy generates shock wave motion, which commonly induces inelastic flow or failure. Several experiments have been carried out onto copper and molybdenum targets. The thermomechanical responses have been studied by registering the rear surface motion of the targets using Michelson laser interferometer or the stress history using quartz piezoelectric gage. Numerical computations of energy deposition and shock wave propagation have been performed in order to better understand the dynamics events. The computational constitutive model used for this work was previously developed for metallic material. The numerical results are in almost good accordance with experimental data. The measurement of energy deposition must be improved in order to identify the Gruneisen coefficient of materials.

Electron acceleration in gas by impulse electric field and its application to selective promotion of an electron–molecule reaction

We have simulated electron acceleration under short intense impulse electric fields of 0.1–1 kV cm−1 strength and ∼1 ns duration in N2 at 65.5 Pa. The electron acceleration is completed in a practically collisionless condition when the impulse duration is comparable with or shorter than the electron mean free time. On the basis of this feature, we have proposed a way to produce a group of monoenergetic electrons. The electron energy after acceleration is controllable by the impulse strength and duration, and we have demonstrated selective promotion of the excitation producing N2(A 3Σu+) metastable excited molecules. The selectivity has been quantified as the efficiency of the electron energy utilized for the target reaction, and efficiencies of 20–30% have been achieved in this simulation. These values are higher than those of electron swarms in DC equilibrium, in which the efficiency is at most 10%. The electron behaviour under impulse electric fields and the dependence of the energy efficiency on the impulse profile have been investigated.

Radiative lifetime measurements in Er II by time-resolved laser spectroscopy

Radiative lifetimes of 30 excited states of Er II, rangingin energy from 25 592 to 36 148 cm−1,have been measured by means of a time-resolved laser-induced fluorescence (LIF)technique. Free, singly ionized erbium ions were produced in a laser-induced plasma. Atunable laser, with 1 ns duration pulse, was employed to selectively excite the Er+ions. The lifetime values were evaluated from the transient LIF signals recordedby a fast detection system. A comparison of the new results with previouslypublished values is given. HFR calculations, including core-polarizationcontributions, are found to be in reasonable agreement with experiment, but thepresent calculations clearly indicate that further laboratory and theoretical workis urgently needed, particularly for the low lying odd configurations of thiscomplex ion.

Diode-pumped Nd:YVO 4 and Nd:KGd(WO 4 ) 2 1.3 μm lasers passively Q-switched with PbS-doped glass

Diode-pumped Nd:YVO4 and Nd:KGW lasers passively Q-switched with PbS-quantum dot-doped phosphate glass were demonstrated. For the Nd:YVO4 laser, pulses 110 ns in duration with a 13% Q-switching efficiency were obtained. The absorption recovery time of the PbS-doped glass was measured to be 27 +/- 4 ps. Some recommendations for more efficient use of PbS-doped glasses for Q-switching of diode-pumped lasers are suggested on the basis of our analysis.

Helix Unfolding and Intramolecular Hydrogen Bond Dynamics in Small α-Helices in Explicit Solvent

The dynamics of folding and unfolding of secondary structural motifs found in proteins is crucial to understanding the protein folding problem. In this paper we study the short-time dynamics of loop unfolding and hydrogen bond formation and breaking in the alanine pentapeptide at room temperature, using several constant-energy molecular dynamics simulations of about 4 ns of duration. We analyze the results in terms of time histories of “core” α-carbon dihedral angles. We also perform a principal component analysis of the data. We find that the time scale for a considerable deformation of the dihedral angles formed by the α-carbons is on the order of 1 ns whereas hydrogen bonds seem to break and form on a shorter time scale.

Nanosecond generation of intense electron currents with high- Tc superconducting rings

Pulse compression of intense electron beams ∼340 keV, ∼1 kA, and ∼10 ns) was observed for the following two superconducting ring lenses (supertrons): an already sintered 1 μm sized powder-pressed Bi(2223)-based ring lens and a double-layered lens having an outer melt-processed Y-based ring. The outer- and innermost diameters of the lenses were 22 and 16 mm, respectively, with a total axial length of 55 mm including a funnel-type inlet. The powder-pressed lens compressed the electron pulses to ∼5 ns in duration at 70 K, whereas the double-layered lens provided ∼3.7 ns pulses. The durations were both longer than those (∼1.4–3.0 ns) for previous double-layered lenses in which the inner rings were made from sintered Bi(2223)-based materials. These results are explained by a ferrite-core model for supertrons.

Nanosized glass-ceramics doped with transition metal ions: nonlinear spectroscopy and possible laser applications

Absorption, luminescence and saturation of absorption for cobalt-doped glass-ceramics were studied. Characteristic times of luminescence decay of 4T1(4P)→4A2 transition for tetrahedrally coordinated Co2+ ions in lithium-, zinc-, magnesium- and magnesium–gallium–aluminosilicate glass-ceramics were evaluated to be in tens of ns time scale. Kinetics of bleaching relaxation of 4A2→4T1(4F) transition for tetrahedrally coordinated Co2+ ions in magnesium–aluminosilicate glass-ceramics was found to be 450±150 and 120±40 ns for 0.1 and 0.3 wt% cobalt concentration, respectively. Ground-state absorption cross-section for Co2+ ions at 1.54 μm was estimated to be ∼3×10−19 cm2. Passively Q-switched pulses from Er:glass laser of 6 mJ in energy and 80 ns in duration using the Co-doped magnesium–aluminosilicate glass-ceramics sample as saturable absorber were demonstrated.

Glass doped with PbS quantum dots as a saturable absorber for 1-μm neodymium lasers

Saturable-absorber mode locking and Q switching of neodymium-doped lasers at 1.06 µm with a PbS-doped glass were demonstrated. Q-switched pulses of 0.3 µJ in energy and 15 ns in duration from a cw diode-pumped Nd3+:KGd(WO4)2 laser and ultrashort pulses of 18 µJ in energy and 70 ps in duration from a Nd3+:Y3Al5O12 laser were obtained. The saturation intensity of the PbS-doped glass (with an average semiconductor crystallite radius of 1.7 nm) was estimated to be 2.3 MW/cm2 at 1.06 µm. The bleaching relaxation time was measured to be 23 ps.