Subnanosecond Time Scale Research Articles

Does addition of 1-octanol as a phase modifier provide radical scavenging radioprotection for N,N,N',N'-tetraoctyldiglycolamide (TODGA)?

To mitigate third phase formation in next generation used nuclear fuel reprocessing technologies, the addition of 1-octanol has been trialed. However, contradictory reports on the radiolytic effect of 1-octanol incorporation on separation ligand degradation need to be resolved. Here, 50 mM N,N,N',N'-tetraoctyldiglycolamide (TODGA) dissolved in n-dodecane was gamma irradiated in the presence and absence of 1-octanol (2.5-10 vol%) and a 3.0 M HNO3 aqueous phase. Radiation-induced TODGA degradation exhibited pseudo-first-order decay kinetics as a function of absorbed gamma dose for all investigated solution and solvent system formulations. The addition of 1-octanol afforded diametrically different effects on the rate of TODGA degradation depending on solvent system formulation. For organic-only irradiations, 1-octanol promoted TODGA degradation (d = 0.0057 kGy-1 for zero 1-octanol present vs.∼0.0073 kGy-1 for 7.5-10 vol%) attributed to a favourable hydrogen atom abstraction reaction free energy (-0.31 eV) and the ability of 1-octanol to access a higher yield of n-dodecane radical cation (RH˙+) at sub-nanosecond timescales. This was rationalized by determination of the rate coefficient (k) for the reaction of 1-octanol with RH˙+, k = (1.23 ± 0.07) × 1010 M-1 s-1. In contrast, irradiation in the presence of 1-octanol and a 3.0 M HNO3 aqueous phase afforded significant radioprotection (d = 0.0054 kGy-1 for zero 1-octanol present vs.≤ 0.0044 kGy-1 for >2.5 vol%) that increases with 1-octanol concentration, relative to the single phase, organic-only solutions. This effect was attributed to the extraction of sufficiently high concentrations of HNO3 and H2O into the organic phase by TODGA and 1-octanol as adducts which interfere with the hydrogen atom abstraction process between the 1-octanol radical and TODGA. Our findings suggest that the addition of 1-octanol as a phase modifier will enhance the radiation robustness of TODGA-based separation technologies under envisioned solvent system conditions in the presence of aqueous HNO3.

Evolution of Diffusion Coefficient of Photoexcited Species in Excimer Forming Organic Thin Films

The diffusion of photogenerated excitons in organic semiconductors represents often a rate-determining process in the exciton population evolution in ultrafast transient absorption experiments, due to an increased mutual exciton interaction probability at typical excitation densities. A simple description of a diffusion process with a time-independent diffusion coefficient often fails to explain the observed kinetics in the highly non-equilibrium system. In this work, we used the concept of time-dependent diffusion and determined the evolution of the diffusion coefficient of photogenerated excited species in thin films of bis(terpyridine-4’-yl)terthiophene from the experimental ultrafast transient absorption spectroscopic data in a sub-nanosecond time scale. We show that the diffusion coefficient decreases in time more rapidly with increasing intensity of the photoexcitation. We explain the dependences by the planarization of photoexcited molecules and excimer formation.

The Interaction of a High-Power Sub-Nanosecond Microwave Pulse With Plasma

We present the results of experiments, analytical modeling, and numerical particle-in-cell (PIC) simulations for the propagation of a high-power, sub-nanosecond microwave pulse through a plasma-filled cylindrical waveguide in a parameter regime not studied before. Depending on the experimental conditions, the non-linear interaction of the high-power microwave (HPM) pulse with the plasma results in self-channeling through it or causes the formation of a wakefield. To study this phenomenon, two backward wave oscillators (BWOs), operating in the super-radiant mode at frequencies of 9.6 and 28.6 GHz, were designed and tested. These BWOs were driven by an electron beam (~280 keV, ~1.5 kA, ~5 ns) generated in a magnetically insulated foil-less diode, from which it propagates through a slow wave structure guided by an external axial magnetic field. Microwave pulses of ~0.4-ns width, up to ~500-MW peak power at 9.6 GHz, and up to ~1.2 GW at 28.6 GHz were obtained. These powerful subnanosecond timescale microwave beam pulses were injected into the neutral gas, plasma, or their mixture in various configurations.

Subnanosecond Fluctuations in Low-Barrier Nanomagnets

Fluctuations in nanomagnets are interesting for application in areas such as probabilistic computing, stochastic optimization, statistical sampling, and cryptography, as these fields rely on random numbers. The authors show theoretically and numerically that superparamagnetic nanomagnets can be used to generate random numbers on a subnanosecond time scale. The order parameters of both thermally excited easy-plane and antiferromagnetically coupled nanomagnets are found to possess similar dynamics. This insight could have an impact on emerging devices such as stochastic magnetic tunnel junctions, where such magnets could be used in the free layer to speed operation by orders of magnitude.

In-cell structural dynamics of an EGF receptor during ligand-induced dimer-oligomer transition.

The epidermal growth factor receptor (EGFR) is a membrane protein that regulates cell proliferation, differentiation and survival, and is a drug target for cancer therapy. Ligand-induced activation of the EGFR kinase is generally regarded to require ligand-bound-dimers, while phosphorylation and down-stream signalling is modulated by oligomers. Recent work has unveiled changes in EGFR dynamics from ligand-induced dimerization in membranes extracted from cells, however, less is known about the changes in EGFR dynamics that accompany the ligand-induced oligomerization in a live cell environment. Here, we determine the dynamics of a c-terminal GFP tag attached to EGFR in the unliganded dimer and in the liganded oligomers. By means of the single-frequency polarized phasor ellipse approach we extracted two correlation times on the sub-nanosecond and super-nanosecond timescales, respectively. EGF binding to the EGFR-GFP dimer lengthened the sub-nanosecond correlation time (from 0.1 to 1.3ns) and shortened the super-nanosecond correlation time (from 210 to 56ns) of the c-terminal GFP probe. The sub-nanosecond depolarization processes were assigned to electronic energy migration between proximal GFPs in the EGFR dimer or oligomer, while the super-nanosecond correlation times were assigned to nanosecond fluctuations of the GFP probe in the EGFR complex. Accordingly, these results show that ligand binding increased the average separation between the c-terminal tags and increased their rotational mobility. We propose that the dynamics are linked to an inhibitory function of the c-terminal tail in the un-liganded dimer and to the requirement of facile stochastic switching between kinase activation and cytoplasmic adaptor/effector binding in the active oligomers.

Micelle Heterogeneity from the 2D Kinetics of Solute Rotation.

The chemical and physical properties of microstructured materials vary with position. The photophysics of solute molecules can measure these local properties, but they often show multiple rates (rate dispersion), which complicates the interpretation. In the case of micelles, rate dispersion in a solute's anisotropy decay has been assigned to either local anisotropy or heterogeneity in the local viscosity. To resolve this conflict, the rotation of PM597 molecules in SDS micelles has been measured by polarized MUPPETS (multiple population-period transient spectroscopy). This 2D technique shows that heterogeneity is strong and that local anisotropy is minimal. The results suggest that on a subnanosecond time scale, the solute sees only one strong fluctuation of the micelle structure. The anisotropic, average structure emerges on longer time scales.

Full Spatiotemporal Control of Laser-Excited Periodic Surface Deformations

We demonstrate full control of acoustic and thermal periodic deformations at solid surfaces down to sub-nanosecond time scales and few-micrometer length scales via independent variation of the temporal and spatial phase of two optical transient grating (TG) excitations. For this purpose, we introduce an experimental setup that exerts control of the spatial phase of subsequent time-delayed TG excitations depending on their polarization state. Specific exemplary coherent control cases are discussed theoretically and corresponding experimental data are presented in which time-resolved x-ray reflectivity measures the spatiotemporal surface distortion of nanolayered heterostructures. Finally, we discuss examples where the application of our method may enable the control of functional material properties via tailored spatiotemporal strain fields.

Sparks as sub-nanosecond, broadband light switches.

We evaluate spark micro-discharges as the working element of laser switches that can operate on sub-nanosecond timescales, function over a broad range of wavelengths, and handle high laser power. Sparks were generated in room temperature argon at 11-51bar and xenon at 3-11.6bar. A continuous 405nm wavelength laser light was focused through the spark gap, and its transmission was recorded by a high-speed photodiode. We demonstrate that the transition to opacity at the higher pressures is faster than our detection circuit's 90%-10% fall time of 400ps, indicating that the true fall time is 100ps or shorter. The challenges to be overcome to improve device reliability are identified.

Picosecond Switchable Azo Dyes.

Azo dyes that combine electron-withdrawing thiazole/benzothiazole heterocycles and electron-donating amino groups within the very same covalent skeleton exhibit relaxation times for their thermal isomerization kinetics within milli- and microsecond timescales at room temperature. Notably, the thermal back reaction of the corresponding benzothiazolium and thiazolium salts occurred much faster, within the picosecond temporal domain. In fact, these new light-sensitive platforms are the first molecular azo derivatives capable of reversible switching between their trans and cis isomers in a subnanosecond timescale under ambient conditions. In addition, theoretical calculations revealed very low activation energies for the isomerization process, in accordance with the fast subnanosecond kinetics that were observed experimentally.

Breakdown of Corner States and Carrier Localization by Monolayer Fluctuations in Radial Nanowire Quantum Wells.

We report a comprehensive study of the impact of the structural properties in radial GaAs-Al0.3Ga0.7As nanowire-quantum well heterostructures on the optical recombination dynamics and electrical transport properties, emphasizing particularly the role of the commonly observed variations of the quantum well thickness at different facets. Typical thickness fluctuations of the radial quantum well observed by transmission electron microscopy lead to pronounced localization. Our optical data exhibit clear spectral shifts and a multipeak structure of the emission for such asymmetric ring structures resulting from spatially separated, yet interconnected quantum well systems. Charge carrier dynamics induced by a surface acoustic wave are resolved and prove efficient carrier exchange on native, subnanosecond time scales within the heterostructure. Experimental findings are corroborated by theoretical modeling, which unambiguously show that electrons and holes localize on facets where the quantum well is the thickest and that even minute deviations of the perfect hexagonal shape strongly perturb the commonly assumed 6-fold symmetric ground state.

Experiments Designed to Study the Non-Linear Transition of High-Power Microwaves through Plasmas and Gases

The interaction of powerful sub-picosecond timescale lasers with neutral gas and plasmas has stimulated enormous interest because of the potential to accelerate particles to extremely large energies by the intense wakefields formed and without being limited by high accelerating gradients as in conventional accelerator cells. The interaction of extremely high-power electromagnetic waves with plasmas is though, of general interest and also to plasma heating and wake-field formation. The study of this subject has become more accessible with the availability of sub-nanosecond timescale GigaWatt (GW) power scale microwave sources. The interaction of such high-power microwaves (HPM) with under-dense plasmas is a scale down of the picosecond laser—dense plasma interaction situation. We present a review of a unique experiment in which such interactions are being studied, some of our results so far including results of our numerical modeling. Such experiments have not been performed before, self-channeling of HPM through gas and plasma and extremely fast plasma electron heating to keV energies have already been observed, wakefields resulting from the transition of HPM through plasma are next and more is expected to be revealed.

New Frontiers in Experimentally Observing Detonation Properties

New Frontiers in Experimentally Observing Detonation Properties

Development of an ultra-fast photomultiplier tube for gamma-ray Cherenkov detectors at the National Ignition Facility (PD-PMT).

A new ultra-fast photomultiplier tube and associated drivers have been developed for use in the next generation of gamma-ray high pressure gas Cherenkov detectors for inertial confinement fusion experiments at the National Ignition Facility. Pulse-dilation technology has been applied to a standard micro-channel-plate-based photomultiplier tube to improve the temporal response by about 10×. The tube has been packaged suitably for deployment on the National Ignition Facility, and remote electronics have been designed to deliver the required non-linear waveforms to the pulse dilation electrode. This is achieved with an avalanche pulse generator system capable of generating fast arbitrary waveforms over the useful parameter space. The pulse is delivered via fast impedance-matching transformers and isolators, allowing the cathode to be ramped on a sub-nanosecond time scale between two high voltages in a controlled non-linear manner. This results in near linear pulse dilation over several ns. The device has a built-in fiducial system that allows easy calibration and testing with fiber optic laser sources. Results are presented demonstrating the greatly improved response time and other parameters of the device.

Energy Efficient Superconducting Neural Networks for High-Speed Intellectual Data Processing Systems

We present the results of circuit simulations for the adiabatic flux-operating neuron. The proposed cell with one-shot calculation of activation function is based on a modified single-junction superconducting quantum interferometer. In comparison, functionally equivalent elements of the artificial neural network (ANN) in the semiconductor-based implementations consist of approximately 20 transistors. Also in the article, we present the connecting synapse based on the adiabatic quantum flux parametron. These neurons and synapses allow constructing ANNs with a magnetic representation of information in the form of direction and/or magnitude of the magnetic flux in the superconducting circuit. We discuss the dissipation of energy during operations in the frame of the proposed concept. This value in superconducting neurons and synapses with sub-nanosecond timescale can be reduced down to 10 and 0.1 aJ, respectively. The use of the adiabatic superconducting logic circuits in our approach promises compatibility with superconducting quantum information processing systems.

Highly sensitive detection of photo-thermal transient stress by a sub-nanosecond pump probe with surface plasmon resonance

Photoacoustic microscopy (PAM) has attracted increasing attention for non-invasive functional imaging in biomedicine and biomedical studies. The surface plasmon resonance (SPR) sensor is a potential acoustical detector with high sensitivity and wide bandwidth for use in high-resolution 3D PAM. In this study, we investigated a simple Kretschmann SPR sensor, demonstrating highly sensitive stress detection in the sub-nanosecond timescale by directly inducing photo-thermal transient stress. To do this, we designed and used a sub-nanosecond pump probe system. The SPR sensor clearly detected the transient response from the thermal elastic effect, while a simple pump-probe sensor without SPR did not detect it. Our experimental results demonstrate a simple SPR sensor that can detect stress with high sensitivity and sub-nanosecond time resolution. This technique could be used in high-resolution 3D PAM by mounting a small biomedical sample on the sensor.

Ultrafast dynamic response of single-crystal β-HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine)

We report experimental and computational studies of shock wave dynamics in single-crystal β-HMX on an ultrafast time scale. Here, a laser-based compression drive (∼1 ns in duration; stresses of up to ∼40 GPa) is used to propagate shock waves normal to the (110) and (010) lattice planes. Ultrafast time-domain interferometry measurements reveal distinct, time-dependent relationships between the shock wave velocity and particle velocity for each crystal orientation, which suggest evolving physical processes on a sub-nanosecond time scale. To help interpret the experimental data, elastic shock wave response was simulated using a finite-strain model of crystal thermoelasticity. At early propagation times (<500 ps), the model is in agreement with the data, which indicates that the mechanical response is dominated by thermoelastic deformation. The model agreement depends on the inclusion of nonlinear elastic effects in both the spherical and deviatoric stress-strain responses. This is achieved by employing an equation-of-state and a pressure-dependent stiffness tensor, which was computed via atomistic simulation. At later times (>500 ps), the crystal samples exhibit signatures of inelastic deformation, structural phase transformation, or chemical reaction, depending on the direction of wave propagation.

Statistical variances of diffusional properties from ab initio molecular dynamics simulations

Ab initio molecular dynamics (AIMD) simulation is widely employed in studying diffusion mechanisms and in quantifying diffusional properties of materials. However, AIMD simulations are often limited to a few hundred atoms and a short, sub-nanosecond physical timescale, which leads to models that include only a limited number of diffusion events. As a result, the diffusional properties obtained from AIMD simulations are often plagued by poor statistics. In this paper, we re-examine the process to estimate diffusivity and ionic conductivity from the AIMD simulations and establish the procedure to minimize the fitting errors. In addition, we propose methods for quantifying the statistical variance of the diffusivity and ionic conductivity from the number of diffusion events observed during the AIMD simulation. Since an adequate number of diffusion events must be sampled, AIMD simulations should be sufficiently long and can only be performed on materials with reasonably fast diffusion. We chart the ranges of materials and physical conditions that can be accessible by AIMD simulations in studying diffusional properties. Our work provides the foundation for quantifying the statistical confidence levels of diffusion results from AIMD simulations and for correctly employing this powerful technique.

A Buried Water Molecule Influences Reactivity in α-Amylase on a Subnanosecond Time Scale

The subset of catalytically competent conformations can be significantly small in comparison with the full conformational landscape of enzyme–substrate complexes. In some enzymes, the probability of finding a reactive conformation can account for up to 4 kcal/mol of activation barrier, even when the substrate remains tightly bound. In this study, we sampled conformations of human pancreatic α-amylase with bound substrate in a molecular dynamics (MD) simulation of over 100 ns and calculated energy profiles along the reaction coordinate. We found that reactive states require a hydrogen bond between a buried water molecule and E233, which is the general acid in the glycolysis mechanism. The effect of this single, nonreactive, intermolecular interaction is as important as the correct positioning and orientation of the reacting residues to achieve a competent energy barrier. This hydrogen bond increases the acidity of E233, facilitating proton transfer to the glycosidic oxygen. In the MD simulation, this requi...

Sideband pump-probe technique resolves nonlinear modulation response of PbS/CdS quantum dots on a silicon nitride waveguide

For possible applications of colloidal nanocrystals in optoelectronics and nanophotonics, it is of high interest to study their response at low excitation intensity with high repetition rates, as switching energies in the pJ/bit to sub-pJ/bit range are targeted. We develop a sensitive pump-probe method to study the carrier dynamics in colloidal PbS/CdS quantum dots deposited on a silicon nitride waveguide after excitation by laser pulses with an average energy of few pJ/pulse. We combine an amplitude modulation of the pump pulse with phase-sensitive heterodyne detection. This approach permits to use co-linearly propagating co-polarized pulses. The method allows resolving transmission changes of the order of 10−5 and phase changes of arcseconds. We find a modulation on a sub-nanosecond time scale caused by Auger processes and biexciton decay in the quantum dots. With ground state lifetimes exceeding 1 μs, these processes become important for possible realizations of opto-electronic switching and modulation based on colloidal quantum dots emitting in the telecommunication wavelength regime.

Effect of pulse asymmetry and nonlinear chirp on the accuracy of ultrafast pulsed laser interferometry.

Ultrafast pulsed laser interferometry (PLI) can measure picometer displacements at sub-nanosecond time scales, such as acoustic waves and vibrations in microstructures. In this Letter, the effects of pulse characteristics on the accuracy of PLI are investigated through measurements and modeling. The results show that the effective wavelength of PLI, λeff, varies significantly as a function of overlap between the interfering pulses due to pulse asymmetry and nonlinear chirp. This variation presents a serious limitation on the accuracy of PLI if unaddressed. However, it is shown that a continuous-wave laser interferometer can be used to determine λeff with an uncertainty near 0.01%, making it possible to use PLI for accurate displacement measurements.