Sub-nanosecond Regime Research Articles

N-Perfluorooctane versus n-Octane: Pyrene Fluorescence to Compare and Contrast Solute Solvation.

Fluorous solvents may offer distinctly different solvation environments to a solute compared to their hydrocarbon analogues due to the inherently high electronegativity associated with fluorine. Solute solvation within n-perfluorooctane (PFO) is compared with that in n-octane using the well-established polycyclic aromatic hydrocarbon (PAH) fluorescence probe pyrene in the temperature range of 288 to 318 K. Both density (ρ) and dynamic viscosity (η) of PFO are considerably higher than those of n-octane. UV-vis molecular absorbance, fluorescence emission/excitation, and excited-state emission intensity decay reveal the cybotactic region of pyrene to be more nonpolar in PFO than that in n-octane. Bimolecular quenching rate constants (kq) for the pyrene-nitrobenzene fluorophore-quencher pair adhere to the Stokes-Einstein formulation; however, they are considerably higher than the estimated rate constants for the diffusion-controlled process (kdiff). This is due to the high electron affinity of nitrobenzene leading to aromatic π-π interactions between pyrene and nitrobenzene. For a nonaromatic low electron affinity quencher, such as nitromethane, while kq < kdiff in n-octane, kq > kdiff in PFO. This is due to the fact that highly electronegative fluorines on PFO stabilize the partial positive charge (δ+) that develops on excited pyrene during electron/charge transfer to the quencher nitromethane, facilitating quenching in the process. Exciplex formation between pyrene and triethylamine (TEA) is more favored in PFO as opposed to n-octane although ηPFO > ηn-octane. The developing charge on the exciplex is stabilized by the electronegative fluorines of the PFO. The pyrene-TEA exciplex appears to form exclusively in the excited state of pyrene, and the kinetics of exciplex formation is in the subnanosecond regime. On the contrary, the efficiency of exciplex formation between pyrene and N,N-dimethylaniline (DMA) is comparable in PFO and n-octane, and the kinetics is slower in comparison to that of the pyrene-TEA exciplex. Certain ground-state heterogeneity is detected for the pyrene-DMA system in PFO due to the low solubilizing ability of the fluorous solvent. Highly electronegative fluorines on perfluorohydrocarbon solvents are found to offer unusual and unique solvation characteristics.

Watt-level deep-UV subnanosecond laser system based on Nd-doped fiber at 229 nm.

We report an efficient deep-UV master-oscillator power amplifier (MOPA) laser system at 229 nm that generates 350 ps pulses at 2 MHz repetition rate with an average power of 1.2 W. The use of a polarization-maintaining large mode area neodymium-doped fiber operating on the 4F3/2→4I9/2 transition allows high-power laser emission of up to 28 W near 915 nm in the sub-nanosecond regime with low spectral broadening. Two nonlinear frequency conversion stages (LBO + BBO crystals) in a single-pass configuration directly convert the IR laser emission to deep UV. This laser demonstrates the great potential of Nd3+-doped fiber lasers to produce high-power deep-UV emission.

Fast and Flexible, Arbitrary Waveform, 20-kV, Solid-State, Impedance-Matched Marx Generator

We developed a new pulsed power supply to study the influence of the high-voltage (HV) pulse shape on the generation of plasma-activated water (PAW). This article shows the design and implementation of the generator and evaluates its performance on resistive loads. The design is an improved version of the solid-state impedance-matched Marx generator (SS-IMG) concept as previously developed at Eindhoven University of Technology. The IMG concept allows for sub-nanosecond rise time pulses, to be able to create a nonthermal plasma very efficiently. A conventional SS-Marx generator (SS-Marx) circuit is taken as the starting point, and a careful implementation is made with most electrical connections analyzed as transmission lines (TLs). All these TLs are impedance-matched to each other and the load. The implemented generator is able to generate 25 ns to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> duration pulses of 20 kV up to 10-kHz repetition rate in a 50- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega $ </tex-math></inline-formula> load with about 8 ns rise time and arbitrary pulse shape. Future improvements are suggested which will increase the repetition rate and decrease the pulse rise time to the sub-nanosecond regime.

Field-Free Type- x Spin-Orbit-Torque Switching by Easy-Axis Engineering

The current-induced type-x spin-orbit-torque (SOT) switching configuration describes the orthogonal relationship between the magnetic easy axis (EA) of a ferromagnetic (FM) layer and the injected spin polarization, $\ensuremath{\sigma}$, from a heavy-metal (HM) layer, which has the potential to eclipse the conventional type-y scenario (EA parallel to $\ensuremath{\sigma}$) at the sub-nanosecond pulse regime. Here, we show that, in HM/FM bilayer heterostructures, current-driven differential planar Hall signals can serve as efficient means to probe type-x SOT switching in a simple measurement fashion. Through this approach, we demonstrate field-free type-x SOT switching in all devices with a canted EA, which are engineered via field-annealing processes. By analyzing the switching-phase diagrams, we further verify that such field-free switching stems from a z-direction effective field contributed to by both the dampinglike torque and the canted EA. Our work indicates that the canted EA, as engineered by a field-annealing process, can give rise to a robust field-free SOT switching and further reduce the critical switching current density.

Laser-induced damage study of bonded material for a high-brightness laser system

In this work we evaluated the laser-induced damage threshold of the interface between two identical YAG crystals, bonded by an inter-layer assisted surface activated bonding method. The experimental results indicate slight damage threshold degradation for both single- and polycrystalline trivalent rear-earth (RE3+)-ion-doped YAG gain media in the sub-nanosecond pulse regime. Moreover, crystal annealing prior to damage testing could provide additional improvement for the laser damage threshold of the bulk and bonded interface.

High-contrast fluorescence polarization microscopy through stimulated emission

In this work, we demonstrate the contrast enhancement through polarization-resolved pump-probe microscopy, implemented by measuring the spontaneous fluorescence loss induced by stimulated emission. The pump-probe measurement is compared with the conventional fluorescence polarization microscopy. The anisotropy values thus obtained are 0.11 and 0.01, respectively. The contrast improvement is attributed to the multiphoton mechanism in sharpening the point-spread function and the polarization resolving. In addition, the pump-probe technique promises higher temporal resolution in lifetime measurements than time-correlated single-photon counting, enabling more precise determination of the fluorescent molecules’ rotational diffusion time constant, which is often in the sub-nanosecond regime.

Field-Free Spin-Orbit Torque Switching in Magnetic Tunnel Junction Structures With Stray Fields

Spin-orbit torque (SOT) offers ultrafast magnetic switching in the subnanosecond regime for magnetoresistive random-access memory (MRAM) as an alternative to future cache memories. However, one main obstacle to the development of perpendicular SOT-MRAM is that a small in-plane external field is required to achieve deterministic magnetization switching. Here we eliminate the external magnetic field for deterministic SOT switching by utilizing the stray field in a magnetic tunnel junction (MTJ) structure. The MTJ consists of a perpendicularly magnetized CoFeB free layer and an in-plane magnetized reference layer that provides a stray field. Unlike usual SOT switching in a single ferromagnet, four switching phases as well as unique critical switching current dependence on the applied in-plane magnetic fields have been observed. This letter demonstrates the effectiveness of stray-field assisted field-free SOT switching in MTJ structures that may extend to eliminate the main obstacle of SOT-MRAM in practical applications.

Understanding the Switching Mechanisms of the Antiferromagnet/Ferromagnet Heterojunction

Electric-field-driven spintronic devices are considered promising candidates for beyond CMOS logic and memory applications thanks to their potential for ultralow energy switching and nonvolatility. In this work, we have developed a comprehensive modeling framework to understand the fundamental physics of the switching mechanisms of the antiferromagnet/ferromagnet heterojunction by taking BiFeO3/CoFe heterojunctions as an example. The models are calibrated with experimental results and demonstrate that the switching of the ferromagnet in the antiferromagnet/ferromagnet heterojunction is caused by the rotation of the Neel vector in the antiferromagnet and is not driven by the unidirectional exchange bias at the interface as was previously speculated. Additionally, we demonstrate that the fundamental limit of the switching time of the ferromagnet is in the subnanosecond regime. The geometric dependence and the thermal stability of the antiferromagnet/ferromagnet heterojunction are also explored. Our simulation results provide the critical metrics for designing magnetoelectric devices.

Real-time laser energy meter of nanosecond and subnanosecond optical pulses

Abstract Energy meters have become one of the most important tools to determine the performance of pulsed lasers. In case of low power semiconductor lasers operating in a nanosecond and subnanosecond regime, the pulse energy measurement is difficult due to low sensitivity of the apparatus. Moreover, the energy measurement of pulses generated with high frequencies is still challenging for many commercially available meters. In this paper, an ultra-sensitive, high repetition rate, real-time and low-cost laser energy meter based on a high-speed photodiode is presented. The meter’s performance was optimized by using circuit simulation software to achieve maximum sensitivity and high-speed operation. In the experimental part, the minimum detectable energy and the maximum pulse repetition rate were determined to be 110 fJ and 2.28 MHz, respectively. To the best of author’s knowledge, the developed meter is the most sensitive and the fastest infrared pulse energy meter ever reported, including commercially-available devices.

Ultrafast and energy-efficient spin–orbit torque switching in compensated ferrimagnets

Spin–orbit torque can be used to manipulate magnetization in spintronic devices. However, conventional ferromagnetic spin–orbit torque systems have intrinsic limitations in terms of operation speed due to their inherent magnetization dynamics. Antiferromagnets and ferrimagnets with antiparallel exchange coupling exhibit faster spin dynamics and could potentially overcome these limitations. Here, we report ultrafast spin–orbit torque-induced magnetization switching in ferrimagnetic cobalt-gadolinium (CoGd) alloy devices. Using a stroboscopic pump–probe technique to perform time-resolved measurements, we show that the switching time in the ferrimagnets can be reduced to the subnanosecond regime and a domain wall velocity of 5.7 km s‒1 can be achieved, which is in agreement with analytical modelling and atomistic spin simulations. We also find that the switching energy efficiency in the ferrimagnets is one to two orders of magnitude higher than that of ferromagnets. Time-resolved measurements show that current-induced magnetization switching in ferrimagnetic devices is faster and more energy-efficient than in ferromagnet devices.

Highly efficient and reproducible evanescent Raman converters based on a silica nanofiber immersed in a liquid

We present highly efficient and reproducible Raman converters built with a silica nanofiber immersed in ethanol. The converters are pumped at 532 nm in the sub-nanosecond regime and the first Stokes order photons are generated in the evanescent field probing the liquid at 630 nm. Two standard fibers (SMF28 and 460HP) are tested and compared. The Raman conversion operating range limited by the damage threshold is optimized, leading to an external Raman conversion efficiency up to 60% with a nanofiber radius of 300 nm and a length of 8 cm. The extracted Stokes energy is 0.29 μJ, which is three times higher than the previous result. We give guidelines for the design of other efficient evanescent Raman converters, opening the way for a new family of all-fibered compact Raman sources.

Shaping EPR: Phase and amplitude modulated microwave pulses.

The advent of fast arbitrary waveform generators in the sub-nanosecond time regime recently enabled new experimental developments in the field of pulsed EPR. In this article, the new possibilities of such fast phase/amplitude modulated microwave pulses are shortly described with respect to applications in pulsed dipolar spectroscopy. Some of the specific challenges of an accurate creation of such pulses in the field of EPR are outlined. Finally, a short outlook of potential applications is given and some specific experimental conditions are discussed, where shaped pulses might have an especially important impact in the future.

Focusing of multi-MeV, subnanosecond proton bunches from a laser-driven source

We report on our latest transverse focusing results of subnanosecond proton bunches achieved with a laser-driven multi-MeV ion beamline. In the frame of the LIGHT collaboration, a target normal sheath acceleration (TNSA) source based 6 m long beamline was installed. In the past years, the laser-driven proton beam was transported and shaped by this beamline. The particle beam is collimated with a pulsed high-field solenoid and rotated in longitudinal phase space with a radio-frequency cavity which leads to an energy compression with an energy spread of $(2.7\ifmmode\pm\else\textpm\fi{}1.7)%$ ($\mathrm{\ensuremath{\Delta}}\mathrm{E}/{\mathrm{E}}_{0}$ at FWHM) or a time compression to the subnanosecond regime. Highest peak intensities in the subnanosecond regime open up an interesting field for several applications, e.g., proton imaging, as injectors in conventional accelerators or precise stopping power measurements in a plasma. We report on achieving highest peak intensities using an installed second solenoid as a final focusing system in our beamline to achieve small focal spot sizes. We measured a focal spot size of $1.1\ifmmode\times\else\texttimes\fi{}1.2\text{ }\mathrm{mm}$ leading to $5.8\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }\mathrm{protons}\text{ }\mathrm{per}\text{ }\mathrm{s}\text{ }{\mathrm{cm}}^{2}$ at a central energy bin of $(9.55\ifmmode\pm\else\textpm\fi{}0.25)\text{ }\mathrm{MeV}$, which can be combined with a bunch duration below 500 ps at FWHM.

Chemical-vapor deposited ultra-fast diamond detectors for temporal measurements of ion bunches.

This article reports on the development of thin diamond detectors and their characterization for their application in temporal profile measurements of subnanosecond ion bunches. Two types of diamonds were used: a 20 μm thin polycrystalline chemical vapor deposited (CVD) diamond and a membrane with a thickness of (5 ± 1) μm etched out of a single crystal (sc) CVD diamond. The combination of a small detector electrode and an impedance matched signal outlet leads to excellent time response properties with a signal pulse resolution (FWHM) of τ = (113 ± 11) ps. Such a fast diamond detector is a perfect device for the time of flight measurements of MeV ions with bunch durations in the subnanosecond regime. The scCVD diamond membrane detector was successfully implemented within the framework of the laser ion generation handling and transport project, in which ion beams are accelerated via a laser-driven source and shaped with conventional accelerator technology. The detector was used to measure subnanosecond proton bunches with an intensity of 108 protons per bunch.

Efficient Raman converter in the yellow range with high spatial and spectral brightness.

We present a Raman converter emitting at 583nm on the second Stokes order of a line of propan-2-ol pumped by a microlaser at 532nm in the sub-nanosecond regime. We used a mixture of liquids to adapt the transmission band of a photonic bandgap fiber. The internal conversion efficiency is 67% in photon numbers, and the output power is 1.06mW, corresponding to a maximum peak power of 338W. The beam delivered by the converter presents a Gaussian spatial structure and a high spectral brightness, typically more than five times higher than supercontinuum sources in this spectral range.

PETRA IV: the ultralow-emittance source project at DESY.

The PETRA IV project aims at upgrading the present synchrotron radiation source PETRA III at DESY into an ultralow-emittance source. Being diffraction limited up to X-rays of about 10 keV, PETRA IV will be ideal for three-dimensional X-ray microscopy of biological, chemical and physical processes under realistic conditions at length scales from atomic dimensions to millimetres and time scales down to the sub-nanosecond regime. In this way, it will enable groundbreaking studies in many fields of science and industry, such as health, energy, earth and environment, mobility and information technology. The science case is reviewed and the current state of the conceptual design is summarized, discussing a reference lattice, a hybrid multi-bend achromat with an interleaved sextupole configuration based on the ESRF-EBS design, in more detail as well as alternative lattice concepts.

Transient Kinetics and Quantum Yield Studies of Nanocrystalline α-Phenyl-Substituted Ketones: Sorting Out Reactions from Singlet and Triplet Excited States.

Recent work has shown that diarylmethyl radicals generated by pulsed laser excitation in nanocrystalline (NC) suspensions of tetraarylacetones constitute a valuable probe for the detailed mechanistic analysis of the solid-state photodecarbonylation reaction. Using a combination of reaction quantum yields and laser flash photolysis in nanocrystalline suspensions of ketones with different substituents on one of the α-carbons, we are able to suggest with confidence that a significant fraction of the initial α-cleavage reaction takes place from the ketone singlet excited state, that the originally formed diarylmethyl-acyl radical pair loses CO in the crystal with time constants in the sub-nanosecond regime, and that the secondary bis(diarylmethyl) triplet radical pair has a lifetime limited by the rate of intersystem crossing of ca. 70 ns.

First application studies at the laser-driven LIGHT beamline: Improving proton beam homogeneity and imaging of a solid target

In the last two decades, the generation of intense ion beams based on laser-driven sources has become an extensively investigated field. The LIGHT collaboration combines a laser-driven intense ion source with conventional accelerator technology based on the expertise of laser, plasma and accelerator physicists. Our collaboration has installed a laser-driven multi-MeV ion beamline at the GSI Helmholtzzentrum für Schwerionenforschung delivering intense proton bunches in the subnanosecond regime. We investigate possible applications for this beamline, especially in this report we focus on the imaging capabilities. We report on our proton beam homogenization and on first imaging results of a solid target.

The γ-γ fast-timing technique and the EXILL&amp;FATIMA campaign

At the Institut Laue-Langevin in Grenoble, germanium-gated γ-γ fast-timing lifetime measurements of nuclear excited states in neutron-rich nuclei have been performed within a prompt γ-ray spectroscopy experimental campaign. We report on results obtained from the cold-neutron induced fission of 235U. The excited secondary fission products were stopped almost instantaneously within the thick target and the γ rays emitted were collected triggerlessly using the EXILL&amp;FATIMA mixed array of HPGe and LaBr3(Ce) detectors. Precise lifetimes could be determined by analysing the γ-γ time difference spectra using the generalized centroid difference method. This picosecondsensitive method provides many advantages and is briefly explained. Still, the major source of systematic errors is related to the contribution of time-correlated Compton background. The EXILL&amp;FATIMA results are discussed with respect to the typical energy-dependent timing behaviour of the background. According to the time response of the background, appropriate methods and a time correction for the sub-nanosecond regime are proposed.

High-speed microstrip multi-anode multichannel plate detector system.

High-speed detector systems with high dynamic range and pulse width characteristics in the sub-nanosecond regime are mandatory for high resolution and highly sensitive time-of-flight mass spectrometers. Typically, for a reasonable detector area, an impedance-matched anode design is necessary to transmit the registered signal fast and distortion-free from the anode to the signal acquisition system. In this report, a high-speed microstrip multi-anode multichannel plate detector is presented and discussed. The anode consists of four separate active concentric anode segments allowing a simultaneous readout of signal with a dynamic range of about eight orders of magnitude. The impedance matched anode segments show pulse width of about 250 ps, measured at full width at half maximum, and rise time of ∼170 ps, measured with an oscilloscope with a sampling rate of 20 GS/s and 4 GHz analogue bandwidth. The usage of multichannel plates as signal amplifier allowed the design of a lightweight, low power consuming, and compact detector system, suitable, e.g., for the integration into space instrumentation or portable systems where size, weight, and power consumption are limited parameters.