Short-pulse Source Research Articles

Computed tomography imaging using a short pulse source with angular discontinuity

This paper deals with an inverse problem that consists of the attenuation coefficient identification for the non-stationary radiation transfer equation. To solve the problem, we propose to use a pulsed radiation source with an angular discontinuity. We show that the solution to the radiation transfer equation is the sum of a discontinuous ballistic component and a continuous scattered one.The representation, allows us to obtain a formula for finding the attenuation coefficient. The uniqueness theorem for the solution of the inverse problem has been proved. Numerical experiments on a digital phantom show that the method proposed improves the reconstruction quality.

On Peculiarities of Betatron Oscillations of Accelerated Electron Bunches in Capillary Waveguides

It is shown that the dynamics of electrons accelerated in narrow capillary waveguides is significantly influenced by the parametric excitation of their betatron oscillations. On the one hand, this excitation can irreversibly spoil the emittance of an accelerated electron bunch that limits the possibilities of their practical use. On the other hand, controlled parametric excitation of betatron oscillations can be used to generate short-pulse sources of synchrotron radiation. The article analyzes the regions of parametric instabilities, their dependence on the parameters of accelerated electron bunches and guiding structures, and their influence on the dynamics of accelerated electrons. The parameters of the generated synchrotron radiation are also estimated. Measurements of the spectral parameters of synchrotron radiation can serve as a tool for diagnostics of betatron oscillations and their excitation in the case of parametric resonances.

Frame overlap Bragg edge imaging

Neutron Bragg edge imaging enables spatially resolved studies of crystalline features through the exploitation and analysis of Bragg edges in the transmission spectra recorded in each pixel of an imaging detector. Studies with high spectral resolutions, as is required e.g. for high-resolution strain mapping, and with large wavelength ranges have been largely reserved to pulsed neutron sources. This is due to the fact, that the efficiency for high wavelength resolution measurements is significantly higher at short pulse sources. At continuous sources a large fraction of the available neutrons must be sacrificed in order to achieve high wavelength resolution for a relevant bandwidth e.g. through a chopper system. Here we introduce a pulse overlap transmission imaging technique, which is suited to increase the available flux of high wavelength resolution time-of-flight neutron Bragg edge imaging at continuous neutron sources about an order of magnitude. Proof-of-principle measurements utilizing a chopper with a fourfold repeated random slit distribution of eight slits were performed at a thermal neutron beamline. It is demonstrated, that disentanglement of the overlapping pulses is achieved with the correlation theorem for signal processing. Thus, the Bragg edge pattern can be reconstructed from the strongly overlapping Bragg edge spectra recorded and the results demonstrate the feasibility of the technique.

Femtosecond laser produced periodic plasma in a colloidal crystal probed by XFEL radiation

With the rapid development of short-pulse intense laser sources, studies of matter under extreme irradiation conditions enter further unexplored regimes. In addition, an application of X-ray Free-Electron Lasers (XFELs) delivering intense femtosecond X-ray pulses, allows to investigate sample evolution in IR pump - X-ray probe experiments with an unprecedented time resolution. Here we present a detailed study of the periodic plasma created from the colloidal crystal. Both experimental data and theory modeling show that the periodicity in the sample survives to a large extent the extreme excitation and shock wave propagation inside the colloidal crystal. This feature enables probing the excited crystal, using the powerful Bragg peak analysis, in contrast to the conventional studies of dense plasma created from bulk samples for which probing with Bragg diffraction technique is not possible. X-ray diffraction measurements of excited colloidal crystals may then lead towards a better understanding of matter phase transitions under extreme irradiation conditions.

Intermode beating mode-locking: Toward compact 2 µm short-pulse all-fiber lasers

Abstract We have successfully applied intermode beating mode-locking (IBML) for short pulse generation in compact Tm3+-doped double clad fiber lasers (TDFLs). This IBML technique is implemented by longitudinal-mode frequency matching between pump source and laser oscillator without extra mode-locker. Here, the achieved IBML TDFLs can respectively generate 2056.47 nm, 2003.88 nm and 1963.79 nm short pulses. Their output characteristics have been respectively investigated. The IBML technique enables the large pulse energy of ~20.8 nJ with pulse repetition rate of ~9.6 MHz. This work may open a new route for next-generation of high performance mid-infrared short-pulse all-fiber laser sources.

The performance of ESS spectrometers in comparison with instruments at a short-pulse source

The performance of ESS spectrometers in comparison with instruments at a short-pulse source

Acoustic response of a laser-excited polycrystalline Au-film studied by ultrafast Debye–Scherrer diffraction at a table-top short-pulse x-ray source

The transient acoustic response of a free-standing, polycrystalline thin Au-film upon femtosecond optical excitation has been studied by time-resolved Debye–Scherrer x-ray diffraction using ultrashort Cu Kα x-ray pulses from a laser-driven plasma x-ray source. The temporal strain evolution has been determined from the transient shifts of multiple Bragg diffraction peaks. The experimental data are in good agreement with the results of calculations based on the two-temperature model and an acoustic model assuming uniaxial strain propagation in the laser-excited thin film.

High-Spatial-Resolution Brillouin Optical Correlation Domain Analysis Using Short-Pulse Optical Sources

High-spatial-resolution distributed optical fiber sensing based on Brillouin optical correlation domain analysis (BOCDA) using short-pulse optical sources is demonstrated. Short optical pulses phase-modulated with a pseudo random bit stream (PRBS) are used for pump and Stokes probe lights for the correlation-domain measurement of distributions of local Brillouin frequency shift (BFS). Because the excitation of acoustic wave is tightly confined in short length along the fiber owing to the short pulse duration, spatial resolution of the measurement is improved as compared with the case using not-return-to-zero pulses. In this experiment, BFS variation in a 2 mm standard single-mode fiber section is clearly detected by the use of an external cavity mode-locked semiconductor laser emitting 3 ps pulses as the light source. Spatial resolution shorter than 1 mm is obtained. Scanning of measurement location is achieved electrically by changing the clock speed of the PRBS, which is an advantage of the proposed scheme over the high-spatial-resolution BOCDA using amplified spontaneous emission as a light source. Theoretical estimation of Brillouin gain that the short-pulse Stokes probe light acquires is presented.

Quantum random numbers from a fiber-optic photon-pair source

We demonstrate a fiber-optic source of random numbers in which random-number generation (RNG) is implemented via an inherently quantum process—detection of correlated photon pairs produced by spontaneous four-wave mixing (FWM) inside an optical fiber. The detection times of individual photon pairs, generated in our RNG scheme via phase-matched FWM in a highly nonlinear photonic-crystal fiber with a suitable dispersion profile, are converted into megabit-size binary sequences, serving as strings of random numbers. The randomness of these bit strings is verified by the National Institute of Standards and Technology Statistical Test Suite. We identify FWM regimes in which mode-locked short-pulse laser sources can provide a high peak power needed to drive photon-pair generation in optical fibers without an excessive loss of randomness due to unwanted patterns in the RNG photon-pair output.

Laser-driven radiation: Biomarkers for molecular imaging of high dose-rate effects.

Recently developed short‐pulsed laser sources garner high dose‐rate beams such as energetic ions and electrons, x rays, and gamma rays. The biological effects of laser‐generated ion beams observed in recent studies are different from those triggered by radiation generated using classical accelerators or sources, and this difference can be used to develop new strategies for cancer radiotherapy. High‐power lasers can now deliver particles in doses of up to several Gy within nanoseconds. The fast interaction of laser‐generated particles with cells alters cell viability via distinct molecular pathways compared to traditional, prolonged radiation exposure. The emerging consensus of recent literature is that the differences are due to the timescales on which reactive molecules are generated and persist, in various forms. Suitable molecular markers have to be adopted to monitor radiation effects, addressing relevant endogenous molecules that are accessible for investigation by noninvasive procedures and enable translation to clinical imaging. High sensitivity has to be attained for imaging molecular biomarkers in cells and in vivo to follow radiation‐induced functional changes. Signal‐enhanced MRI biomarkers enriched with stable magnetic nuclear isotopes can be used to monitor radiation effects, as demonstrated recently by the use of dynamic nuclear polarization (DNP) for biomolecular observations in vivo. In this context, nanoparticles can also be used as radiation enhancers or biomarker carriers. The radiobiology‐relevant features of high dose‐rate secondary radiation generated using high‐power lasers and the importance of noninvasive biomarkers for real‐time monitoring the biological effects of radiation early on during radiation pulse sequences are discussed.

A compact Fourier transform spectrometer with no moving parts for laser induced breakdown spectroscopy

We report the development of a portable single-shot Fourier transform spectrometer for application in laser induced breakdown spectroscopy (LIBS) with a particular emphasis on remote or standoff LIBS. The device, based on a design by Harvey and Padgett[1], has a number of inherent features that give it performance advantages over a portable grating spectrometer, in particular a gain in signal to noise ratio (SNR). Tests with low-pressure discharge lamps show a gain in SNR of at least five times that of a traditional portable grating spectrometer in stand-off mode. In addition, the instrument has also demonstrated the ability to measure the spectrum from a short or ultrashort pulsed source in a single shot, e.g. a laser produced plasma.

High precision laser macrodrilling of carbon fiber reinforced plastics with a new nanosecond pulsed laser—Optimized toward industrial needs

Due to their outstanding strength-to-weight ratio, carbon fiber reinforced plastics (CFRP) are of high interest for lightweight construction. Within industrial fields requiring lightweight design and energy efficiency, the demand for CFRP increases continuously. Until now, machining of CFRP, as a crucial step in the production chain of composite parts, is mainly performed by conventional techniques such as drilling, milling, and abrasive water jet cutting. These techniques come with known drawbacks, including force input, handling of auxiliaries, and tool wear. The laser cutting of CFRP has already shown its high potential to be a practical alternative due to wear-free and contactless processing. However, if sufficient cutting rates are required, laser cutting has to overcome the challenge of either nonorthogonal cutting edges when using short pulsed high power lasers or heat input into the material when using continuously emitting (continuous wave) high power singlemode laser sources, as shown by Staehr et al. [J. Laser Appl. 28, 022203 (2016)]. Within this investigation, the machining with a novel short pulsed high power laser source by Trumpf Laser GmbH emitting at λ = 1030 nm will be examined. The lab-state laser source provides nanosecond pulses at tp = 20 ns, at a maximum pulse energy of Ep = 100 mJ, and a maximum average power of Pavg = 1.5 kW, while maintaining a very good beam quality allowing for small focus diameters down to df = 84 μm with the focusing optics used. It was shown that processing with small focus diameters results in narrow cutting kerfs contributing to the cutting edge angle. In addition, the investigations revealed a minimized thermal load resulting from the short pulses. At the same time, challenges resulting from shielding effects when specific parameters are used will be displayed and discussed. Different processing strategies for adjusting the cutting edge angle toward orthogonality will be presented in this paper. This optimization procedure aims at the drilling of macroscopic holes with a diameter of Ø > 4 mm in CFRP laminates with a thickness of s ≥ 2 mm at very high precision.

Computation of intensity and polarization state of diffracted fields from reflective gratings in conical geometry.

Conical diffraction geometry has interesting properties for monochromatization of short pulse sources of extreme-UV or soft X-ray radiation, like high harmonic generation in gases. It enables high diffraction efficiency at the cost of a reduced angular dispersion. Computation of grating efficiency is complex when the incidence plane is not perpendicular to the grating lines because S and P polarizations are coupled. We have therefore developed a special code, named COROX, which implements a rigorous coupled-wave analysis algorithm. The code is able to compute the complex diffracted field from any incidence direction and polarization on any kind of grating (thick lamellar and blazed gratings with single or multilayer coating). COROX is developed with C++ and uses an eigen linear algebra library. This paper outlines the method of computation, and the results are given for a 150 l/mm blazed grating in a conical Littrow configuration.

Quality Control of High-Speed Photon Detectors

High-speed-photon detectors are some of the most important tools for observations of high energy cosmic rays. As technologies of photon detectors and their read-out electronics improved rapidly, the time resolution of some cosmic ray detectors became better than one nanosecond. To utilize such devices effectively, calibrations using a short-pulse light source are necessary. We have developed a pulsed laser of 80 picosecond width and adjustable peak intensity up to 100 mW. This pulsed laser is composed of a simple electric circuit and a laser diode. Details of this pulsed laser and its application for quality controls of photon detectors are reported in this contribution.

Atomic-scale imaging of ultrafast materials dynamics

Abstract

The ultrafast X-ray spectroscopic revolution in chemical dynamics

The past two decades have seen rapid developments in short-pulse X-ray sources, which have enabled the study of nuclear and electronic dynamics by ultrafast X-ray spectroscopies with unprecedented time resolution ranging from nanoseconds to attoseconds. In this Perspective, we discuss some of the major achievements in the study of nuclear and electronic dynamics with X-ray pulses produced by high-harmonic, free-electron-laser and synchrotron sources. The particular dynamic processes probed by X-ray radiation highlighted in this Perspective are electronic coherences on attosecond to femtosecond timescales, chemical reactions, such as dissociations, and pericyclic ring-openings, spin-crossover dynamics, ligand-exchange dynamics and structural deformations in excited states. X-ray spectroscopic probing of chemical dynamics holds great promise for the future owing to the ongoing developments of new spectroscopies, such as four-wave mixing, and the continuous improvements in emerging laboratory-based, high-harmonic sources and large-scale, facility-based, free-electron lasers.

Continuous-wave multiphoton photoemission from plasmonic nanostars

Highly nonlinear optical processes require high intensities, typically achieved with ultrashort laser pulses, and hence, they were first observed with the advent of picosecond laser technology. An alternative approach for reaching the required field intensities is offered by localized optical resonances in tailored plasmonic nanostructures, enabling the enhancement of a multitude of nonlinear phenomena. However, so far, plasmon-enhanced high-order nonlinear effects have been restricted to experiments involving short-pulsed and ultrafast laser sources. Here, we demonstrate localized three-photon photoemission from chemically synthesized plasmonic gold nanostars under continuous-wave illumination at sub-MWcm−2 incident intensities. Intensity- and polarization-dependent measurements confirm the nonlinearity of the photoemission process and agree with quantum mechanical calculations of the electron yield from nanostar tips with features smaller than 5 nm, which facilitate local intensity enhancement factors exceeding 1000. Our results open up new avenues for the design of accessible nanoscale coherent electron sources, with potential applications in microscopy, spectroscopy, sensing, and signal processing.

Giant collimated gamma-ray flashes

Bright sources of high energy electromagnetic radiation are widely employed in fundamental research as well as in industry and medicine. This steadily growing interest motivated the construction of several facilities aiming at the realisation of sources of intense X- and gamma-ray pulses. To date, free electron lasers and synchrotrons provide intense sources of photons with energies up to 10-100 keV. Facilities under construction based on incoherent Compton back scattering of an optical laser pulse off an electron beam are expected to yield photon beams with energy up to 19.5 MeV and peak brilliance in the range 10$^{20}$-10$^{23}$ photons s$^{-1}$ mrad$^{-2}$ mm$^{-2}$ per 0.1% bandwidth. Here, we demonstrate a novel mechanism based on the strongly amplified synchrotron emission which occurs when a sufficiently dense electron beam interacts with a millimetre thickness solid target. For electron beam densities exceeding approximately $3\times10^{19}\text{ cm$^{-3}$}$ filamentation instability occurs with the self-generation of 10$^{7}$-10$^{8}$ gauss magnetic fields where the electrons of the beam are trapped. This results into a giant amplification of synchrotron emission with the production of collimated gamma-ray pulses with peak brilliance above $10^{25}$ photons s$^{-1}$ mrad$^{-2}$ mm$^{-2}$ per 0.1% bandwidth and photon energies ranging from 200 keV up to several hundreds MeV. These findings pave the way to compact, high-repetition-rate (kHz) sources of short (30 fs), collimated (mrad) and high flux ($>10^{12}$ photons/s) gamma-ray pulses.

Evaluation of temperature history of a spherical nanosystem irradiated with various short-pulse laser sources.

Spatiotemporal thermal response and characteristics of net entropy production rate of a gold nanosphere (radius: 50-200 nm), subjected to a short-pulse, femtosecond laser is reported. In order to correctly illustrate the temperature history of laser-metal interaction(s) at picoseconds transient with a comprehensive single temperature definition in macroscale and to further understand how the thermophysical response of the single-phase lag (SPL) and dual-phase lag (DPL) frameworks (with various lag-ratios') differs, governing energy equations derived from these benchmark non-Fourier frameworks are numerically solved and thermodynamic assessment under both the classical irreversible thermodynamics (CIT) as well as extended irreversible thermodynamics (EIT) frameworks is subsequently carried out. Under the frameworks of SPL and DPL with small lag ratio, thermophysical anomalies such as temperature overshooting characterized by adverse temperature gradient is observed to violate the local thermodynamic equilibrium (LTE) hypothesis. The EIT framework, however, justifies the compatibility of overshooting of temperature with the second law of thermodynamics under a nonequilibrium paradigm. The DPL framework with higher lag ratio was however observed to remain free from temperature overshooting and finds suitable consistency with LTE hypothesis. In order to solve the dimensional non-Fourier governing energy equation with volumetric laser-irradiation source term(s), the lattice Boltzmann method (LBM) is extended and a three-time level, fully implicit, second order accurate finite difference method (FDM) is illustrated. For all situations under observation, the LBM scheme is featured to be computationally superior to remaining FDM schemes. With detailed prediction of maximum temperature rise and the corresponding peaking time by all the numerical schemes, effects of the change of radius of the gold nanosphere, the magnitude of fluence of laser, and laser irradiation with multiple pulses on thermal energy transport and lagging behavior (if any) are further elucidated at different radial locations of the gold nanosphere. Last, efforts are further made to address the thermophysical characteristics when effective thermal conductivity (with temporal and size effects) is considered instead of the usual bulk thermal conductivity.

Characterization of a high repetition-rate laser-driven short-pulsed neutron source

We demonstrate a repetitive, high flux, short-pulsed laser-driven neutron source using a heavy-water jet target. We measure neutron generation at 1/2 kHz repetition rate using several-mJ pulse energies, yielding a time-averaged neutron flux of 2 × 105 neutrons s−1 (into 4π steradians). Deuteron spectra are also measured in order to understand source characteristics. Analyses of time-of-flight neutron spectra indicate that two separate populations of neutrons, ‘prompt’ and ‘delayed’, are generated at different locations. Gamma-ray emission from neutron capture 1H(n,γ) is also measured to confirm the neutron flux.