Characteristics of X/γ-ray radiations by intense laser interactions with high-Z solids: The role of bremsstrahlung and radiation reactions

In this work, characteristics of X/γ-ray radiations by intense laser interactions with high-Z solids are investigated by means of a newlydeveloped particle-in-cell (PIC) simulation code. The PIC code takes advantage of the recently developed ionization and collision dynamicsmodels, which make it possible to model different types of materials based on their intrinsic atomic properties. Within the simulations, bothbremsstrahlung and nonlinear Compton scatterings have been included. Different target materials and laser intensities are considered forstudying the parameter-dependent features of X/γ-ray radiations. The relative strength and angular distributions of X/γ ray productions frombremsstrahlung and nonlinear Compton scatterings are compared to each other. The threshold under which the nonlinear Compton scatteringsbecome dominant over bremsstrahlung is also outlined.

Temperature and structure measurements of heavy-ion-heated diamond using in situ X-ray diagnostics

We present in situ measurements of spectrally resolved X-ray scattering and X-ray diffraction from monocrystalline diamond samples heatedwith an intense pulse of heavy ions.In this way,we determine the samples’heating dynamics and their microscopic and macroscopic structuralintegrity over a timespan of several microseconds.Connecting the ratio of elastic to inelastic scattering with state-of-the-art density functionaltheory molecular dynamics simulations allows the inference of average temperatures around 1300 K,in agreement with predictions fromstopping power calculations.The simultaneous diffraction measurements show no hints of any volumetric graphitization of the material,butdo indicate the onset of fracture in the diamond sample.Our experiments pave the way for future studies at the Facility for Antiproton andIon Research,where a substantially increased intensity of the heavy ion beam will be available.

Underwater ghost imaging with pseudo-Bessel-ring modulation pattern

In this study,we propose an underwater ghost-imaging scheme using a modulation pattern combining offset-position pseudo-Bessel-ring(OPBR)and random binary(RB)speckle pattern illumination.We design the experiments based on modulation rules to order the OPBR speckle patterns.We retrieve ghost images by OPBR beam with different modulation speckle sizes.The obtained ghost images have a better contrast-to-noise rate compared to RB beam ghost imaging under the same conditions.We verify the results both in the experiment and simulation.In addition,we also check the image quality at different turbidities.Furthermore,we demonstrate that the OPBR speckle pattern also provides better image quality in other objects.The proposed method promises wide applications in highly scattering media,atmosphere,turbid water,etc.

Yb-doped large-pitch fibres: effective single-mode operation based on higher-order mode delocalisation

Rare earth-doped fibres are a diode-pumped,solid-state laser architecture that is highly scalable in average power.The performance of pulsed fibre laser systems is restricted due to nonlinear effects.Hence,fibre designs that allow for very large mode areas at high average powers with diffraction-limited beam quality are of enormous interest.Ytterbium-doped,rod-type,large-pitch fibres(LPF)enable extreme fibre dimensions,i.e.,effective single-mode fibres with mode sizes exceeding 100 times the wavelength of the guided radiation,by exploiting the novel concept of delocalisation of higher-order transverse modes.The non-resonant nature of the operating principle makes LPF suitable for high power extraction.This design allows for an unparalleled level of performance in pulsed fibre lasers.

A concept for multiterawatt fibre lasers based on coherent pulse stacking in passive cavities

Since the advent of femtosecond lasers,performance improvements have constantly impacted on existing applications and enabled novel applications.However,one performance feature bearing the potential of a quantum leap for high-field applications is still not available:the simultaneous emission of extremely high peak and average powers.Emerging applications such as laser particle acceleration require exactly this performance regime and,therefore,challenge laser technology at large.On the one hand,canonical bulk systems can provide pulse peak powers in the multi-terawatt to petawatt range,while on the other hand,advanced solid-state-laser concepts such as the thin disk,slab or fibre are well known for their high efficiency and their ability to emit high average powers in the kilowatt range with excellent beam quality.In this contribution,a compact laser system capable of simultaneously providing high peak and average powers with high wall-plug efficiency is proposed and analysed.The concept is based on the temporal coherent combination(pulse stacking)of a pulse train emitted from a high-repetition-rate femtosecond laser system in a passive enhancement cavity.Thus,the pulse energy is increased at the cost of the repetition rate while almost preserving the average power.The concept relies on a fast switching element for dumping the enhanced pulse out of the cavity.The switch constitutes the key challenge of our proposal.Addressing this challenge could,for the first time,allow the highly efficient dumping of joule-class pulses at megawatt average power levels and lead to unprecedented laser parameters.

Ultrahigh temporal contrast performance of the PHELIX petawatt facility

We report on the temporal contrast performance of the PHELIX facility in view of the requirements imposed by solidtarget interaction experiments. The requirement analysis for the nanosecond and picosecond temporal contrast is derived from empirical data and simple theoretical modeling, while the realization shows that using an ultrafast optical parametric amplifier and plasma mirrors enables meeting this specification.

Implementation of a phase plate for the generation of homogeneous focal-spot intensity distributions at the high-energy short-pulse laser facility PHELIX

We propose and demonstrate the use of random phase plates(RPPs)for high-energy sub-picosecond lasers.Contrarily to previous work related to nanosecond lasers,an RPP poses technical challenges with ultrashort-pulse lasers.Here,we implement the RPP near the beginning of the amplifier and image-relay it throughout the laser amplifier.With this,we obtain a uniform intensity distribution in the focus over an area 1600 times the diffraction limit.This method shows no significant drawbacks for the laser and it has been implemented at the PHELIX laser facility where it is now available for users.

Resonance-enhanced multi-octave supercontinuum generation in antiresonant hollow-core fibers

Ultrafast supercontinuum generation in gas-filled waveguides is an enabling technology for many intriguing applications ranging from attosecond metrology towards biophotonics,with the amount of spectral broadening crucially depending on the pulse dispersion of the propagating mode.In this study,we show that structural resonances in a gas-filled antiresonant hollow core optical fiber provide an additional degree of freedom in dispersion engineering,which enables the generation of more than three octaves of broadband light that ranges from deep UV wavelengths to near infrared.Our observation relies on the introduction of a geometric-induced resonance in the spectral vicinity of the ultrafast pump laser,outperforming gas dispersion and yielding a unique dispersion profile independent of core size,which is highly relevant for scaling input powers.Using a krypton-filled fiber,we observe spectral broadening from 200 nm to 1.7μm at an output energy of B 23μJ within a single optical mode across the entire spectral bandwidth.Simulations show that the frequency generation results from an accelerated fission process of solitonlike waveforms in a non-adiabatic dispersion regime associated with the emission of multiple phase-matched Cherenkov radiations on both sides of the resonance.This effect,along with the dispersion tuning and scaling capabilities of the fiber geometry,enables coherent ultra-broadband and high-energy sources,which range from the UV to the mid‐infrared spectral range.

Petawatt and exawatt class lasers worldwide

In the 2015 review paper‘Petawatt Class Lasers Worldwide’a comprehensive overview of the current status of highpower facilities of>200 TW was presented.This was largely based on facility specifications,with some description of their uses,for instance in fundamental ultra-high-intensity interactions,secondary source generation,and inertial confinement fusion(ICF).With the 2018 Nobel Prize in Physics being awarded to Professors Donna Strickland and Gerard Mourou for the development of the technique of chirped pulse amplification(CPA),which made these lasers possible,we celebrate by providing a comprehensive update of the current status of ultra-high-power lasers and demonstrate how the technology has developed.We are now in the era of multi-petawatt facilities coming online,with 100 PW lasers being proposed and even under construction.In addition to this there is a pull towards development of industrial and multi-disciplinary applications,which demands much higher repetition rates,delivering high-average powers with higher efficiencies and the use of alternative wavelengths:mid-IR facilities.So apart from a comprehensive update of the current global status,we want to look at what technologies are to be deployed to get to these new regimes,and some of the critical issues facing their development.

Accelerating ions with high-energy short laser pulses from submicrometer thick targets

Using the example of the PHELIX high-energy short pulse laser we discuss the technical preconditions to investigate ion acceleration with submicrometer thick targets. We show how the temporal contrast of this system was improved to prevent pre-ionization of such targets on the nanosecond timescale. Furthermore the influence of typical fluctuations or uncertainties of the on-target intensity on ion acceleration experiments is discussed. We report how these uncertainties were reduced by improving the assessment and control of the on-shot intensity and by optimizing the positioning of the target into the focal plane. Finally we report on experimental results showing maximum proton energies in excess of 85 MeV for ion acceleration via the target normal sheath acceleration mechanism using target thicknesses on the order of one micrometer.

Nonlinear ionization dynamics of hot dense plasma observed in a laser-plasma amplifier

Understanding the behaviour of matter under conditions of extreme temperature,pressure,density and electromagnetic fields has profound effects on our understanding of cosmologic objects and the formation of the universe.Lacking direct access to such objects,our interpretation of observed data mainly relies on theoretical models.However,such models,which need to encompass nuclear physics,atomic physics and plasma physics over a huge dynamic range in the dimensions of energy and time,can only provide reliable information if we can benchmark them to experiments under well-defined laboratory conditions.Due to the plethora of effects occurring in this kind of highly excited matter,characterizing isolated dynamics or obtaining direct insight remains challenging.High-density plasmas are turbulent and opaque for radiation below the plasma frequency and allow only near-surface insight into ionization processes with visible wavelengths.Here,the output of a high-harmonic seeded laser-plasma amplifier using eightfold ionized krypton as the gain medium operating at a 32.8 nm wavelength is ptychographically imaged.A complexvalued wavefront is observed in the extreme ultraviolet(XUV)beam with high resolution.Ab initio spatio-temporal Maxwell–Bloch simulations show excellent agreement with the experimental observations,revealing overionization of krypton in the plasma channel due to nonlinear laser-plasma interactions,successfully validating this four-dimensional multiscale model.This constitutes the first experimental observation of the laser ion abundance reshaping a laserplasma amplifier.The presented approach shows the possibility of directly modelling light-plasma interactions in extreme conditions,such as those present during the early times of the universe,with direct experimental verification.

Factor 30 Pulse Compression by Hybrid Multipass Multiplate Spectral Broadening

As ultrafast laser technology advances towards ever higher peak and average powers,generating sub-50 fs pulses from laser architectures that exhibit best power-scaling capabilities remains a major challenge.Here,we present a very compact and highly robust method to compress 1.24 ps pulses to 39fs by means of only a single spectral broadening stage which neither requires vacuum parts nor custom-made optics.Our approach is based on the hybridization of the multiplate continuum and.the multipass cell spectral broadening techniques.Their combination leads to significantly higher spectral broadening factors in bulk material than what has been reported from either method alone.Moreover,our approach efficiently suppresses adverse features of single-pass bulk spectral broadening.We use a burst-mode Yb:YAG laser emitting pulses with 80 MW peak power that are enhanced to more than 1 GW after postcompression.With only 0.19%rms pulse-to-pulse energy fluctuations,the technique exhibits excellent stability.Furthermore,we have measured state-of-the-art spectral-spatial homogeneity and good beam quality of M^(2)=1.2 up to a spectral broadening factor of 30.Due to the method's simplicity,compactness,and scalability,it is highly attractive for turning a picosecond laser into an ultrafast light source that generates pulses of only a few tens of femtoseconds duration.

Optical probing of ultrafast laser-induced solid-to-overdense-plasma transitions

Understanding the solid target dynamics resulting from the interaction with an ultrashort laser pulse is a challenging fundamental multi-physics problem involving atomic and solid-state physics,plasma physics,and laser physics.Knowledge of the initial interplay of the underlying processes is essential to many applications ranging from lowpower laser regimes like laser-induced ablation to high-power laser regimes like laser-driven ion acceleration.Accessing the properties of the so-called pre-plasma formed as the laser pulse’s rising edge ionizes the target is complicated from the theoretical and experimental point of view,and many aspects of this laser-induced transition from solid to overdense plasma over picosecond timescales are still open questions.On the one hand,laser-driven ion acceleration requires precise control of the pre-plasma because the efficiency of the acceleration process crucially depends on the target properties at the arrival of the relativistic intensity peak of the pulse.On the other hand,efficient laser ablation requires,for example,preventing the so-called“plasma shielding”.By capturing the dynamics of the initial stage of the interaction,we report on a detailed visualization of the pre-plasma formation and evolution.Nanometer-thin diamond-like carbon foils are shown to transition from solid to plasma during the laser rising edge with intensities<10^(16)W/cm^(2).Single-shot near-infrared probe transmission measurements evidence sub-picosecond dynamics of an expanding plasma with densities above 10^(23)cm^(−3)(about 100 times the critical plasma density).The complementarity of a solid-state interaction model and kinetic plasma description provides deep insight into the interplay of initial ionization,collisions,and expansion.