arXiv subjects

S. Kahaly

Publications and source records attributed to S. Kahaly.

12 recordsLinked to original sources

arXiv2023

Energetic, tunable, highly-elliptically polarized higher harmonics generated by intense two-color counter rotating laser fields

In this work, we demonstrate experimentally the efficient generation and tunability of energetic highly-elliptical high-harmonics in Ar gas, driven by intense two-color counter rotating laser electric fields. A bi-chromatic beam tailored by a MAZEL-TOV apparatus generates HHG, where the output spectrum of the highly elliptical HHG radiation can be tuned for a energy range of \DeltaE\HF150meV in the spectral range of ~20 eV with energy per pulse EXUV~400 nJ at the source. Furthermore we employ time-dependent density functional simulations to probe in-depth the dependence of the harmonic ellipticity and the strength of the isolated atto pulses on the driving field parameters and demonstrate the robustness of the HHG with the bichromatic field. We show how by properly tuning the central frequency of the second harmonic, the central frequency of the extreme ultraviolet (XUV) high harmonic radiation is continuously tuned. The demonstrated energy values largely exceed the output energy from many other laser driven attosecond sources reported so far and prove to be sufficient for inducing (along with tight XUV focusing geometries) the nonlinear processes in the atomic system. We envisage that such tunable energetic highly-elliptical HHG spectra can remove the facility restrictions from requirements of few-cycle driving pulses for isolated circular attosecond pulse generation.

physics.optics↗

arXiv2022

Two-XUV-photon double ionization of Neon studied at the Extreme Light Infrastructure (ELI-ALPS)

Two XUV-photon double ionization of Ne, induced by an intense few-pulse attosecond train with a ~ 4 fs envelope duration is investigated experimentally and theoretically. The experiment is performed at ELI-ALPS utilizing the recently constructed 10 Hz gas phase high-order harmonic generation SYLOS GHHG-COMPACT beamline. A total pulse energy up to ~1 μJ generated in Argon in conjunction with high reflectivity optics in the XUV region, allowed the observation of the doubly charged state of Ne induced by 40 eV central XUV photon energies. The interaction of the intense attosecond pulse train with Ne is also theoretically studied via a second-order time dependent perturbation theory equations-of-motion. The results of this work, combined with the feasibility of conducting XUV-pump-XUV-probe experiments, constitute a powerful tool for many potential applications. Those include attosecond pulse metrology as well as time resolved investigations of the dynamics underlying direct and sequential double ionization and their electron correlation effects.

physics.atom-ph↗

arXiv2020

Quantum path interferences in high-order harmonic generation from aligned diatomic molecules

Electron quantum path interferences in strongly laser-driven aligned molecules and their dependence on the molecular alignment is an essential open problem in strong-field molecular physics. Here, we demonstrate an approach which provides direct access to the observation of these interference processes. The approach is based on the combination of the time-gated-ion-microscopy technique with a pump-probe arrangement used to align the molecules and generate high-order harmonics. By spatially resolving the interference pattern produced by the spatiotemporal overlap of the harmonics emitted by the short and long electron quantum paths, we have succeeded in measuring in situ their phase difference and disclose their dependence on molecular alignment. The findings constitute a vital step towards an understanding of strong-field molecular physics and the development of attosecond spectroscopy approaches without the use of auxiliary atomic references.

physics.atom-ph↗

arXiv2019

Towards temporal characterization of intense isolated attosecond pulses from relativistic surface high harmonics

Relativistic surface high harmonics have been considered a unique source for the generation of intense isolated attosecond pulses in the extreme ultra-violet (XUV) and X-ray spectral range. However, its experimental realization is still a challenging task requiring identification of the optimum conditions for the generation of isolated attosecond pulses as well as their temporal characterization. Here, we demonstrate measurements in both directions. Particularly, we have made a first step towards the temporal characterization of the emitted XUV radiation by adapting the attosecond streak camera concept to identify the time domain characteristics of relativistic surface high harmonics. The results, supported by PIC simulations, set the upper limit for the averaged (over many shots) XUV duration to <6 fs, even when driven by not CEP controlled relativistic few-cycle optical pulses. Moreover, by measuring the dependence of the spectrum of the relativistic surface high harmonics on the carrier envelope phase (CEP) of the driving infrared laser field, we experimentally determined the optimum conditions for the generation of intense isolated attosecond pulses.

physics.plasm-ph↗

arXiv2018

Sub-cycle quantum electrodynamics in strongly laser-driven semiconductors

Electrodynamical processes induced in complex systems like semiconductors by strong electromagnetic fields, have traditionally/conventionally been described using semi-classical approaches. Although these approaches, allowed the investigation of ultrafast dynamics in solids culminating in multi-petahertz electronics, they do not provide any access in the quantum optical nature of the interaction as they treat the driving-field classically and unaffected by the interaction. Here, using a full quantum-optical approach, we demonstrate that the sub-cycle electronic response in a strongly driven semiconductor crystal is imprinted in the quantum-state of the driving-field resulting in non-classical light-states carrying the information of the interaction. This vital step towards strong-field ultrafast quantum electrodynamics unravels information inaccessible by conventional approaches and leads to the development of a new class non-classical light sources.

physics.optics↗

arXiv2015

Vacuum laser acceleration of relativistic electrons using plasma mirror injectors

Accelerating particles to relativistic energies over very short distances using lasers has been a long standing goal in physics. Among the various schemes proposed for electrons, vacuum laser acceleration has attracted considerable interest and has been extensively studied theoretically because of its appealing simplicity: electrons interact with an intense laser field in vacuum and can be continuously accelerated, provided they remain at a given phase of the field until they escape the laser beam. But demonstrating this effect experimentally has proved extremely challenging, as it imposes stringent requirements on the conditions of injection of electrons in the laser field. Here, we solve this long-standing experimental problem for the first time by using a plasma mirror to inject electrons in an ultraintense laser field, and obtain clear evidence of vacuum laser acceleration. With the advent of PetaWatt class lasers, this scheme could provide a competitive source of very high charge (nC) and ultrashort relativistic electron beams.

physics.plasm-ph↗

arXiv2015

Persistence of magnetic field driven by relativistic electrons in a plasma

The onset and evolution of magnetic fields in laboratory and astrophysical plasmas is determined by several mechanisms, including instabilities, dynamo effects and ultra-high energy particle flows through gas, plasma and interstellar-media. These processes are relevant over a wide range of conditions, from cosmic ray acceleration and gamma ray bursts to nuclear fusion in stars. The disparate temporal and spatial scales where each operates can be reconciled by scaling parameters that enable to recreate astrophysical conditions in the laboratory. Here we unveil a new mechanism by which the flow of ultra-energetic particles can strongly magnetize the boundary between the plasma and the non-ionized gas to magnetic fields up to 10-100 Tesla (micro Tesla in astrophysical conditions). The physics is observed from the first time-resolved large scale magnetic field measurements obtained in a laser wakefield accelerator. Particle-in-cell simulations capturing the global plasma and field dynamics over the full plasma length confirm the experimental measurements. These results open new paths for the exploration and modelling of ultra high energy particle driven magnetic field generation in the laboratory.

physics.plasm-ph↗

arXiv2013

Optical Properties of Relativistic Plasma Mirrors

The advent of ultrahigh-power femtosecond lasers creates a need for optical components suitable to handle ultrahigh light intensities. Due to the unavoidable laser-induced ionization of matter, these components will have to be based on a plasma medium. An archetype of such optical elements is a plasma mirror, created when an intense femtosecond laser pulse impinges on a solid target. It consists of a dense plasma, formed by the laser field itself, which specularly reflects the main part of the pulse. Plasma mirrors have major potential applications as active optical elements to manipulate the temporal and spatial properties of intense laser beams, in particular for the generation of intense attosecond pulses of light. We investigate the basic physics involved in the deformation of a plasma mirror resulting from the light pressure exerted by the ultraintense laser during reflection, by deriving a simple model of this fundamental process, which we validate both numerically and experimentally. The understanding of this deformation is essential for all future applications of plasma mirrors, especially for the generation of collimated attosecond beams. We show how its effect on the attosecond beam divergence can be mitigated by using the laser phase, thus providing crucial control for future applications in attosecond science.

physics.plasm-ph↗

arXiv2012

Short Intense Laser Pulse Collapse in Near-Critical Plasma

It is observed that the interaction of an intense ultra-short laser pulse with an overdense gas jet results in the pulse collapse and the deposition of a significant part of energy in a small and well localized volume in the rising part of the gas jet, where the electrons are efficiently accelerated and heated. A collisionless plasma expansion over 150 microns at a sub-relativistic velocity (~c/3) has been optically monitored in time and space, and attributed to the quasistatic field ionization of the gas associated to the hot electron current. Numerical simulations in good agreement with the observations suggest the acceleration in the collapse region of relativistic electrons, along with the excitation of a sizeable magnetic dipole that sustains the electron current over several picoseconds. Perspectives of ion beam generation at high repetition rate directly from gas jets are discussed.

physics.plasm-ph↗

arXiv2011

Anticorrelation between Ion Acceleration and Nonlinear Coherent Structures from Laser-Underdense Plasma Interaction

In laser-plasma experiments, we observed that ion acceleration from the Coulomb explosion of the plasma channel bored by the laser, is prevented when multiple plasma instabilities such as filamentation and hosing, and nonlinear coherent structures (vortices/post-solitons) appear in the wake of an ultrashort laser pulse. The tailoring of the longitudinal plasma density ramp allows us to control the onset of these insabilities. We deduced that the laser pulse is depleted into these structures in our conditions, when a plasma at about 10% of the critical density exhibits a gradient on the order of 250 μm (gaussian fit), thus hindering the acceleration. A promising experimental setup with a long pulse is demonstrated enabling the excitation of an isolated coherent structure for polarimetric measurements and, in further perspectives, parametric studies of ion plasma acceleration efficiency.

physics.plasm-ph↗

arXiv2005

Enhanced hard X-ray emission from femtosecond laser irradiated microdroplets

We make a comparative study of hard x-ray emission from 15 $μ$m methanol microdroplets and a plain slab target of similar atomic composition at similar laser intensities. The hard X-ray yield from droplet plasmas is $\simeq$ 35 times more than that obtained from solid plasmas. A prepulse that is about 10ns and about 5% of the main pulse is essential for hard x-ray generation from the droplets. A hot electron temperature of 36 keV is measured from the droplets at 8$\times10^{14}$ W cm$^{-2}$; three times higher intensity is needed to obtain similar hot electron temperature from solid plasmas with similar composition. We use 1D PIC simulation to obtain qualitative correlation to the experimental observations.

physics.plasm-ph↗

arXiv2005

Nanostructures and enhanced absorption in intense laser interaction with matter: effect of laser prepulses

Hard x-ray emission (20 - 200 keV) from plasmas produced by intense laser pulses on nanoparticle coated targets is compared with that from optically polished targets. The yield enhancement offered by nanoparticles is studied under different prepulse conditions. It is observed that the enhancement reduces when the nanoparticle coated target is irradiated with a prepulse with intensity greater than 10^13 W/cm^2. When the prepulse intensity exceeds 10^14 W/cm^2, the enhancement vanishes completely. This is attributed to preplasma formation on nanoparticles their subsequent structural modifcation before the arrival of the main pulse. It is suggested that high-contrast ultrashort pulses are essential for nanoparticles to function as yield enhancers.

physics.plasm-ph↗