Pulse-length dependence of the anisotropy of laser-driven cluster explosions: transition to the impulsive regime for pulses approaching the few-cycle limit

Unveiling the Inhomogeneous Nature of Strong Field Ionization in Extended Systems

Intense light-induced fragmentation of spherical clusters produces highly energetic ions with characteristic spatial distributions. By subjecting argon clusters to a wavelength tunable laser, we show that ion emission energy and anisotropy can be controlled through the wavelength-isotropic and energetic for shorter wavelengths and increasingly anisotropic at longer wavelengths. The anisotropic part of the energy spectrum, consisting of multiply charged high-energy ions, is considerably more prominent at longer wavelengths. Classical molecular dynamics simulations reveal that cluster ionization occurs inhomogeneously producing a columnlike charge distribution along the laser polarization direction. This previously unknown distribution results from the dipole response of the neutral cluster which creates an enhanced field at the surface, preferentially triggering ionization at the poles. The subsequently formed nanoplasma provides an additional wavelength-dependent ionization mechanism through collisional ionization, efficiently homogenizing the system only at short wavelengths close to resonance. Our results open the door to studying polarization induced effects in nanostructures and complex molecules and provide a missing piece in our understanding of anisotropic ion emission.

Anisotropic Surface Broadening and Core Depletion during the Evolution of a Strong-Field Induced Nanoplasma

Strong-field ionization of nanoscale clusters provides excellent opportunities to study the complex correlated electronic and nuclear dynamics of near-solid density plasmas. Yet, monitoring ultrafast, nanoscopic dynamics in real-time is challenging, which often complicates a direct comparison between theory and experiment. Here, near-infrared laser-induced plasma dynamics in ∼600  nm diameter helium droplets are studied by femtosecond time-resolved x-ray coherent diffractive imaging. An anisotropic, ∼20  nm wide surface region, defined as the range where the density lies between 10% and 90% of the core value, is established within ∼100  fs, in qualitative agreement with theoretical predictions. At longer timescales, however, the width of this region remains largely constant while the radius of the dense plasma core shrinks at average rates of ≈71  nm/ps along and ≈33  nm/ps perpendicular to the laser polarization. These dynamics are not captured by previous plasma expansion models. The observations are phenomenologically described within a numerical simulation; details of the underlying physics, however, remain to be explored.

Structure and energetics of microscopically inhomogeneous nanoplasmas in exploding clusters

We present a theoretical-computational study of the formation, structure, composition, energetics, dynamics and expansion of nanoplasmas consisting of high-energy matter on the nanoscale of ions and electrons. Molecular dynamics simulations explored the structure and energetics of hydrogen and neon persistent nanoplasmas formed under the condition of incomplete outer ionization by the laser field. We observed a marked microscopic inhomogeneity of the structure and the charge distribution of exploding nanoplasmas on the nanoscale. This is characterized by a nearly neutral, uniform, interior domain observed for the first time, and a highly positively charged, exterior domain, with these two domains being separated by a transition domain. We established the universality of the general features of the shape of the charge distribution, as well as of the energetics and dynamics of individual ions in expanding persistent nanoplasmas containing different positive ions. The inhomogeneous three-domain shell structure of exploding nanoplasmas exerts major effects on the local ion energies, which are larger by one order of magnitude in the exterior, electron-depleted domain than in the interior, electron-rich domain, with the major contribution to the ion energies originating from electrostatic interactions. The radial structural inhomogeneity of exploding nanoplasmas bears analogy to the inhomogeneous transport regime in expanded and supercritical metals undergoing metal-nonmetal transition.

Evaporation of an anisotropic nanoplasma

Intense laser induced plasma dynamics in sub-micron scale helium droplets are monitored by femtosecond time-resolved X-ray coherent diffractive imaging. Anisotropic surface softening and strongly anisotropic shrinking of the plasma core are observed.

Attosecond physics at the nanoscale

Recently two emerging areas of research, attosecond and nanoscale physics, have started to come together. Attosecond physics deals with phenomena occurring when ultrashort laser pulses, with duration on the femto- and sub-femtosecond time scales, interact with atoms, molecules or solids. The laser-induced electron dynamics occurs natively on a timescale down to a few hundred or even tens of attoseconds (1 attosecond  =  1 as  =  10^-18 s), which is comparable with the optical field. For comparison, the revolution of an electron on a 1s orbital of a hydrogen atom is  ∼152 as. On the other hand, the second branch involves the manipulation and engineering of mesoscopic systems, such as solids, metals and dielectrics, with nanometric precision. Although nano-engineering is a vast and well-established research field on its own, the merger with intense laser physics is relatively recent. In this report on progress we present a comprehensive experimental and theoretical overview of physics that takes place when short and intense laser pulses interact with nanosystems, such as metallic and dielectric nanostructures. In particular we elucidate how the spatially inhomogeneous laser induced fields at a nanometer scale modify the laser-driven electron dynamics. Consequently, this has important impact on pivotal processes such as above-threshold ionization and high-order harmonic generation. The deep understanding of the coupled dynamics between these spatially inhomogeneous fields and matter configures a promising way to new avenues of research and applications. Thanks to the maturity that attosecond physics has reached, together with the tremendous advance in material engineering and manipulation techniques, the age of atto-nanophysics has begun, but it is in the initial stage. We present thus some of the open questions, challenges and prospects for experimental confirmation of theoretical predictions, as well as experiments aimed at characterizing the induced fields and the unique electron dynamics initiated by them with high temporal and spatial resolution.

Strong-field ionization of clusters using two-cycle pulses at 1.8 μm

The interaction of intense laser pulses with nanoscale particles leads to the production of high-energy electrons, ions, neutral atoms, neutrons and photons. Up to now, investigations have focused on near-infrared to X-ray laser pulses consisting of many optical cycles. Here we study strong-field ionization of rare-gas clusters (103 to 105 atoms) using two-cycle 1.8 μm laser pulses to access a new interaction regime in the limit where the electron dynamics are dominated by the laser field and the cluster atoms do not have time to move significantly. The emission of fast electrons with kinetic energies exceeding 3 keV is observed using laser pulses with a wavelength of 1.8 μm and an intensity of 1 × 1015 W/cm2, whereas only electrons below 500 eV are observed at 800 nm using a similar intensity and pulse duration. Fast electrons are preferentially emitted along the laser polarization direction, showing that they are driven out from the cluster by the laser field. In addition to direct electron emission, an electron rescattering plateau is observed. Scaling to even longer wavelengths is expected to result in a highly directional current of energetic electrons on a few-femtosecond timescale.

High performance charge-state resolving ion energy analyzer optimized for intense laser studies on low-density cluster targets

We report on a versatile ion analyzer which is capable to resolve ion charge states and energies with a resolution of E/ΔE = 100 at 75 keV/nucleon. Charge states are identified by their characteristic deflection in a magnetic field, whereas the ion energies are independently determined by a time-of-flight measurement. To monitor the signals a delay-line detector is used which records ion impact positions and times in each laser shot. Compared to conventional Thomson parabola spectrometers our instrument provides a low background measurement, hence a superior dynamic range. Further features are an improved energy resolution and a significantly increased transmission. We demonstrate the performance by showing charge-state resolved ion energy spectra from the Coulomb explosion of a low-density target, i.e., silver clusters exposed to intense femtosecond laser pulses.

Size-dependent high-order harmonic generation in rare-gas clusters

High-order harmonic generation (HHG) is investigated in rare-gas clusters as a function of the cluster size using 0.8 and 1.3  μm femtosecond lasers. A characteristic, species-dependent knee structure in the single particle response is observed. A 1D recollision model qualitatively reproduces this behavior and associates it to the degree of delocalization of the initial wave function. Small clusters are observed to have a higher efficiency than monomers but rapidly lose this advantage as the size increases. The implications of these findings on the HHG mechanism in clusters are discussed.

Time-resolved x-ray imaging of anisotropic nanoplasma expansion

A complete time-resolved x-ray imaging experiment of laser heated solid-density hydrogen clusters is modeled by microscopic particle-in-cell simulations that account self-consistently for the microscopic cluster dynamics and electromagnetic wave evolution. A technique is developed to retrieve the anisotropic nanoplasma expansion from the elastic and inelastic x-ray scattering data. Our method takes advantage of the self-similar evolution of the nanoplasma density and enables us to make movies of ultrafast nanoplasma dynamics from pump-probe x-ray imaging experiments.

Photoionization of clusters in intense few-cycle near infrared femtosecond pulses

In this article we present a perspective on the current state of the art in the photoionization of atomic clusters in few-cycle near-infrared laser pulses.

Inhomogeneous high harmonic generation in krypton clusters

High order harmonic generation from clusters is a controversial topic: conflicting theories exist, with different explanations for similar experimental observations. From an experimental point of view, separating the contributions from monomers and clusters is challenging. By performing a spectrally and spatially resolved study in a controlled mixture of clusters and monomers, we are able to isolate a region of the spectrum where the emission purely originates from clusters. Surprisingly, the emission from clusters is depolarized, which is the signature of statistical inhomogeneous emission from a low-density source. The harmonic response to laser ellipticity shows that this generation is produced by a new recollisional mechanism, which opens the way to future theoretical studies.

Coulomb explosion of uniformly charged spheroids

A simple, semianalytical model is proposed for nonrelativistic Coulomb explosion of a uniformly charged spheroid. This model allows us to derive the time-dependent particle energy distributions. Simple expressions are also given for the characteristic explosion time and maximum particle energies in the limits of extreme prolate and oblate spheroids as well as for the sphere. Results of particle simulations are found to be in remarkably good agreement with the model.

Monitoring atomic cluster expansion by high-harmonic generation

High-harmonic generation is shown to be capable of providing time-resolved information about the particle density of a complex system. As an example, we study numerically high-harmonic generation from expanding xenon clusters in a pump-probe laser scheme, where the pump laser pulse induces the cluster explosion and the probe pulse generates harmonics in the expanding cluster. We show that the high-harmonic spectra characterize the properties of the expanding cluster. Hence, measuring the dependence of the harmonic signal on the pump-probe delay suggests itself as an experimental tool to monitor many-particle dynamics with unique temporal resolution; based on optical measurements, this technique is naturally free from any spatial charge effects.

Time-resolved analysis of strong-field induced plasmon oscillations in metal clusters by spectral interferometry with few-cycle laser fields

We propose a scheme for ultrafast real-time imaging of laser-induced collective electron oscillations (Mie plasmons) in gas phase metal clusters by interferometrically stable scanning of two intense few-cycle optical laser pulses. The feasibility of our nonlinear spectral interferometry method with experimentally accessible observables is tested in a theoretical case study on simple-metal clusters (Na(147)). The results show that the plasmon period and lifetime as well as the phase and relative amplitude of the collective electron motion can be extracted with sub-fs resolution. The access to nonlinear response effects, as the demonstrated increase of the plasmon lifetime with laser intensity due to ionization-induced contraction of the electron cloud, opens up vast opportunities for interrogating ultrafast many-particle dynamics in nanosystems under strong laser fields with unprecedented resolution.