Steplike intensity threshold behavior of extreme ionization in laser-driven xenon clusters

Transition from Surface to Volume Expansion in Argon Clusters Coulomb Explosion

The intensity-difference spectrum technique is applied to record charge-state resolved ion energy spectra from the Coulomb explosion of small Ar clusters under well-resolved laser intensity conditions. The far-reaching control of the experimental parameters permits us to identify a striking change in the expansion pattern of the nanoplasma beyond a given intensity. The simultaneous characterization of ion charge state and energy uncovers that a reduction of the laser intensity leads to a development of low energy cuts in the ion yields, not present at higher fluence. The complex interplay of outer ionization, recombination, ion screening, and the phenomenon of ionization saturation favors a surface-driven expansion at low plasma electron temperatures. With increasing laser intensity a transition into a volume-driven Coulomb explosion is observed.

Phase-of-the-Phase Electron Momentum Spectroscopy on Single Metal Atoms in Helium Nanodroplets

Magnesium atoms fully embedded in helium nanodroplets are exposed to two-color laser pulses, which trigger multiphoton above-threshold ionization (ATI). This allows exemplary study of the contribution of a dense, neutral, and finite medium on single electron propagation. The angular-resolved photoelectron spectra show striking differences with respect to results obtained on free atoms. Scattering of the individual Mg photoelectrons, when traversing the neutral helium environment, causes the angular distribution to become almost isotropic. Furthermore, the appearance of higher-energy electrons is observed, indicating the impact of the droplet on the concerted emission process. Phase-of-the-phase spectroscopy, however, reveals a marked loss in the 2ω-ω phase dependence of the electron signal. Taking into account sideband formation on a quantitative level, a Monte Carlo simulation which includes laser-assisted electron scattering can reproduce the experimental spectra and give insights into the strong-field-induced electron emission from disordered systems.

Volume averaging effect in nonlinear processes of focused laser fields

We report theoretical derivations and experimental results on the volume averaging effect of nonlinear processes in focused laser fields. This effect is considered detrimental in revealing the intensity dependence of a nonlinear process, caused by the intensity variation across the sampled volume of a focused laser. Following the treatment in the literature, we prove that if the signal dependence can be expressed as a simple power function of the laser intensity and if the detection region encompasses effectively the whole volume, volume average does not affect the final conclusion on the derived exponent. However, to reveal the detailed saturation effect of a multi-photon process, intensity selective scans involving spatial filters and displacement of the laser focus (z-scan) are required. Moreover, to fully capture the dependence of the signal on the variation of the laser intensity, the degree of spatial discrimination and the corresponding range of the z-scan need to be modeled carefully. Limitations in the dynamic range of the detector or the laser power, however, can thwart the desired scan range, resulting in erroneous fitting exponents. Using our nanosecond laser with a non-ideal Gaussian beam profile based on multiphoton ionization of argon atoms from a collimated molecular beam and from ambient argon gas, we report experimental measurements of the beam waist and Rayleigh range and compare the experimental intensity dependence of Ar⁺ with theoretical values. Agreements between theory and experiment are remarkable.

Temporal Development of a Laser-Induced Helium Nanoplasma Measured through Auger Emission and Above-Threshold Ionization

Femtosecond pump-probe electron and ion spectroscopy is applied to study the development of a helium nanoplasma up to the nanosecond timescale. Electrons, bound by the deep confining mean-field potential, are elevated toward the vacuum level in the nanoplasma expansion. Subsequent electron recombination gives rise to transitions between He^{+} states, resulting in autoionization. The time-resolved analysis of the energy transfer to quasifree electrons reveals a transient depletion of the Auger emission, which allows for a temporal gate to map the distribution of delocalized electrons in the developing mean field. Furthermore, we trace the recombination of delocalized electrons near the vacuum level into highly excited Rydberg states. Transient above-threshold ionization is introduced as a diagnostic tool to resolve the dynamics. Thus, the development of the electron distribution in the nanoplasma mean-field potential can be monitored via the features observed in the emission spectra.

Comparison of Electron and Ion Emission from Xenon Cluster-Induced Ignition of Helium Nanodroplets

The charging dynamics of helium droplets driven by embedded xenon cluster ignition in strong laser fields is studied by comparing the abundances of helium and highly charged Xe ions to the electron signal. Femtosecond pump-probe experiments show that near the optimal delay for highly charged xenon the electron yield increases, especially at low energies. The electron signature can be traced back to the ionization of the helium environment by Xe seed electrons. Accompanying molecular dynamics simulations suggest a two-step ionization scenario in the Xe-He core-shell system. In contrast to xenon, the experimental signal of the helium ions, as well as low-energy electron emission show a deviating delay dependence, indicating differences in the temporal and spacial development of the charge state distribution of Xe core and He surrounding. From the pump-probe dependence of the electron emission, effective temperatures can be extracted, indicating the nanoplasma decay.

Low-Energy Electron Emission in the Strong-Field Ionization of Rare Gas Clusters

Clusters and nanoparticles have been widely investigated to determine how plasmonic near fields influence the strong-field induced energetic electron emission from finite systems. We focus on the contrary, i.e., the slow electrons, and discuss a hitherto unidentified low-energy structure (LES) in the photoemission spectra of rare gas clusters in intense near-infrared laser pulses. For Ar and Kr clusters we find, besides field-driven fast electrons, a robust and nearly isotropic emission of electrons with <4  eV kinetic energies that dominates the total yield. Molecular dynamics simulations reveal a correlated few-body decay process involving quasifree electrons and multiply excited ions in the nonequilibrium nanoplasma that results in a dominant LES feature. Our results indicate that the LES emission occurs after significant nanoplasma expansion, and that it is a generic phenomenon in intense laser nanoparticle interactions, which is likely to influence the formation of highly charged ions.

Highly Charged Rydberg Ions from the Coulomb Explosion of Clusters

Ion emission from a nanoplasma produced in the interaction of intense optical laser pulses with argon clusters is studied resolving simultaneously charge states and recoil energies. By applying appropriate static electric fields we observe that a significant fraction of the ions Ar^{q+} (q=1-7) has electrons with binding energies lower than 150 meV; i.e., n_{Ryd}≥15 levels are populated. Charge state changes observed on a μs time scale can be attributed to electron emission due to autoionizing Rydberg states, indicating that high-ℓ Rydberg levels are populated as well. The experiments support theoretical predictions that a significant fraction of delocalized electrons, which are bound with hundreds of eV to the nanoplasma after the laser exposure, fill up meV bound ion states in the adiabatic expansion. We expect the process to be relevant for the long-term evolution of expanding laser-induced dense plasmas in general.

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.

Ionization Avalanching in Clusters Ignited by Extreme-Ultraviolet Driven Seed Electrons

We study the ionization dynamics of Ar clusters exposed to ultrashort near-infrared (NIR) laser pulses for intensities well below the threshold at which tunnel ionization ignites nanoplasma formation. We find that the emission of highly charged ions up to Ar^{8+} can be switched on with unit contrast by generating only a few seed electrons with an ultrashort extreme-ultraviolet (XUV) pulse prior to the NIR field. Molecular dynamics simulations can explain the experimental observations and predict a generic scenario where efficient heating via inverse bremsstrahlung and NIR avalanching is followed by resonant collective nanoplasma heating. The temporally and spatially well-controlled injection of the XUV seed electrons opens new routes for controlling avalanching and heating phenomena in nanostructures and solids, with implications for both fundamental and applied laser-matter science.

Mapping nanoscale absorption of femtosecond laser pulses using plasma explosion imaging

We make direct observations of localized light absorption in a single nanostructure irradiated by a strong femtosecond laser field, by developing and applying a technique that we refer to as plasma explosion imaging. By imaging the photoion momentum distribution resulting from plasma formation in a laser-irradiated nanostructure, we map the spatial location of the highly localized plasma and thereby image the nanoscale light absorption. Our method probes individual, isolated nanoparticles in vacuum, which allows us to observe how small variations in the composition, shape, and orientation of the nanostructures lead to vastly different light absorption. Here, we study four different nanoparticle samples with overall dimensions of ∼100 nm and find that each sample exhibits distinct light absorption mechanisms despite their similar size. Specifically, we observe subwavelength focusing in single NaCl crystals, symmetric absorption in TiO2 aggregates, surface enhancement in dielectric particles containing a single gold nanoparticle, and interparticle hot spots in dielectric particles containing multiple smaller gold nanoparticles. These observations demonstrate how plasma explosion imaging directly reveals the diverse ways in which nanoparticles respond to strong laser fields, a process that is notoriously challenging to model because of the rapid evolution of materials properties that takes place on the femtosecond time scale as a solid nanostructure is transformed into a dense plasma.

Observation and control of shock waves in individual nanoplasmas

Using an apparatus that images the momentum distribution of individual, isolated 100-nm-scale plasmas, we make the first experimental observation of shock waves in nanoplasmas. We demonstrate that the introduction of a heating pulse prior to the main laser pulse increases the intensity of the shock wave, producing a strong burst of quasimonoenergetic ions with an energy spread of less than 15%. Numerical hydrodynamic calculations confirm the appearance of accelerating shock waves and provide a mechanism for the generation and control of these shock waves. This observation of distinct shock waves in dense plasmas enables the control, study, and exploitation of nanoscale shock phenomena with tabletop-scale lasers.

Plasma-formation dynamics in intense laser-droplet interaction

We study the ionization dynamics in intense laser-droplet interaction using three-dimensional, relativistic particle-in-cell simulations. Of particular interest is the laser intensity and frequency regime for which initially transparent, wavelength-sized targets are not homogeneously ionized. Instead, the charge distribution changes both in space and in time on a subcycle scale. One may call this the extreme nonlinear Mie-optics regime. We find that--despite the fact that the plasma created at the droplet surface is overdense--oscillating electric fields may penetrate into the droplet under a certain angle, ionize, and propagate in the just generated plasma. This effect can be attributed to the local field enhancements at the droplet surface predicted by standard Mie theory. The penetration of the fields into the droplet leads to the formation of a highly inhomogeneous charge density distribution in the droplet interior, concentrated mostly in the polarization plane. We present a self-similar, exponential fit of the fractional ionization degree which depends only on a dimensionless combination of electric field amplitude, droplet radius, and plasma frequency with only a weak dependence on the laser frequency in the overdense regime.