Multiple-harmonic generation in rare gases at high laser intensity

Benchmarking Functionals for Strong-Field Light-Matter Interactions in Adiabatic Time-Dependent Density Functional Theory

In recent years, time-dependent density functional theory (TDDFT) has been extensively employed for highly nonlinear optics in molecules and solids, including high harmonic generation (HHG), photoemission, and more. TDDFT exhibits a relatively low numerical cost while still describing both light-matter and electron-electron interactions ab initio, making it highly appealing. However, the majority of implementations of the theory utilize the simplest possible approximations for the exchange-correlation (XC) functional-either the local density or generalized gradient approximations, which are traditionally considered to have rather poor chemical accuracy. We present the first systematic study of the XC functional effect on molecular HHG, testing various levels of theory. Our numerical results suggest justification for using simpler approximations for the XC functional, showing that hybrid and meta functionals (as well as Hartree-Fock) can, at times, lead to poor and unphysical results. The specific source of the failure in more elaborate functionals should be topic of future work, but we hypothesize that its origin might be connected to the adiabatic approximation of TDDFT.

Efficient single-cycle mid-infrared femtosecond laser pulse generation by spectrally temporally cascaded optical parametric amplification with pump energy recycling

We proposed spectrally temporally cascaded optical parametric amplification (STOPA) using pump energy recycling to simultaneously increase spectral bandwidth and conversion efficiency in optical parametric amplification (OPA). Using BiB3O6 and KTiOAsO4 nonlinear crystals, near-single-cycle mid-infrared (MIR) pulses with maximum energy conversion efficiencies exceeding 25% were obtained in simulations. We successfully demonstrated sub-two-cycle, CEP-stable pulse generation at 1.8 µm using a four-step STOPA system in the experiment. This method provides a solution to solve the limitations of the gain bandwidth of nonlinear crystals and the low conversion efficiency in broadband OPA systems, which is helpful for intense attosecond pulse generation and strong laser field physics studies.

Modeling of High-Harmonic Generation in the C60 Fullerene Using Ab Initio, DFT-Based, and Semiempirical Methods

We report calculations of the high-harmonic generation spectra of the C60 fullerene molecule carried out by employing a diverse set of real-time time-dependent quantum chemical methods. All methodologies involve expanding the propagated electronic wave function in bases consisting of the ground and singly excited time-independent eigenstates obtained through the solution of the corresponding linear-response equations. We identify the correlation and exchange effect in the spectra by comparing the results from methods relying on the Hartree-Fock reference determinant with those obtained using approaches based on the density functional theory with different exchange-correlation functionals. The effect of the full random-phase approximation treatment of the excited electronic states is also analyzed and compared with the configuration interaction singles and the Tamm-Dancoff approximation. We also showcase the fact that the real-time extension of the semiempirical method INDO/S can be effectively applied for an approximate description of laser-driven dynamics in large systems.

Highly efficient and aberration-free off-plane grating spectrometer and monochromator for EUV-soft X-ray applications

We demonstrate a novel flat-field, dual-optic imaging EUV-soft X-ray spectrometer and monochromator that attains an unprecedented throughput efficiency exceeding 60% by design, along with a superb spectral resolution of λ/Δλ > 200 accomplished without employing variable line spacing gratings. Exploiting the benefits of the conical diffraction geometry, the optical system is globally optimized in multidimensional parameter space to guarantee optimal imaging performance over a broad spectral range while maintaining circular and elliptical polarization states at the first, second, and third diffraction orders. Moreover, our analysis indicates minimal temporal dispersion, with pulse broadening confined within 80 fs tail-to-tail and an FWHM value of 29 fs, which enables ultrafast spectroscopic and pump-probe studies with femtosecond accuracy. Furthermore, the spectrometer can be effortlessly transformed into a monochromator spanning the EUV-soft X-ray spectral region using a single grating with an aberration-free spatial profile. Such capability allows coherent diffractive imaging applications to be conducted with highly monochromatic light in a broad spectral range and extended to the soft X-ray region with minimal photon loss, thus facilitating state-of-the-art imaging of intricate nano- and bio-systems, with a significantly enhanced spatiotemporal resolution, down to the nanometer-femtosecond level.

Photon Pathways and the Nonperturbative Scaling Law of High Harmonic Generation

High harmonic generation (HHG) has become a core pillar of attosecond science. Traditionally described with field-based models, HHG can also be viewed in a parametric picture, which predicts all properties of the emitted photons, but not the nonperturbative efficiency of the process. Driving HHG with two noncollinear beams and deriving analytically the corresponding yield scaling laws for any intensity ratio, we herein reconcile the two interpretations, introducing a generalized photonic description of HHG. It is in full agreement with field-based simulations and experimental data, opening the route to smart engineering of HHG with multiple driving beams.

Ultrasound harmonic generation and atomic layer deposition of multilayer, deep-UV mirrors and filters with microcavity plasma arrays

Abstract

In honor of Professor Kurt Becker's pioneering contributions to microplasma physics and applications, we report the capabilities of arrays of microcavity plasmas in two emerging and disparate applications. The first of these is the generation of ultrasound radiation in the 20-240 kHz spectral range with microplasmas in either a static or jet configuration. When a 10 × 10 array of microplasma jets is driven by a 20-kHz sinusoidal voltage, for example, harmonics as high as m = 12 are detected and fractional harmonics are produced by controlling the spatial symmetry of the emitter array. The preferential emission of ultrasound in an inverted cone having an angle of ± 45 ∘ with respect to the surface normal of the jet array's exit face is attributed to interference between spatially periodic, outward-propagating waves generated by the arrays. The spatial distribution of ultrasound generated by the arrays is analogous to the radiation patterns produced by Yagi-Uda phased array antennas at RF frequencies for which radiation is emitted broadside to arrays of parallel electric dipoles. Also, the nonperturbative envelope of the ultrasound harmonic spectrum resembles that for high-order harmonic generation at optical frequencies in rare gas plasmas and attests to the strong nonlinearity provided by the pulsed microplasmas in the sub-250-kHz region. Specifically, the relative intensities of the second and third harmonics exceed that for the fundamental, and a "plateau" region is observed extending from the 5th through the 8th harmonics. A strong plasma nonlinearity appears to be responsible for both the appearance of fractional harmonics and the nonperturbative nature of the acoustic harmonic spectrum. Multilayer metal-oxide optical filters designed to have peak transmission near 222 nm in the deep-UV region of the spectrum have been fabricated by microplasma-assisted atomic layer deposition. Alternating layers of ZrO2 and Al2 O3 , each having a thickness in the 20-50 nm range, were grown on quartz and silicon substrates by successively exposing the substrate to the Zr or Al precursor (tetrakis(dimethylamino) zirconium or trimethylaluminum, respectively) and the products of an oxygen microplasma while maintaining the substrate temperature at 300 K. Bandpass filters comprising 9 cycles of 30-nm-thick ZrO2 /50-nm-thick Al2 O3 film pairs transmit 80% at 235 nm but < 35% in the 250-280 nm interval. Such multilayer reflectors appear to be of significant value in several applications, including bandpass filters suppressing long wavelength (240-270 nm) radiation emitted by KrCl (222) lamps.

Graphical abstract

Role of fractional high harmonics with non-integer OAM on the generation of a helical attosecond pulse train

Extreme-ultraviolet pulses of attosecond duration carrying orbital angular momentum (OAM) can be produced by spectrally filtering vortex high harmonics generated in a gas medium. Here we reveal that fractional high harmonics (FHHs) with non-integer OAM generated by a short duration Laguerre-Gaussian laser beam are origins for the change of helical attosecond pulse train (APT) with azimuthal angle. We show that these harmonics have gap and minimum structures in the annular intensity profile and discontinue phase distribution along azimuthal angle. And each FHH can be expressed as a superposition of OAM modes with integer topological charges. Features of FHH can be identified by coherently combining selected OAM modes. We also uncover that these features are formed after FHH is propagated in gas medium and in vacuum. We finally demonstrate that the generation of FHHs and the dependence of helical APTs on azimuthal angle are changed by varying the macroscopic condition.

Cross-polarized common-path temporal interferometry for high-sensitivity strong-field ionization measurements

Absolute density measurements of low-ionization-degree or low-density plasmas ionized by lasers are very important for understanding strong-field physics, atmospheric propagation of intense laser pulses, Lidar etc. A cross-polarized common-path temporal interferometer using balanced detection was developed for measuring plasma density with a sensitivity of ∼0.6 mrad, equivalent to a plasma density-length product of ∼2.6 × 10¹³ cm^-2 if using an 800 nm probe laser. By using this interferometer, we have investigated strong-field ionization yield versus intensity for various noble gases (Ar, Kr, and Xe) using 800 nm, 55 fs laser pulses with both linear (LP) and circular (CP) polarization. The experimental results were compared to the theoretical models of Ammosov-Delone-Krainov (ADK) and Perelomov-Popov-Terent'ev (PPT). We find that the measured phase change induced by plasma formation can be explained by the ADK theory in the adiabatic tunneling ionization regime, while PPT model can be applied to all different regimes. We have also measured the photoionization and fractional photodissociation of molecular (MO) hydrogen. By comparing our experimental results with PPT and MO-PPT models, we have determined the likely ionization pathways when using three different pump laser wavelengths of 800 nm, 400 nm, and 267 nm.

High-repetition rate attosecond beamline for multi-particle coincidence experiments

In this paper, a 3-dimensional photoelectron/ion momentum spectrometer (reaction microscope) combined with a table-top attosecond beamline based on a high-repetition rate (49 kHz) laser source is presented. The beamline is designed to achieve a temporal stability below 50 attoseconds. Results from measurements on systems like molecular hydrogen and argon dimers demonstrate the capabilities of this setup in observing the attosecond dynamics in 3D while covering the full solid angle for ionization processes having low cross-sections.

Spatiospectral characterization of ultrafast pulse-beams by multiplexed broadband ptychography

Ultrafast pulse-beam characterization is critical for diverse scientific and industrial applications from micromachining to generating the highest intensity laser pulses. The four-dimensional structure of a pulse-beam, E~(x,y,z,ω), can be fully characterized by coupling spatiospectral metrology with spectral phase measurement. When temporal pulse dynamics are not of primary interest, spatiospectral characterization of a pulse-beam provides crucial information even without spectral phase. Here we demonstrate spatiospectral characterization of pulse-beams via multiplexed broadband ptychography. The complex spatial profiles of multiple spectral components, E~(x,y,ω), from modelocked Ti:sapphire and from extreme ultra-violet pulse-beams are reconstructed with minimum intervening optics and no refocusing. Critically, our technique does not require spectral filters, interferometers, or reference pulses.

Phase-matching control of high-order harmonics with circular Airy-Gaussian beams

We investigate the phase-matching of the high harmonics (HHG) driven by the circular Airy-Gaussian beams (CAiGB), which abruptly auto-focus and subsequently propagate without diffraction. The results show that the harmonics corresponding to both short and long quantum paths can be well phase-matched after the focusing point of the CAiGB. Therefore, the effective interaction length of HHG for CAiGB is much longer than that for the conventional Gaussian beams with the same size of the waist. Our numerical simulations reveal that the harmonics continuously gain up to 1 cm of the propagation distance. This work provides a route to enhance the conversion efficiency of HHG by the coherent control of abrupt auto-focusing beams.

All-optical attosecond time domain interferometry

Interferometry, a key technique in modern precision measurements, has been used for length measurement in engineering metrology and astronomy. An analogous time-domain interferometric technique would represent a significant complement to spatial domain applications and require the manipulation of interference on extreme time and energy scales. Here, we report an all-optical interferometer using laser-driven high order harmonics as attosecond temporal slits. By controlling the phase of the temporal slits with an external field, a time domain interferometer that preserves both attosecond temporal resolution and hundreds of meV energy resolution is implemented. We apply this exceptional temporal resolution to reconstruct the waveform of an arbitrarily polarized optical pulse, and utilize the provided energy resolution to interrogate the abnormal character of the transition dipole near the Cooper minimum in argon. This novel attosecond interferometry paves the way for high precision measurements in the time-energy domain using all-optical approaches.

Control of High-Harmonic Generation by Tuning the Electronic Structure and Carrier Injection

High-harmonic generation (HHG), which is the generation of light with multiple optical harmonics, is an unconventional nonlinear optical phenomenon beyond the perturbation regime. HHG, which was initially observed in gaseous media, has recently been demonstrated in solid-state materials. Determining how to control such extreme nonlinear optical phenomena is a challenging subject. Here, we demonstrate the control of HHG through tuning the electronic structure and carrier injection using single-walled carbon nanotubes (SWCNTs). We reveal systematic changes in the high-harmonic spectra of SWCNTs with a series of electronic structures ranging from a metal structure to a semiconductor structure. We demonstrate enhancement or reduction of harmonic generation by more than 1 order of magnitude by tuning the electron and hole injection into the semiconductor SWCNTs through electrolyte gating. These results open a path toward the control of HHG in the context of field-effect transistor devices.

Time-resolved XUV ARPES with tunable 24-33 eV laser pulses at 30 meV resolution

High harmonic generation of ultrafast laser pulses can be used to perform angle-resolved photoemission spectroscopy (ARPES) to map the electronic band structure of materials with femtosecond time resolution. However, currently it is difficult to reach high momenta with narrow energy resolution. Here, we combine a gas phase extreme ultraviolet (XUV) femtosecond light source, an XUV monochromator, and a time-of-flight electron analyzer to develop XUV-based time-resolved ARPES. Our technique can produce tunable photon energy between 24-33 eV with an unprecedented energy resolution of 30 meV and time resolution of 200 fs. This technique enables time-, energy- and momentum-resolved investigation of the nonequilibrium dynamics of electrons in materials with a full access to their first Brillouin zone. We evaluate the performance of this setup through exemplary measurements on various quantum materials, including WTe₂, WSe₂, TiSe₂, and Bi₂Sr₂CaCu₂O_8+δ.

Recent advances in ultrafast X-ray sources

Over more than a century, X-rays have transformed our understanding of the fundamental structure of matter and have been an indispensable tool for chemistry, physics, biology, materials science and related fields. Recent advances in ultrafast X-ray sources operating in the femtosecond to attosecond regimes have opened an important new frontier in X-ray science. These advances now enable: (i) sensitive probing of structural dynamics in matter on the fundamental timescales of atomic motion, (ii) element-specific probing of electronic structure and charge dynamics on fundamental timescales of electronic motion, and (iii) powerful new approaches for unravelling the coupling between electronic and atomic structural dynamics that underpin the properties and function of matter. Most notable is the recent realization of X-ray free-electron lasers (XFELs) with numerous new XFEL facilities in operation or under development worldwide. Advances in XFELs are complemented by advances in synchrotron-based and table-top laser-plasma X-ray sources now operating in the femtosecond regime, and laser-based high-order harmonic XUV sources operating in the attosecond regime. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

Spin-ARPES EUV Beamline for Ultrafast Materials Research and Development

A new femtosecond, Extreme Ultraviolet (EUV), Time Resolved Spin-Angle Resolved Photo-Emission Spectroscopy (TR-Spin-ARPES) beamline was developed for ultrafast materials research and development. This 50-fs laser-driven, table-top beamline is an integral part of the “Ultrafast Spintronic Materials Facility”, dedicated to engineering ultrafast materials. This facility provides a fast and in-situ analysis and development of new materials. The EUV source based on high harmonic generation process emits 2.3 × 1011 photons/second (2.3 × 108 photons/pulse) at H23 (35.7 eV) and its photon energy ranges from 10 eV to 75 eV, which enables surface sensitive studies of the electronic structure dynamics. The EUV monochromator provides the narrow bandwidth of the EUV beamline while preserving its pulse duration in an energy range of 10–100 eV. Ultrafast surface photovoltaic effect with ~650 fs rise-time was observed in p-GaAs (100) from time-resolved ARPES spectra. The data acquisition time could be reduced by over two orders of magnitude by scaling the laser driver from 1 KHz, 4W to MHz, KW average power.

Control of quasi-phase-matching of high-harmonics in a spatially structured plasma

High-harmonic generation is widely used for providing extreme ultraviolet radiation in attosecond science. Such experiments include photoelectron spectroscopy, diffractive imaging, or the investigation of spin dynamics. Many applications are restricted by a low photon flux which originates from the low efficiency of the generation process. In this article an effective method based on the quasi-phase-matched generation of high harmonics in spatially structured, laser ablated plasma is demonstrated. Through a proper dimensioning of the plasma structure, the harmonic yield is optimized for a controllable range of harmonic orders. By using four coherent zones, the intensity of a single harmonic is increased to a maximal possible value of 16 compared to using a single zone. The Gouy phase shift of the fundamental field is identified as the primary effect responsible for constructive interference of the harmonic fields generated in the individual plasma jets of the plasma structure.

Photoelectron spectrometer for liquid and gas-phase attosecond spectroscopy with field-free and magnetic bottle operation modes

A compact time-of-flight spectrometer for applications in attosecond spectroscopy in the liquid and gas phases is presented. It allows for altering the collection efficiency by transitioning between field-free and magnetic-bottle operation modes. High energy resolution (ΔE/E = 0.03 for kinetic energies >20 eV) is achieved despite the short flight-tube length through a homogeneous deceleration potential at the beginning of the flight tube. A closing mechanism allows isolating the vacuum system of the flight tube from the interaction region in order to efficiently perform liquid-microjet experiments. The capabilities of the instrument are demonstrated through photoelectron spectra from multiphoton ionization of argon and xenon, as well as photoelectron spectra of liquid and gaseous water generated by an attosecond pulse train.

Long-Range Coulomb Effect in Intense Laser-Driven Photoelectron Dynamics

In strong field atomic physics community, long-range Coulomb interaction has for a long time been overlooked and its significant role in intense laser-driven photoelectron dynamics eluded experimental observations. Here we report an experimental investigation of the effect of long-range Coulomb potential on the dynamics of near-zero-momentum photoelectrons produced in photo-ionization process of noble gas atoms in intense midinfrared laser pulses. By exploring the dependence of photoelectron distributions near zero momentum on laser intensity and wavelength, we unambiguously demonstrate that the long-range tail of the Coulomb potential (i.e., up to several hundreds atomic units) plays an important role in determining the photoelectron dynamics after the pulse ends.

Charge Transfer Dynamics at Dye-Sensitized ZnO and TiO2 Interfaces Studied by Ultrafast XUV Photoelectron Spectroscopy

Interfacial charge transfer from photoexcited ruthenium-based N3 dye molecules into ZnO thin films received controversial interpretations. To identify the physical origin for the delayed electron transfer in ZnO compared to TiO2, we probe directly the electronic structure at both dye-semiconductor interfaces by applying ultrafast XUV photoemission spectroscopy. In the range of pump-probe time delays between 0.5 to 1.0 ps, the transient signal of the intermediate states was compared, revealing a distinct difference in their electron binding energies of 0.4 eV. This finding strongly indicates the nature of the charge injection at the ZnO interface associated with the formation of an interfacial electron-cation complex. It further highlights that the energetic alignment between the dye donor and semiconductor acceptor states appears to be of minor importance for the injection kinetics and that the injection efficiency is dominated by the electronic coupling.

Ultrafast excited states dynamics of [Ru(bpy)3]2+ dissolved in ionic liquids

In this work, we demonstrate the potential of room-temperature ionic liquids as solvents to investigate the excited states dynamics of [Ru(bpy)₃]²⁺ by means of time-resolved photoelectron spectroscopy.

Coherent diffractive imaging beyond the projection approximation: waveguiding at extreme ultraviolet wavelengths

We study extreme-ultraviolet wave propagation within optically thick nanostructures by means of high-resolution coherent diffractive imaging using high-harmonic radiation. Exit waves from different objects are reconstructed by phase retrieval algorithms, and are shown to be dominated by waveguiding within the sample. The experiments provide a direct visualization of extreme-ultraviolet guided modes, and demonstrate that multiple scattering is a generic feature in extruded nanoscale geometries. The observations are successfully reproduced in numerical and semi-analytical simulations.

Generation of bright isolated attosecond soft X-ray pulses driven by multicycle midinfrared lasers

High harmonic generation driven by femtosecond lasers makes it possible to capture the fastest dynamics in molecules and materials. However, to date the shortest subfemtosecond (attosecond, 10(-18) s) pulses have been produced only in the extreme UV region of the spectrum below 100 eV, which limits the range of materials and molecular systems that can be explored. Here we experimentally demonstrate a remarkable convergence of physics: when midinfrared lasers are used to drive high harmonic generation, the conditions for optimal bright, soft X-ray generation naturally coincide with the generation of isolated attosecond pulses. The temporal window over which phase matching occurs shrinks rapidly with increasing driving laser wavelength, to the extent that bright isolated attosecond pulses are the norm for 2-µm driving lasers. Harnessing this realization, we experimentally demonstrate the generation of isolated soft X-ray attosecond pulses at photon energies up to 180 eV for the first time, to our knowledge, with a transform limit of 35 attoseconds (as), and a predicted linear chirp of 300 as. Most surprisingly, advanced theory shows that in contrast with as pulse generation in the extreme UV, long-duration, 10-cycle, driving laser pulses are required to generate isolated soft X-ray bursts efficiently, to mitigate group velocity walk-off between the laser and the X-ray fields that otherwise limit the conversion efficiency. Our work demonstrates a clear and straightforward approach for robustly generating bright isolated attosecond pulses of electromagnetic radiation throughout the soft X-ray region of the spectrum.

Hydrogen bond dynamics of superheated water and methanol by ultrafast IR-pump and EUV-photoelectron probe spectroscopy

Snapshot of superheated water 40 ps after fs-IR laser excitation; representative aggregates formed during the simulation (close-up) compared to one obtained from superheated methanol phase (inset).

Attosecond extreme ultraviolet vortices from high-order harmonic generation

We present a theoretical study of high-order harmonic generation (HHG) and propagation driven by an infrared field carrying orbital angular momentum (OAM). Our calculations unveil the following relevant phenomena: extreme-ultraviolet harmonic vortices are generated and survive to the propagation effects, vortices transport high-OAM multiples of the corresponding OAM of the driving field and, finally, the different harmonic vortices are emitted with similar divergence. We also show the possibility of combining OAM and HHG phase locking to produce attosecond pulses with helical pulse structure.

Generation and bistability of a waveguide nanoplasma observed by enhanced extreme-ultraviolet fluorescence

We present a study of the highly nonlinear optical excitation of noble gases in tapered hollow waveguides using few-femtosecond laser pulses. The local plasmonic field enhancement induces the generation of a nanometric plasma, resulting in incoherent extreme-ultraviolet fluorescence from optical transitions of neutral and ionized xenon, argon, and neon. Despite sufficient intensity in the waveguide, high-order harmonic generation is not observed. The fluorescent emission exhibits a strong bistability manifest as an intensity hysteresis, giving strong indications for multistep collisional excitations.

Isolated attosecond pulses from laser-driven synchrotron radiation

A quantitative theory of attosecond pulse generation in relativistically driven overdense plasma slabs is presented based on an explicit analysis of synchrotron-type electron trajectories. The subcycle, field-controlled release, and subsequent nanometer-scale acceleration of relativistic electron bunches under the combined action of the laser and ionic potentials give rise to coherent radiation with a high-frequency cutoff, intensity, and radiation pattern explained in terms of the basic laws of synchrotron radiation. The emerging radiation is confined to time intervals much shorter than the half-cycle of the driver field. This intuitive approach will be instrumental in analyzing and optimizing few-cycle-laser-driven relativistic sources of intense isolated extreme ultraviolet and x-ray pulses.

Coherent control of high harmonic generation via dual-gas multijet arrays

High harmonic generation (HHG) is a central driver of the rapidly growing field of ultrafast science. We present a novel quasiphase-matching (QPM) concept with a dual-gas multijet target leading, for the first time, to remarkable phase control between multiple HHG sources (>2) within the Rayleigh range. The alternating jet structure with driving and matching zones shows perfect coherent buildup for up to six QPM periods. Although not in the focus of the proof-of-principle studies presented here, we achieved competitive conversion efficiencies already in this early stage of development.

Controlling the length of plasma waveguide up to 5 mm, produced by femtosecond laser pulses in atomic clustered gas

We report the observation of longitudinally uniform plasma waveguide with a controlled length of up to nearly 5 mm, in argon clustered gas jet. This self-channeling plasma is obtained using a 35 mJ, 30 fs FWHM pulse as a pump laser pulse to create the plasma channel. A 1 mJ pulse of the same laser is used for probing the plasma channels using interferometric diagnostics. The radial distribution of the electron density confirms the formation of a plasma waveguide. Clustered argon enhances the absorption efficiency of femtosecond pulses which enables the use of pump pulses of only 35 mJ, approximately 10 times less energy than required for heating conventional gas targets. The plasma channel length is controlled by the laser focus point (F), the laser intensity (I), the pump-probe delay time (t) and the laser height from a nozzle (z). The variation of the electron density for these parameters is also studied. We found that the highest density of 1.2 x 10(19) cm(-3) was obtained at I = 5.2 x 10(16) W/cm(2), z = 2 mm and t = 7.6 ns. It was demonstrated that by using a clustered jet, both the plasma waveguide length and the plasma density could be controlled.

Harmonic generation in ablation plasmas of wide bandgap semiconductors

Third and fifth harmonic generation of an IR (1.064 μm) pulsed laser has been produced in ablation plasmas of the wide bandgap semiconductors CdS and ZnS. The study of the temporal behaviour of the harmonic emission has revealed the presence of distinct compositional populations in these complex plasmas. Species ranging from atoms to nanometre-sized particles have been identified as emitters, and their nonlinear optical properties can be studied separately due to strongly differing temporal behaviour. At short distances from the target (<1 mm), atomic species are mostly responsible for harmonic generation at early times (<500 ns), while clusters and nanoaggregates mostly contribute at longer times (>1 μs). Harmonic generation thus emerges as a powerful and universal technique for ablation plasma diagnosis and as a tool to determine the nonlinear optical susceptibility of ejected clusters or nanoparticles.

Saturation of the all-optical Kerr effect

Saturation of the intensity dependence of the refractive index is directly computed from ionization rates via a Kramers-Kronig transform. The linear intensity dependence and its dispersion are found to be in excellent agreement with complete quantum mechanical orbital computations. Higher-order terms concur with solutions of the time-dependent Schrödinger equation. Expanding the formalism to all orders up to the ionization potential of the atom, we derive a model for saturation of the Kerr effect. This model widely confirms recently published and controversially discussed experimental data and corroborates the importance of higher-order Kerr terms for filamentation.

Spatial fingerprint of quantum path interferences in high order harmonic generation

We have spatially and spectrally resolved the high order harmonic emission from an argon gas target. Under proper phase matching conditions we were able to observe for the first time the spatial fine structure originating from the interference of the two shortest quantum paths in the harmonic beam. The structure can be explained by the intensity-dependent harmonic phase of the contributions from the two paths. The spatially and spectrally resolved measurements are consistent with previous spatially integrated results. Our measurement method represents a new tool to clearly distinguish between different interference effects and to potentially observe higher order trajectories in the future with improved detection sensitivity. Here, we demonstrate additional experimental evidence that the observed interference pattern is only due to quantum-path interferences and cannot be explained by a phase modulation effect. Our experimental results are fully supported by simulations using the strong field approximation and including propagation.

Time-Resolved ESCA: a Novel Probe for Chemical Dynamics

Electron spectroscopy for chemical analysis (ESCA) is a well established tool for quantitative studies of the composition and the chemical environment of molecular systems. Recent developments in the generation and utilization of ultrashort X-ray pulses now add the dimension of time to this technique and will expand the possibilities of femtochemistry in terms of chemical selectivity, quality of information, and temporal resolution. The properties and capabilities of various X-ray pulse sources are discussed, along with their prospects for dynamical studies. Examples of time-resolved electron spectroscopy are presented in the femtosecond (1 fs = 10−15 s) as well as the attosecond (1 as = 10−18 s) regime, the latter marking the current ultimate limit for the time-resolution in pump-probe experiments.