A. Staudte

Attosecond ionization and tunneling delay time measurements in helium.

It is well established that electrons can escape from atoms through tunneling under the influence of strong laser fields, but the timing of the process has been controversial and far too rapid to probe in detail. We used attosecond angular streaking to place an upper limit of 34 attoseconds and an intensity-averaged upper limit of 12 attoseconds on the tunneling delay time in strong field ionization of a helium atom. The ionization field derives from 5.5-femtosecond-long near-infrared laser pulses with peak intensities ranging from 2.3 × 1014 to 3.5 × 1014 watts per square centimeter (corresponding to a Keldysh parameter variation from 1.45 to 1.17, associated with the onset of efficient tunneling). The technique relies on establishing an absolute reference point in the laboratory frame by elliptical polarization of the laser pulse, from which field-induced momentum shifts of the emergent electron can be assigned to a temporal delay on the basis of the known oscillation of the field vector.

Laser-Induced Electron Tunneling and Diffraction

Molecular structure is usually determined by measuring the diffraction pattern the molecule impresses on x-rays or electrons. We used a laser field to extract electrons from the molecule itself, accelerate them, and in some cases force them to recollide with and diffract from the parent ion, all within a fraction of a laser period. Here, we show that the momentum distribution of the extracted electron carries the fingerprint of the highest occupied molecular orbital, whereas the elastically scattered electrons reveal the position of the nuclear components of the molecule. Thus, in one comprehensive technology, the photoelectrons give detailed information about the electronic orbital and the position of the nuclei.

Correlated electron emission in multiphoton double ionization

Electronic correlations govern the dynamics of many phenomena in nature, such as chemical reactions and solid state effects, including superconductivity. Such correlation effects can be most clearly investigated in processes involving single atoms. In particular, the emission of two electrons from an atom—induced by the impact of a single photon, a charged particle or by a short laser pulse—has become the standard process for studies of dynamical electron correlations. Atoms and molecules exposed to laser fields that are comparable in intensity to the nuclear fields have extremely high probabilities for double ionization; this has been attributed to electron–electron interaction. Here we report a strong correlation between the magnitude and the direction of the momentum of two electrons that are emitted from an argon atom, driven by a femtosecond laser pulse (at 38 TW cm-2). Increasing the laser intensity causes the momentum correlation between the electrons to be lost, implying that a transition in the laser–atom coupling mechanism takes place.

Laser Tunnel Ionization from Multiple Orbitals in HCl

A Lower Tunnel Among the peculiarities inherent in quantum mechanics is the ability of particles to tunnel through barriers that they lack the energy to surmount classically, as happens during radioactive decay. Strong laser fields can liberate electrons in this way from atoms and molecules. Akagi et al. (p. 1364) elegantly confirm that tunneling is not limited to the highest-energy electrons in a system by mapping the energy and momentum of both the ejected electron and positive ion produced when an intense laser pulse impinges on hydrogen chloride. When the molecule adopts specific orientations relative to the laser field, tunneling occurs from lower-lying states, as well as the highest-energy occupied orbital. This raises the possibility of tunneling microscopy capable of imaging the electronic structure of single molecules. Ion imaging shows that electrons can tunnel out of states below the highest occupied orbital of a molecule. Tunneling, one of the most striking manifestations of quantum mechanics, influences the electronic structure of many molecules and solids and is responsible for radioactive decay. Much of the interaction of intense light pulses with matter commences with electrons tunneling from atoms or molecules to the continuum. Until recently, this starting point was assumed to be the highest occupied orbital of a given system. We have now observed tunneling from a lower-lying state in hydrogen chloride (HCl). Analyzing two independent experimental observables allowed us to isolate (via fragment ions), identify (via molecular frame photoelectron angular distributions), and, with the help of ab initio simulations, quantify the contribution of lower-lying orbitals to the total and angle-dependent tunneling current of the molecule. Our results bolster the emerging tenet that the coherent interaction between different orbitals—which can amplify the impact of lower orbitals—must be considered in tunneling processes.

Partitioning of the Linear Photon Momentum in Multiphoton Ionization

The balance of the linear photon momentum in multiphoton ionization is studied experimentally. In the experiment argon and neon atoms are singly ionized by circularly polarized laser pulses with a wavelength of 800 and 1400 nm in the intensity range of 10(14)-10(15)  W/cm2. The photoelectrons are measured using velocity map imaging. We find that the photoelectrons carry linear momentum corresponding to the photons absorbed above the field free ionization threshold. Our finding has implications for concurrent models of the generation of terahertz radiation in filaments.

Momentum space tomographic imaging of photoelectrons

We apply tomography, a general method for reconstructing 3-D distributions from multiple projections, to reconstruct the momentum distribution of electrons produced via strong field photoionization. The projections are obtained by rotating the electron distribution via the polarization of the ionizing laser beam and recording a momentum spectrum at each angle with a 2-D velocity map imaging spectrometer. For linearly polarized light the tomographic reconstruction agrees with the distribution obtained using an Abel inversion. Electron tomography, which can be applied to any polarization, will simplify the technology of electron imaging. The method can be directly generalized to other charged particles.

Sequential and nonsequential contributions to double ionization in strong laser fields

We demonstrate experimentally the difference between a sequential interaction of a femtosecond laser field with two electrons and a nonsequential process of double ionization mediated by electron-electron correlation. This is possible by observing the momentum distribution of doubly charged argon ions created in the laser field. In the regime of laser intensities where the nonsequential process dominates, an increase in laser power leads to an increase in the observed ion momenta. At the onset of the sequential process, however, a higher laser power leads to colder ions. The momentum distributions of the ions from the sequential process can be modelled by convolving the single-ionization distribution with itself.

Precise in-situ measurement of laser pulse intensity using strong field ionization

Building on the work of Alnaser et al. [Phys. Rev. A 70, 023413 (2004)], we devise an improved method for an in-situ measurement of the peak intensity in a focused, femtosecond infrared laser pulse. The method is shown to be effective with both photoion and photoelectron imaging devices. The model used to fit the experimental data has no unphysical free parameters used in fitting. The accuracy of the fit is 4% and the overall accuracy of the measurement is 8%.

Tailored semiconductors for high-harmonic optoelectronics

Hitting the highs in solid state The ability to generate high harmonics of optical frequencies through the nonlinear interaction between intense light pulses and gas atoms has opened up the area of ultrafast optics and spectroscopy. Sivis et al. now show that high harmonics can also be generated with a solid-state sample. They used nanofabricated structured targets of ZnO and varied the chemical composition of the sample to demonstrate that (modest) high harmonics can be generated as the light interacts with the target materials. The results present the possibility of developing solid-state ultrafast optical devices. Science, this issue p. 303 Nanofabricated structures and chemical composition can tune the generation of high harmonics from solid-state targets. The advent of high-harmonic generation in gases 30 years ago set the foundation for attosecond science and facilitated ultrafast spectroscopy in atoms, molecules, and solids. We explore high-harmonic generation in the solid state by means of nanostructured and ion-implanted semiconductors. We use wavelength-selective microscopic imaging to map enhanced harmonic emission and show that the generation medium and the driving field can be locally tailored in solids by modifying the chemical composition and morphology. This enables the control of high-harmonic technology within precisely engineered solid targets. We demonstrate customized high-harmonic wave fields with wavelengths down to 225 nanometers (ninth-harmonic order of 2-micrometer laser pulses) and present an integrated Fresnel zone plate target in silicon, which leads to diffraction-limited self-focusing of the generated harmonics down to 1-micrometer spot sizes.

Probing Molecular Dynamics by Laser-Induced Backscattering Holography.

We use differential holography to overcome the forward scattering problem in strong-field photoelectron holography. Our differential holograms of H_{2} and D_{2} molecules exhibit a fishbonelike structure, which arises from the backscattered part of the recolliding photoelectron wave packet. We demonstrate that the backscattering hologram can resolve the different nuclear dynamics between H_{2} and D_{2} with subangstrom spatial and subcycle temporal resolution. In addition, we show that attosecond electron dynamics can be resolved. These results open a new avenue for ultrafast studies of molecular dynamics in small molecules.