Propagation effects in polarization-gated attosecond soft-X-ray pulse generation.

Accurate estimation of the duration of soft-x-ray pulses from high-harmonic generation (HHG) remains challenging given their higher photon energies and broad spectral bandwidth. The carrier-envelope-phase (CEP) dependence of generated soft-x-ray spectra is indicative of attosecond pulse generation, but advanced simulations are needed to infer the pulse duration from such data. Here, we employ macroscopic propagation simulations to reproduce experimental polarization-gated CEP-dependent soft-x-ray spectra. The simulations indicate chirped pulses, which we theoretically find to be compressible in hydrogen plasmas, suggesting this as a viable compression scheme for broadband soft-x-rays from HHG.

Quantum control of flying doughnut terahertz pulses.

The ability to manipulate the multiple properties of light diversifies light-matter interaction and light-driven applications. Here, using quantum control, we introduce an approach that enables the amplitude, sign, and even configuration of the generated light fields to be manipulated in an all-optical manner. Following this approach, we demonstrate the generation of "flying doughnut" terahertz (THz) pulses. We show that the single-cycle THz pulse radiated from the dynamic ring current has an electric field structure that is azimuthally polarized and that the space- and time-resolved magnetic field has a strong, isolated longitudinal component. We apply the flying doughnut pulse for a spectroscopic measurement of the water vapor in ambient air. Pulses such as these will serve as unique probes for spectroscopy, imaging, telecommunications, and magnetic materials.

Reconfigurable terahertz metasurfaces coherently controlled by wavelength-scale-structured light.

Structuring light-matter interaction at a deeply subwavelength scale is fundamental to optical metamaterials and metasurfaces. Conventionally, the operation of a metasurface is determined by the collective electric polarization response of its lithographically defined structures. The inseparability of electric polarization and current density provides the opportunity to construct metasurfaces from current elements instead of nanostructures. Here, we realize metasurfaces using structured light rather than structured materials. Using coherent control, we transfer structure from light to transient currents in a semiconductor, which act as a source for terahertz radiation. A spatial light modulator is used to control the spatial structure of the currents and the resulting terahertz radiation with a resolution of 5.6 ± 0.8 µm , or approximately λ / 54 at a frequency of 1 THz. The independence of the currents from any predefined structures and the maturity of spatial light modulator technology enable this metasurface to be reconfigured with unprecedented flexibility.

Energy deposition and incubation effects of nonlinear absorption of ultrashort laser pulses in dielectrics.

Although ultrashort laser has been widely employed in micromachining thanks to its excellent processing precision, one of the main challenges it faces when applied to 3D modification inside dielectrics is its processing efficiency. Many applications require multiple pulses to achieve significant modification to create structure such as microlenses. We report incubation experiments on energy deposition and the control of material modification in fused silica. This allows us to develop a practical incubation model by taking account different ionization mechanisms, in which coefficients relating to multiphoton and avalanche ionization change with laser shots due to accumulating defects. We then extend our study to the scheme where a pre-pulse is used to limit the absorption volume through pre-seeding. Both experiments and simulations show that the efficiency of laser processing can be significantly improved without sacrificing the spatial resolution with this method, especially for longer pulses.

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.

Subcycle control of electron-electron correlation in double ionization.

Double ionization of neon with orthogonally polarized two-color (OTC) laser fields is investigated using coincidence momentum imaging. We show that the two-electron emission dynamics in nonsequential double ionization can be controlled by tuning the subcycle shape of the electric field of the OTC pulses. We demonstrate experimentally switching from correlated to anticorrelated two-electron emission, and control over the directionality of the two-electron emission. Simulations based on a semiclassical trajectory model qualitatively explain the experimental results by a subcycle dependence of the electron recollision time on the OTC field shape.

Photon momentum sharing between an electron and an ion in photoionization: from one-photon (photoelectric effect) to multiphoton absorption.

We investigate photon-momentum sharing between an electron and an ion following different photoionization regimes. We find very different partitioning of the photon momentum in one-photon ionization (the photoelectric effect) as compared to multiphoton processes. In the photoelectric effect, the electron acquires a momentum that is much greater than the single photon momentum ℏω/c [up to (8/5) ℏω/c] whereas in the strong-field ionization regime, the photoelectron only acquires the momentum corresponding to the photons absorbed above the field-free ionization threshold plus a momentum corresponding to a fraction (3/10) of the ionization potential Ip. In both cases, due to the smallness of the electron-ion mass ratio, the ion takes nearly the entire momentum of all absorbed N photons (via the electron-ion center of mass). Additionally, the ion takes, as a recoil, the photoelectron momentum resulting from mutual electron-ion interaction in the electromagnetic field. Consequently, the momentum partitioning of the photofragments is very different in both regimes. This suggests that there is a rich, unexplored physics to be studied between these two limits which can be generated with current ultrafast laser technology.

High-pressure gas phase femtosecond laser ionization mass spectrometry.

We describe a novel ion source for analytical mass spectrometry based on femtosecond laser ionization at pressures at and above atmospheric and characterize its performance when coupled to a tandem quadrupole/time-of-flight mass spectrometer. We assess source saturation limits, ionization and sampling efficiencies, the effective ionization volume, and limits of detection. We demonstrate 100% efficient ionization for a set of organic compounds and show that the degree of ion fragmentation over a range of laser powers is favorable compared to electron impact ionization, especially in that a substantial parent ion signal is always observed. We show how collisional cooling plays a role in controlling fragmentation at high pressures and address how ion-molecule chemistry can be controlled or exploited. High-pressure femtosecond laser ionization will allow "universal" and efficient ionization, presenting a research direction that will broaden the options for gas phase analysis beyond the capabilities of electron impact ionization.

CEP stable 1.6 cycle laser pulses at 1.8 µm.

By using the novel approach for pulse compression that combines spectral broadening in hollow-core fiber (HCF) with linear propagation in fused silica (FS), we generate 1.6 cycle 0.24 mJ laser pulses at 1.8 µm wavelength with a repetition rate of 1 kHz. These pulses are obtained with a white light seeded optical parametric amplifier (OPA) and shown to be passively carrier envelope phase (CEP) stable.

Muonic molecules in superintense laser fields.

We study theoretically the ionization and dissociation of muonic molecular ions (e.g., dd mu) in superintense laser fields. We predict that the bond breaks by tunneling of the lightest ion through a bond-softened barrier at intensity I > or =10(21) W/cm(2). Ionization of the muonic atomic fragment occurs at much higher intensity I > or =6 x 10(22) W/cm(2). Since the field controls the ion trajectory after dissociation, it forces recollision of a approximately 10(5)-10(6) eV ion with the muonic atom. Recollision can trigger a nuclear reaction with sub-laser-cycle precision. In general, molecules can serve as precursors for laser control of nuclear processes.

How to use lasers for imaging attosecond dynamics of nuclear processes.

We identify a laser configuration in which attosecond electron wave packets are ionized, accelerated to multi-MeV energies, and refocused onto their parent ion. Magnetic focusing of the electron wave packet results in return currents comparable with large scale accelerator facilities. This technique opens an avenue towards imaging attosecond dynamics of nuclear processes.

Optically timed submillimeter time-of-flight mass spectrometry.

We demonstrate that using intense femtosecond laser pulses to optically time ion flight can lead to a miniature time-of-flight mass spectrometer. After laser ionization, the molecular ion is accelerated by a static electric field and detected using a second, delayed laser pulse. The relative positions of the two laser foci determine the ion flight distance while the time separation of the laser pulses fixes the ion flight time. We mass-resolve CS(2) or C(6)H(6) isotopes after a flight distance of 360 microm using either double ionization or Coulomb explosion detection.