The Grotthuss mechanism

Homochiral Metal–Organic Frameworks for Asymmetric Heterogeneous Catalysis

The rise time of intense radiation determines the maximum field strength atoms can be exposed to before their polarizability dramatically drops due to the detachment of an outer electron. Recent progress in ultrafast optics has allowed the generation of ultraintense light pulses comprising merely a few field oscillation cycles. The arising intensity gradient allows electrons to survive in their bound atomic state up to external field strengths many times higher than the binding Coulomb field and gives rise to ionization rates comparable to the light frequency, resulting in a significant extension of the frontiers of nonlinear optics and (nonrelativistic) high-field physics. Implications include the generation of coherent harmonic radiation up to kiloelectronvolt photon energies and control of the atomic dipole moment on a subfemtosecond $(1{\mathrm{f}\mathrm{s}=10}^{\mathrm{\ensuremath{-}}15}\mathrm{}\mathrm{s})$ time scale. This review presents the landmarks of the 30-odd-year evolution of ultrashort-pulse laser physics and technology culminating in the generation of intense few-cycle light pulses and discusses the impact of these pulses on high-field physics. Particular emphasis is placed on high-order harmonic emission and single subfemtosecond extreme ultraviolet/x-ray pulse generation. These as well as other strong-field processes are governed directly by the electric-field evolution, and hence their full control requires access to the (absolute) phase of the light carrier. We shall discuss routes to its determination and control, which will, for the first time, allow access to the electromagnetic fields in light waves and control of high-field interactions with never-before-achieved precision.

Intense few-cycle laser fields: Frontiers of nonlinear optics

Femtosecond filamentation in transparent media

In principle, the temporal beating of superposed high harmonics obtained by focusing a femtosecond laser pulse in a gas jet can produce a train of very short intensity spikes, depending on the relative phases of the harmonics. We present a method to measure such phases through two-photon, two-color photoionization. We found that the harmonics are locked in phase and form a train of 250-attosecond pulses in the time domain. Harmonic generation may be a promising source for attosecond time-resolved measurements.

https://science.sciencemag.org/content/sci/292/5522/1689.full.pdf

Observation of a Train of Attosecond Pulses from High Harmonic Generation

The localization and solvation of excess electrons in pure water have been resolved at the femtosecond time scale. Before it becomes solvated, the electron thermalizes and reaches in 110 fs a localized state absorbing in the infrared. This transient species with lifetime 240 fs has been postulated to exist but has not been observed previously in liquid water.

Excess electrons in liquid water: First evidence of a prehydrated state with femtosecond lifetime

The extension of time-resolved X-ray diffraction to the subpicosecond domain is an important challenge, as the nature of chemical reactions and phase transitions is determined by atomic motions on these timescales. An ultimate goal is to study the structure of transient states with a time resolution shorter than the typical period of vibration along a reaction coordinate (around 100 fs). Biological processes that can be initiated optically have been studied extensively by ultrafast infrared, visible and ultraviolet spectroscopy1. But these techniques probe only electronic states, whereas time-resolved crystallography should be able to directly monitor atomic positions. Here we show that changes in the X-ray diffraction pattern from an organic film heated by a laser pulse can be monitored on a timescale of less than a picosecond. We have studied the response of a Langmuir–Blodgett multilayer film of cadmium arachidate to laser heating by observing changes in the intensity of one Bragg peak for different delays between the perturbing optical pulse and the X-ray probe pulse. A strong decrease in intensity is seen within a picosecond of heating, resulting from disorder introduced to the layers of cadmium atoms before thermal expansion of the film (which ultimately leads to its destruction) has time to occur.

Femtosecond time-resolved X-ray diffraction from laser-heated organic films

A very large high-energy shift of exciton resonances in GaAs multiple-quantum-well structures is observed during irradiation of the sample with femtosecond nonresonant radiation. This effect is discussed in terms of excitons "dressed" by photons.

"Dressed excitons" in a multiple-quantum-well structure: Evidence for an optical Stark effect with femtosecond response time.

Protontherapy is a well-established approach to treat cancer due to the favorable ballistic properties of proton beams. Nevertheless, this treatment is today only possible with large scale accelerator facilities which are very difficult to install at existing hospitals. In this article we report on a new approach for proton acceleration up to energies within the therapeutic window between 60 and 200 MeV by using modern, high intensity and compact laser systems. By focusing such laser beams onto thin foils we obtained on target intensities of 6×10 19 W/cm 2 , which is sufficient to produce a well-collimated proton beam with an energy of up to 10 MeV. These results are in agreement with numerical simulations and indicate that proton energies within the therapeutic window should be obtained in the very near future using such economical and very compact laser systems. Hence, this approach could revolutionize cancer treatment by bringing the “lab to the hospital—rather than the hospital to the lab.”

Practicability of protontherapy using compact laser systems

Subpicosecond infrared pulses were used to study in GaAs at 15 K the spectral dependence of the absorption saturation around the excitation-pulse wavelength. The nonthermal part of the carrier energy distribution is found to peak at the excitation frequency. A line-shape analysis allows the determination of the electron-hole-pair dephasing time.

Andre Antonetti

Papers

Excess electrons in liquid water: First evidence of a prehydrated state with femtosecond lifetime

Femtosecond time-resolved X-ray diffraction from laser-heated organic films

"Dressed excitons" in a multiple-quantum-well structure: Evidence for an optical Stark effect with femtosecond response time.

Practicability of protontherapy using compact laser systems

Subpicosecond spectral hole burning due to nonthermalized photoexcited carriers in GaAs.