Matthias Lezius

Attosecond real-time observation of electron tunnelling in atoms

Atoms exposed to intense light lose one or more electrons and become ions. In strong fields, the process is predicted to occur via tunnelling through the binding potential that is suppressed by the light field near the peaks of its oscillations. Here we report the real-time observation of this most elementary step in strong-field interactions: light-induced electron tunnelling. The process is found to deplete atomic bound states in sharp steps lasting several hundred attoseconds. This suggests a new technique, attosecond tunnelling, for probing short-lived, transient states of atoms or molecules with high temporal resolution. The utility of attosecond tunnelling is demonstrated by capturing multi-electron excitation (shake-up) and relaxation (cascaded Auger decay) processes with subfemtosecond resolution.

Photonic microwave signals with zeptosecond-level absolute timing noise

Ultralow-noise microwave signals are generated at 12 GHz by a low-noise fibre-based frequency comb and cutting-edge photodetection techniques. The microwave signals have a fractional frequency stability below 6.5 × 10–16 at 1 s and a timing noise floor below 41 zs Hz–1/2. Photonic synthesis of radiofrequency (RF) waveforms revived the quest for unrivalled microwave purity because of its ability to convey the benefits of optics to the microwave world1,2,3,4,5,6,7,8,9,10,11. In this work, we perform a high-fidelity transfer of frequency stability between an optical reference and a microwave signal via a low-noise fibre-based frequency comb and cutting-edge photodetection techniques. We demonstrate the generation of the purest microwave signal with a fractional frequency stability below 6.5 × 10−16 at 1 s and a timing noise floor below 41 zs Hz−1/2 (phase noise below −173 dBc Hz−1 for a 12 GHz carrier). This outperforms existing sources and promises a new era for state-of-the-art microwave generation. The characterization is achieved through a heterodyne cross-correlation scheme with the lowermost detection noise. This unprecedented level of purity can impact domains such as radar systems12, telecommunications13 and time–frequency metrology2,14. The measurement methods developed here can benefit the characterization of a broad range of signals.

Pulse compression with time-domain optimized chirped mirrors.

Dispersive optical interference coatings (chirped mirrors - CMs) are designed by computer optimization of an analytically calculated initial multilayer. Traditionally, the relevant properties of the CM (reflectance and the frequency-dependence of the phase shift upon reflection) are optimized to match frequency-domain targets. We propose a novel target function that quantifies directly the capability of a multilayer to control the temporal shape of the reflected optical pulse. Employing this time-domain analysis/optimization one can design dispersive multilayers having air as medium of incidence and supporting the generation of pulses with durations in the sub-5-fs-range, as demonstrated in a proof-of-principle compression experiment.

Space-borne frequency comb metrology

Precision time references in space are of major importance to satellite-based fundamental science, global satellite navigation, earth observation, and satellite formation flying. Here we report on the operation of a compact, rugged, and automated optical frequency comb setup on a sounding rocket in space under microgravity. The experiment compared two clocks, one based on the optical D2 transition in Rb, and another on hyperfine splitting in Cs. This represents the first frequency comb based optical clock operation in space, which is an important milestone for future satellite-based precision metrology. Based on the approach demonstrated here, future space-based precision metrology can be improved by orders of magnitude when referencing to state-of-the-art optical clock transitions.

Ultrafast dynamics in acetylene clocked in a femtosecond XUV stopwatch

Few-photon induced ultrafast dynamics in acetylene (C2H2) leading to several dissociation channels—deprotonation (H++C2H+ and H++C2H2+), symmetric break-up (CH++CH+) and isomerization (C++CH2+)-–were investigated employing the (XUV; extreme ultra-violet)-pump–(XUV; extreme ultra-violet)-probe scheme at the free-electron laser in Hamburg, combined with multi-hit coincidence detection. The kinetic energy releases and fragment-ion momentum distributions for various decay channels are presented. The C++CH2+ and H++C2H2+ channels reveal clear signatures of ultrafast molecular mechanisms, demonstrating potential applications of our pump-probe technique to complex systems in order to study a large variety of ultrafast phenomena in the XUV regime.

Strong-field control of electron localisation during molecular dissociation

We demonstrate how the waveform of light can be used to control a molecular dissociation by the steering and localisation of electrons. Experimental results have been obtained for the dissociative ionisation of the homonuclear and heteronuclear hydrogen derivates D2 and HD. Asymmetric ejection of the ionic fragments reveals that light-driven electronic motion prior to dissociation localises the electron on one of the two ions in diatomic molecular ions in a controlled way. Extension of these results to electron transfer in complex molecules suggests a new paradigm for controlling photochemistry.

Radiation Induced Absorption in Rare Earth Doped Optical Fibers

We have investigated the radiation induced absorption (RIA) of optical fibers with high active ion concentration. Comparing our results to the literature leads us to the conclusion that RIA appears to be only weakly dependent on the rare earth dopant concentration. Instead, co-dopants like Al, Ge, or P and manufacturing processes seem to play the major role for the radiation sensitivity. It is also observed that different types of irradiation cause very similar RIA at the same dose applied, with the exception at very high dose rates. It has been studied how RIA can be efficiently reduced via moderate heating. Recovery of up to 70% of the original transmission has been reached after annealing at 450 K. We conclude that radiation induced color centers have weak binding energies between 20 and 40 meV. This suggests that annealing could become a key strategy for an improved survival of rare earth doped fibers in radiative environments, opening up new possibilities for long-term missions in space.

Few-cycle carrier envelope phase-dependent stereo detection of electrons

The spatial distribution of electrons emitted from atoms by few-cycle optical fields is known to be dependent on the carrier envelope phase, i.e., the phase of the field with respect to the pulse envelope. With respect to Paulus et al. [Phys. Rev. Lett.91, 253004 (2003)] we propose a greatly simplified device to measure and control the carrier envelope phase of few-cycle pulses with an accuracy of better than pi/10 based on this principle. We compared different schemes to control the carrier envelope phase of our pulses.

Attosecond control of electronic motion using light wave synthesis

We demonstrate that the motion and recollision of an electron after tunnelling in the field of the laser can be fully determined by phase-locked, orthogonal two-colour laser fields. Design rules for the field to arbitrarily define the instants of ionization and return, the angle between ejection and recollision and the return energy are given and applications of this technique to attosecond physics are discussed.

A frequency comb and precision spectroscopy experiment in space

A frequency comb, DFB diode laser and rubidium spectroscopy cell have been developed and commissioned on a sounding rocket mission to demonstrate their technological maturity. The first laser spectroscopy experiment on an optical transition in space is performed.