Matthias Uiberacker

Single-cycle nonlinear optics

Nonlinear optics plays a central role in the advancement of optical science and laser-based technologies. We report on the confinement of the nonlinear interaction of light with matter to a single wave cycle and demonstrate its utility for time-resolved and strong-field science. The electric field of 3.3-femtosecond, 0.72-micron laser pulses with a controlled and measured waveform ionizes atoms near the crests of the central wave cycle, with ionization being virtually switched off outside this interval. Isolated sub-100-attosecond pulses of extreme ultraviolet light (photon energy ∼ 80 electron volts), containing ∼0.5 nanojoule of energy, emerge from the interaction with a conversion efficiency of ∼10–6. These tools enable the study of the precision control of electron motion with light fields and electron-electron interactions with a resolution approaching the atomic unit of time (∼24 attoseconds).

Atomic transient recorder

In Bohrs model of the hydrogen atom, the electron takes about 150 attoseconds (1 as = 10-18 s) to orbit around the proton, defining the characteristic timescale for dynamics in the electronic shell of atoms. Recording atomic transients in real time requires excitation and probing on this scale. The recent observation of single sub-femtosecond (1 fs = 10-15 s) extreme ultraviolet (XUV) light pulses has stimulated the extension of techniques of femtochemistry into the attosecond regime. Here we demonstrate the generation and measurement of single 250-attosecond XUV pulses. We use these pulses to excite atoms, which in turn emit electrons. An intense, waveform-controlled, few cycle laser pulse obtains ‘tomographic images’ of the time-momentum distribution of the ejected electrons. Tomographic images of primary (photo)electrons yield accurate information of the duration and frequency sweep of the excitation pulse, whereas the same measurements on secondary (Auger) electrons will provide insight into the relaxation dynamics of the electronic shell following excitation. With the current ∼750-nm laser probe and ∼100-eV excitation, our transient recorder is capable of resolving atomic electron dynamics within the Bohr orbit time.

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.

Phase-controlled amplification of few-cycle laser pulses

Several methods of carrier-envelope-phase stabilization of amplified ultrashort laser pulses are presented. Full temporal characterization of the resultant intense few-cycle light waveforms, reproducible with every laser shot, was achieved by combining conventional pulse characterization techniques with observing the signature of soft-X- ray emission. Many demanding experiments in strong-field laser physics and coherent control of light-matter interactions would greatly benefit from the use of haser amplifiers that deliver ultrashort optical pulses with a stabilized carrier-envelope phase (CEP). Whereas CEP-stabilized oscillators (I) are now routinely used by several research groups, subsequent pulse amplification to sub-mJ and mJ energy levels remains a difficult technical issue. In this talk, two different approaches to CEP-controlled pulse amplification will be treated and compared with each other: i) optical parametric amplifiers (OPA) with a passive CEP-locking mechanism and ii) conventional amplifiers based on a laser gain medium and equipped with an active CEP stabilization loop. Additionally, practical diagnostics methods for tracking the pulse-to-pulse slippage of CEP in kHz-repetition-rate amplifiers will be reviewed. CEP stabilization in an OPA can be attained in an all-optical way, i.e. without an active feedback loop. The link between the phases of all three pulses taking part in a parametric interaction can be described by a simplified phase equation w, =-lr/2 +y,, -y,, where w stands for a pulse-to-pulse fluctuation of CEP, and I, S, and P denote idler, signal, and pump, respectively. Therefore, CEP self-stabilization of the idler is achieved in an OPA seeded with a white light that is derived from the pump pulse. Alternatively, if a frequency-broadened version of the fundamental pulse is employed as a seed and the second harmonic is used as a pump, such an OPA transfers the CEP fluctuation of the fundamental pulse to both the idler and the signal waves (phase-repeating OPA). Experimental observation of CEP stabilization in 1-pJ pulses and a comparison of both OPA types will be presented. Whereas parametric CEF control in an OPA is straightforward, its significant drawback is the inevitable frequency shift of the CEP-stabilized pulse with respect to the input laser pulse. This complicates further pulse amplification. To maintain the same frequency, a chain of laser amplifiers can be seeded with pulses from an actively CEP-stabilized MHz-repetition-rate oscillator, provided the phase coherence is not destroyed in the amplification process. It will be shown that with such a seed CEP control in a kHz I-mJ multipass amplifier becomes feasible after adding a servo loop that corrt:cts for the slow CEP drift of the amplified pulses. Furthermore, the generation of intense phase-reproducible pulses opens the way to calibrate the actual value of the CEP, which is required to obtain the full information on the shape of the electric field. Among several candidate methods to determine the value of CEP, soft-X-ray emission in a noble gas was chosen for the intuitive dependence between the spectral shape of the highest-energy radiation and the magnitude of the peak optical oscillation of a few-cycle pulse (2). Figure 1 shows the experimental results of the first-time CEP calibration. XI1 Yll 11lil 111) 1211 130 IJII lill -15 .IO -5 I1 5 Io 15 Photon enerqy (eV) Time (Is)

Intense 1.5-cycle near infrared laser waveforms and their use for the generation of ultra-broadband soft-x-ray harmonic continua

We demonstrate sub-millijoule-energy, sub-4?fs-duration near-infrared laser pulses with a controlled waveform comprised of approximately 1.5 optical cycles within the full-width at half-maximum (FWHM) of their temporal intensity profile. We further demonstrate the utility of these pulses for producing high-order harmonic continua of unprecedented bandwidth at photon energies around 100?eV. Ultra-broadband coherent continua extending from 90?eV to more than 130?eV with smooth spectral intensity distributions that exhibit dramatic, never-before-observed sensitivity to the carrier-envelope offset (CEO) phase of the driver laser pulse were generated. These results suggest the feasibility of sub-100-attosecond XUV pulse generation for attosecond spectroscopy in the 100?eV range, and of a simple yet highly sensitive on-line CEO phase detector with sub-50-ms response time.

Powerful 170-attosecond XUV pulses generated with few-cycle laser pulses and broadband multilayer optics

Single 170-as extreme ultraviolet (XUV) pulses delivering more than 10 6 photons/pulse at ∼100 eV at a repetition rate of 3 kHz are produced by ionizing neon with waveform-controlled sub-5 fs near-infrared (NIR) laser pulses and spectrally filtering the emerging near-cutoff high-harmonic continuum with a broadband, chirped multilayer molybdenum-silicon (Mo/Si) mirror.

Extending the supercontinuum spectrum down to 200 nm with few-cycle pulses

By focusing 805 nm pulses of low energy (0.2-1 mJ) into atmospheric-pressure argon, a supercontinuum is generated with a short- wavelength cutoff of 640, 250 and 210 nm for initial pulse durations of 45, 10 and 6 fs, respectively. It is shown numerically that the large shift of the UV cutoff and many features of the spectrum are caused by terms beyond the slowly-varying- envelope approximation (SVEA). Their effect on pulse compression and filament length is also discussed.

Design, fabrication, and analysis of chirped multilayer mirrors for reflection of extreme-ultraviolet attosecond pulses

Chirped Mo/Si multilayer coatings have been designed, fabricated, and characterized for use in extreme-ultraviolet attosecond experiments. By numerically simulating the reflection of the attosecond pulse from a multilayer mirror during the optimization procedure based on a genetic algorithm, we obtain optimized layer designs. We show that normal incidence chirped multilayer mirrors capable of reflecting pulses of approximately 100 attoseconds (as) duration can be designed by enhancing the reflectivity bandwidth and optimizing the phase-shift behavior. The chirped multilayer coatings have been fabricated by electron-beam evaporation in an ultrahigh vacuum in combination with ion-beam polishing of the interfaces and in situ reflectivity measurement for layer thickness control. To analyze the aperiodic layer structure by hard-x-ray reflectometry, we have developed an automatic fitting procedure that allows us to determine the individual layer thicknesses with an error of less than 0.05 nm. The fabricated chirped mirror may be used for production of 150-160 as pulses.

Ion-charge-state chronoscopy of cascaded atomic Auger decay

It has recently been demonstrated that apart from the electron detection realized in the attosecond streak camera, also ion detection can be used for establishing extreme-ultraviolet pump/visible probe experiments, temporally resolving the dynamics of atomic inner-shell relaxation processes. We utilize this method for studying the Auger decay of krypton atoms following the creation of vacancy states in the 3d shell. It is shown that the electronic relaxation occurs through different pathways, each involving cascades of sequential steps which are followed in their native temporal succession.

Attosecond physics comes of age: from tracing to steering electrons at sub-atomic scales

Efforts to access ever shorter time scales are motivated by the endeavour to explore the microcosm in ever smaller dimensions. At the turn of the millennium, one and a half decades after the first real-time observation of molecular dynamics with femtosecond laser pulses (1 fs  = 10-15 s), we witnessed the emergence of sub-femtosecond (that is: attosecond) pulses (1 as  = 10-18 s). They have been produced in the extreme ultraviolet regime by nonlinear frequency conversion of femtosecond laser pulses. A precise control of the hyperfast electric field oscillations of the driving femtosecond pulses not only allowed the controlled generation of single attosecond pulses and their full characterization but also, for the first time, steering and tracing the atomic-scale motion of electrons.