Reinhard Kienberger

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).

Time-resolved atomic inner-shell spectroscopy

The characteristic time constants of the relaxation dynamics of core-excited atoms have hitherto been inferred from the linewidths of electronic transitions measured by continuous-wave extreme ultraviolet or X-ray spectroscopy. Here we demonstrate that a laser-based sampling system, consisting of a few-femtosecond visible light pulse and a synchronized sub-femtosecond soft X-ray pulse, allows us to trace these dynamics directly in the time domain with attosecond resolution. We have measured a lifetime of 7.9-0.9+1.0 fs of M-shell vacancies of krypton in such a pump–probe experiment.

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 spectroscopy in condensed matter

Comprehensive knowledge of the dynamic behaviour of electrons in condensed-matter systems is pertinent to the development of many modern technologies, such as semiconductor and molecular electronics, optoelectronics, information processing and photovoltaics. Yet it remains challenging to probe electronic processes, many of which take place in the attosecond (1 as = 10-18 s) regime. In contrast, atomic motion occurs on the femtosecond (1 fs = 10-15 s) timescale and has been mapped in solids in real time using femtosecond X-ray sources. Here we extend the attosecond techniques previously used to study isolated atoms in the gas phase to observe electron motion in condensed-matter systems and on surfaces in real time. We demonstrate our ability to obtain direct time-domain access to charge dynamics with attosecond resolution by probing photoelectron emission from single-crystal tungsten. Our data reveal a delay of approximately 100 attoseconds between the emission of photoelectrons that originate from localized core states of the metal, and those that are freed from delocalized conduction-band states. These results illustrate that attosecond metrology constitutes a powerful tool for exploring not only gas-phase systems, but also fundamental electronic processes occurring on the attosecond timescale in condensed-matter systems and on surfaces.

Delay in Photoemission

Defining Time-Zero When a high-energy photon hits an atom and is absorbed, the result can be the excitation and emission of an electron. This photoemission, or photoelectric effect, is generally assumed to occur instantaneously, and represents the definition of “time-zero” in clocking such ultrafast events. Schultze et al. (p. 1658, see the cover; see the Perspective by van der Hart) use ultrafast spectroscopy, with light pulses on the time scale of several tens of attoseconds, to test this assumption directly. They excite neon atoms with 100 eV photons and find that there is a small (20-attosecond) time delay between the emission of electrons from the 2s and 2p orbitals of the atoms. These results should have implications in modeling electron dynamics occurring on ultrafast time scales. Ultrafast metrology reveals a 20-attosecond delay between photoemission from different electronic orbitals in neon atoms. Photoemission from atoms is assumed to occur instantly in response to incident radiation and provides the basis for setting the zero of time in clocking atomic-scale electron motion. We used attosecond metrology to reveal a delay of 21±5 attoseconds in the emission of electrons liberated from the 2p orbitals of neon atoms with respect to those released from the 2s orbital by the same 100–electron volt light pulse. Small differences in the timing of photoemission from different quantum states provide a probe for modeling many-electron dynamics. Theoretical models refined with the help of attosecond timing metrology may provide insight into electron correlations and allow the setting of the zero of time in atomic-scale chronoscopy with a precision of a few attoseconds.

Optical-field-induced current in dielectrics

The time it takes to switch on and off electric current determines the rate at which signals can be processed and sampled in modern information technology. Field-effect transistors are able to control currents at frequencies of the order of or higher than 100 gigahertz, but electric interconnects may hamper progress towards reaching the terahertz (1012 hertz) range. All-optical injection of currents through interfering photoexcitation pathways or photoconductive switching of terahertz transients has made it possible to control electric current on a subpicosecond timescale in semiconductors. Insulators have been deemed unsuitable for both methods, because of the need for either ultraviolet light or strong fields, which induce slow damage or ultrafast breakdown, respectively. Here we report the feasibility of electric signal manipulation in a dielectric. A few-cycle optical waveform reversibly increases—free from breakdown—the a.c. conductivity of amorphous silicon dioxide (fused silica) by more than 18 orders of magnitude within 1 femtosecond, allowing electric currents to be driven, directed and switched by the instantaneous light field. Our work opens the way to extending electronic signal processing and high-speed metrology into the petahertz (1015 hertz) domain.

Controlling dielectrics with the electric field of light

The control of the electric and optical properties of semiconductors with microwave fields forms the basis of modern electronics, information processing and optical communications. The extension of such control to optical frequencies calls for wideband materials such as dielectrics, which require strong electric fields to alter their physical properties. Few-cycle laser pulses permit damage-free exposure of dielectrics to electric fields of several volts per ångström and significant modifications in their electronic system. Fields of such strength and temporal confinement can turn a dielectric from an insulating state to a conducting state within the optical period. However, to extend electric signal control and processing to light frequencies depends on the feasibility of reversing these effects approximately as fast as they can be induced. Here we study the underlying electron processes with sub-femtosecond solid-state spectroscopy, which reveals the feasibility of manipulating the electronic structure and electric polarizability of a dielectric reversibly with the electric field of light. We irradiate a dielectric (fused silica) with a waveform-controlled near-infrared few-cycle light field of several volts per angström and probe changes in extreme-ultraviolet absorptivity and near-infrared reflectivity on a timescale of approximately a hundred attoseconds to a few femtoseconds. The field-induced changes follow, in a highly nonlinear fashion, the turn-on and turn-off behaviour of the driving field, in agreement with the predictions of a quantum mechanical model. The ultrafast reversibility of the effects implies that the physical properties of a dielectric can be controlled with the electric field of light, offering the potential for petahertz-bandwidth signal manipulation.

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.

High-repetition-rate picosecond pump laser based on a Yb:YAG disk amplifier for optical parametric amplification

We report an optically synchronized picosecond pump laser for optical parametric amplifiers based on an Yb:YAG thin-disk amplifier. At 3 kHz repetition rate, pulse energies of 25 mJ with 1.6 ps pulse duration were achieved with an rms fluctuation in pulse energy of <0.7% by utilizing a broadly intermittent single-energy regime in the deterministic chaos of a continuously pumped regenerative amplifier.