Attosecond physics: the first decade
Ferenc Krausz1,2,3 1) Max-Planck-Institut für Quantenoptik, Garching, Germany. 2) Ludwig-Maximilians-Universität München, München, Germany 3) www.attoworld.de
Electron motion and light waves form the basis of life: the microscopic motion of electrons creates light, which supplies our globe with life-giving energy from the sun; electrons transform light into biological energy during photosynthesis and into biological signal endowing us with the capability of seeing the world around us. Upon their motion inside and between atoms, electrons emit light, carry and process information in biological systems and man-made devices; create, destroy, or modify molecules, affecting thereby biological function. Consequently, they are key players in physical, chemical, and life sciences; information, industrial, and medical technologies likewise. During the past ten years (2001-2011), advances in laser science opened to door to watching and controlling these hitherto inaccessible dynamics: the motion of electrons at the atomic scale and light wave oscillations (being mutually the cause of each other) evolving on attosecond time scales. Key tools include waveform-controlled few-cycle laser light and attosecond pulses of extreme ultraviolet and soft-X-ray light. They provide a force capable of steering electrons inside and between atoms and a probe for tracking their motion. Insight into and control over microscopic electron motion are likely to be important for developing brilliant sources of X-rays, understanding molecular processes relevant to the curing effects of drugs, the transport of bioinformation, or the damage and repair mechanisms of DNA, at the most fundamental level, where the borders between physics, chemistry and biology disappear. Once implemented in condensed matter, the new technology will be instrumental in advancing electronics and electron-based information technologies to their ultimate speed: from microwave towards lightwave frequencies.
Attosecond Electron Interferometry
Johan Mauritsson Lund University, P. O. Box 118, SE-221 00 Lund, Sweden Johan.Mauritsson@fysik.lth.se
To understand dynamic processes involving atoms, molecules, clusters, and their ions the underlying electron dynamics has to be captured and ultimately controlled. Here we present three different interferometric pump-probe methods aiming to access not only the temporal dynamics, but also state specific phase information after excitation/ionization using attosecond pulses. These pulses have intrinsically very broad coherent bandwidths and in order to obtain state specific information we need to achieve a spectral resolution much better than the inverse of the pulse duration. We do this using either a train of pulses, which corresponds to a frequency comb in the spectral domain, or a pair of pulses, analog to traditional Ramsey spectroscopy, with the difference that the pulses have different frequencies. In the experiments we: 1) measure the intensity dependence of the 2s-3p transition energy in helium using resonant two-color two-photon ionization [1]; 2) characterize an excited electron wave packet in helium by interfering it with a known reference [2]; and 3) measure the difference in time delay between electrons emitted from different sub-shells in argon [3]. The spectral resolution in the experiments is then given either by the number of attosecond pulses [1] and [3] or the inverse of the pump-probe delay [2], which can easily be two orders of magnitude better than the Fourier limit of the excitation pulse.
[3] K. Klünder, et al., “Probing Single-Photon Ionization on the Attosecond Time Scale”, Phys. Rev. Lett. 106, 143002 (2011)
When does an electron exit a tunneling barrier?
Dror Shafir1 , Hadas Soifer1 , Barry D. Bruner1 , Michal Dagan1 , Yann Mairesse 2 , Serguei Patchkovskii3 , Misha Yu. Ivanov4,5, Olga Smirnova5 and Nirit Dudovich1
5. Max-Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born-Strasse 2A, D-12489 Berlin, Germany
Tunneling of a particle through a barrier is one of the most fundamental and ubiquitous quantum processes. When induced by an intense laser field, electron tunneling from atoms and molecules initiates a broad range of processes that evolve on an attosecond time-scale. As the liberated electron is driven by the laser field, it can return to the parent ion and recombine to the initial (ground) state, releasing its energy in an attosecond burst of light. This process, known as High Harmonic Generation (HHG), serves as an excellent spatio-temporal filter for the electron motion. The angstrom-scale spatial resolution is determined by the size of the atomic ground state to which the electron must recombine. The attosecond temporal resolution arises from the mapping between the photon energy (harmonic order) and the return time of the corresponding electron trajectory. In my talk I will describe how adding a weak perturbation allows us to probe both the ionization times and the recollision times in simple atomic systems. Our results, which deviate from the simple classical model, are in good agreement with quantum path analysis. The next step is the probing of molecular systems where more than one ionization channel participates in the process. As an example, I will show how multiple channel ionization is probed in aligned CO2 molecules. I will describe how the high sensitivity of the measurement allows us to probe subtle differences between two ionization channels. This experiment provides an additional, important step toward achieving the ability to resolve multielectron phenomena -- a long-term goal of attosecond studies.