The present proposal focuses on the study and control of strong field processes via the application of attosecond pulses in combination with a variety of other intense, ultrafast sources. The short duration of the pulses and the strength of the applied fields mean that accurate calculations require a nonperturbative solution of the laser-matter interaction, which is achieved by direct integration of the time-dependent Schroedinger equation. All of the calculations envisioned are directly relevant to ongoing collaborations between the principle investigator and experimental groups at the forefront of strong field physics and attosecond science. Investigations to control strong field processes by quantum path selection using attosecond pulse trains synchronized to strong infrared fields, as well as a novel attosecond electron wave packet interferometer which uses the same configuration, are undertaken. In addition studies of the interaction of atoms with attosecond pulse trains having just one pulse per infraredcycle, an arrangement that can be used to freeze the motion of electrons periodically released into an infrared field, will be examined. Explorations of multiphoton physics in a new regime, the deep tunneling limit, reached when a high intensity, few cycle mid-infrared laser is used to ionize rare gas atoms, is also on the agenda. Of special interest is the possibility that these pulses can be used to generate attosecond pulses with a low average frequency, which would make them very useful for controlling ionization processes in atoms and molecules.
Our project focused on the theoretical study and control of strong field processes using a new form of ultrafast light: attosecond pulses. The name ``strong field physics'' refers to the interaction of intense, ultrafast near-infrared laser pulses with atomic and molecular systems. Strong field physics takes place in the interesting regime where the electron-ion and electron-laser interactions are of competing strengths. This gives rise to a host of time-dependent and highly non- linear phenomenon. During this project we extended our earlier studies of strong field phenomena by incorporating attosecond pulses in combination with intense laser pulses. An attosecond is just 1/1000th of a femtosecond, and attosecond pulses are the shortest light pulses that can be made. Though they are intrinsically weak, the attosecond pulses have wavelengths in the extreme ultraviolet (photon energies of 20-100 eV) and so can drive one photon ionization processes in the valence shells of atoms very efficiently. The attosecond pulses used in laboratories can be precisely synchronized to the phase of a laser field, which means that the they can be used to drive ionization at specific times during a laser cycle. This gives unprecedented control over strong field processes and opens the possibility of (i) better understanding of strong field processes such as high harmonic generation; (ii) the possibility of controlling strong field processes by, for example, seeding of high harmonic generation; and (iii) new processes that combine attosecond light and laser light. The findings and outcomes of our work are presented in 24 refereed publications, many appearing in expedited journals. Among the most noteworthy are: (i) The development of the theory of an "attosecond quantum stroboscope" that allowed for the study of ionizing electrons with sub-femtosecond time resolution. We pioneered the theory of these experiments, and collaborated with colleagues at Lund University in Sweden on the first experimental demonstration of the AQS. (ii) The discovery that ionization processes can be controlled with sub-femtosecond precision by creating multiple, interfering electron wave packets in a moderately strong laser field. The electron wave packets were created by a train of attosecond pulses that lunched electrons into a laser field just below the ionization threshold of the atom (helium). Our colleagues at Lund University demonstrated this effect for the first time as well. (iii) The theoretical description of a method for using electron wave packet interferometry to measure an attosecond electron wave packet. We described how a version of holography, familiar from ordinary light pulses, could be applied to electron wave packets. The idea is to launch two different electron wave packets from an atom toward a detector at two different times. If the electron wave packets have the same energy then they will create an interference pattern at the detector. This can be used to describe the motion of the electron during the time that it was bound to the atom. This concept was experimentally verified by an international team of experimental researchers based in Europe. (iv) We pioneered the development of attosecond transient absorption studies in atoms, in a form we called "time-dependent attosecond absorption" (TDAA). This is a form of absorption spectroscopy where the absorption of attosecond extreme ultraviolet light is monitored as a function of the delay between the attosecond light and a laser field that dresses the atomic transition. The combination of the very broad spectrum of the attosecond pulse and its synchronization to the field oscillations of the laser pulse make this a very useful tool for studying electron dynamics in strongly driven systems. Among the concepts we demonstrated are the existence of light-induced states in the transient spectra, and the possibility of interfering multiphoton processes, which can be used to time the absorption of laser photons in an atom. Experimental confirmation of these ideas came from our collaborators at the University of California Berkeley. These findings and outcomes align very well with the intellectual merit of the research, which is the advancement of attosecond science, especially as it relates to the study and control of electron dynamics in the valence shell of atoms and molecules. The impact of our work is chiefly in establishing several new ways in which strong field processes can be studied and controlled. These include electron wave packet interferometry and attosecond transient absorption. Throughout our work the training of graduate students and mentoring of postdoctoral researchers has been a priority. Occasionally, undergraduate students were involved in the research as well. The principle investigator also incorporated the themes and methods of the research into the teaching of time-dependent quantum mechanics in both graduate and undergraduate quantum mechanics courses.
