The research supported by this award addresses the exciting phenomena resulting from the extremely nonlinear interaction of a macroscopic number of atoms with an intense laser pulse. The coherent extreme ultraviolet (XUV) radiation generated in this process presents a unique and versatile source for spectroscopy, coherent control, and femtosecond/attosecond dynamics. Some of the sub-projects of the award will explore novel ways to optimize and control the XUV generation process, and other sub-projects aim at using the XUV light as a diagnostic of the quantum dynamics that produced it. The work will be accomplished through the continued development and application of an ab-initio, non-adiabatic theoretical model for the interaction of atoms in the gas phase with ultrashort laser pulses. This "numerical non-linear medium" allows for the complete simulation of the most advanced short pulse laser experiments from first principles. The majority of the calculations envisioned are directly relevant to ongoing collaborations with four experimental groups at the forefront of ultrafast strong field physics.
Coherent, ultrafast XUV pulses allow for probing and manipulating atomic, molecular and biological processes at few-femtosecond and sub-femtosecond time scales, yielding new ways to understand and control matter via its interaction with light. Ultrafast strong-field physics continues to be dominated by European groups, with strong efforts also underway in Asia. The research supported under this award will strengthen the US presence in this field, and will help train the next generation of scientists in this interdisciplinary field of atomic, molecular and optical science. Postdoctoral researchers and graduate/undergraduate students will gain a broad education in both atomic physics and high intensity laser science. Through close collaborations with experimental groups they will have a thorough understanding of the experimental as well as the theoretical issues involved.
Coherent, ultrafast extreme ultraviolet (XUV) pulses allow for probing and manipulating atomic, molecular and biological processes at few-femtosecond and subfemtosecond time scales, yielding new ways to understand and control matter via its interaction with light. The natural time scale for the dynamics of electrons in small quantum systems is the attosecond to few-femtosecond time scale, and a natural way to probe this dynamics is via ultra short pulses of light. Our theory group has two main focus areas which can be broadly characterized as (i) the production of attosecond laser pulses in the XUV regime, which happens through the highly nonlinear process of high harmonic generation (HHG), and (ii) the application of attosecond pulses to initiate ultrafast dynamics in atoms or molecules, which we have studied predominantly through the process of attosecond transient absorption. The primary theoretical tool we are continuously using and developing is a numerical non-linear medium (NNLM) which solves the coupled time-dependent Schrödinger equation and the Maxwell wave equation in order to describe the microscopic and macroscopic effects of intense, ultrafast laser pulses interacting with a non-linear gas medium. The research outcomes of this project have been shared with the scientific community through peer-reviewed publications (13), a book chapter, invited talks at national and international conferences (six by the PI, one by a post-doc), and contributed talks (two) and posters (four) by junior group members. A large fraction of the work was performed in collaboration with experimental groups at other universities in the US and Europe. A brief overview of the particular results from this project is given below, divided into areas (i) and (ii) as characterized above. (i) There is interest in the strong field community in using laser pulses with mid-infrared (MIR) wavelengths to drive HHG because of their potential for extending the harmonic radiation spectrum to much higher photon energies. In collaboration with experimental colleagues, we studied MIR-HHG in argon and found that characteristics of the argon atomic structure was imprinted on the harmonic spectral ampitude and phase, which led to a reshaping of the generated attosecond pulses in the time domain [PRL 112, 152001 (2014)]. In a collaboration with other theory colleagues, we have investigated the effect on HHG of a permanent dipole in polar molecules [PRA 84, 023418 (2011), PRA 86, 023818 (2012)]. Finally, we have investigated HHG in different experimental geometries in which the propagation of the driving laser pulse is complex and has a profound influence on the characteristics of the generated harmonics [PRA 83, 053804 (2011), NJP 13, 073035 (2011), Opt. Expr. 19, 19495 (2011)]. (ii) Absorption and dispersion are usually thought of as linear processes and described in the frequency domain. However, in the presence of a strong field and a range of XUV frequencies in the form of an attosecond pulse (or an attosecond pulse train), an atom may absorb light at multiple frequencies and in multiple orders of nonlinearity. For this project, we developed a framework for calculating absorption probabilities for an atom interacting with such ultrafast, multi-color laser fields [PRA 83, 013419 (2011)]. We have also extensively tested that this framework works in connection with the NNLM so that we are also able to describe the macroscopic aspects of the dynamical absorption processes. We have used this framework in a number of studies of the transient absorption of attosecond XUV pulses by helium atoms interacting with a moderately strong IR field [OL 37, 2211 (2012), PRA 86, 063408 (2012), PRA 87, 013828 (2013), PRA 87, 033408 (2013), PRA 88, 033409 (2013), PRA 88, 043416 (2013)]. The attached figure demonstrates several examples by showing the helium absorption spectrum as a function of the IR-XUV delay. In terms of work-force development, a number of junior researchers have been involved in this project: One postdoc, four graduate students (two from other universities involved through collaborations), and two undergraduate students. These junior researchers have been mentored and trained in a variety of ways from learning best practices in forefront science, over highly specialized knowledge in ultrafast AMO physics and high performance computing, and to the dissemination of research results to the scientific community. Junior researchers have presented their work at national and international meetings as described above. The training of the next generation of scientists will be an important aspect of maintaining a presence of ultrafast AMO science in the United States. Finally, the PI served as chair of one of the 2014 regional Conferences for Undergraduate Women in Physics (CUWiP). The CUWiP allow undergraduate women physicists to meet, network with, and be inspired by both peers and established women scientists. Maintaining and increasing gender diversity in physics is desirable, and the CUWiP aim to help by inspiring and encouraging women to continue and utilize their physics careers.
