High harmonic generation (HHG) is a highly nonlinear process by which laser light with a wavelength in the infrared regime can undergo frequency up-conversion into the extreme ultraviolet (XUV) or even soft X-ray regime. HHG is also at the heart of the production of attosecond pulses, which are the shortest bursts of light ever produced, with durations that are more than a billion times shorter than a microsecond. The natural application of attosecond pulses is in the study of electron dynamics, which occurs on this time scale. Several decades of work have been dedicated to the development and application of attosecond light sources, and the proposed work addresses aspects of both development and applications. One of the proposed projects involves the study of ultrafast correlation between electrons during HHG in an atom. The correlation is expected to impart a signature on the time structure of the emitted attosecond XUV radiation. This project thus uses the process of HHG as a probe of the underlying electron dynamics. In another project HHG and attosecond pulse production in transparent solids, which have recently been experientally found to produce relatively high order harmonics, will be investigated. Although the process of HHG in dilute atomic gases is well understood, the efficiency of and the mechanism by which HHG are produced in these transparent solids is largely unknown. This project is thus a study in attosecond source development. All the proposed projects will be undertaken through large-scale calculations within a theoretical framework in which it is possible to describe the HHG process from the ab-initio quantum level to the macroscopic production of a measurable electric field. The proposed work will involve several new theoretical developments as well as a number of applications of the existing theoretical framework. The calculations envisioned are ambitious and will represent the state-of-the-art for the theory and computation of ultrafast laser-matter interactions. The majority of the proposed work is directly relevant to ongoing collaborations with experimental groups at the forefront of ultrafast strong field physics. The broader impacts of this project are diverse and involve the development of both scientific and human resources: (i) The development and application of ultrafast, coherent XUV sources further our fundamental understanding of the dynamics of atomic, molecular, and biological processes at the few-femtosecond and sub-femtosecond time scale, (ii) the proposed work helps to maintain a presence of ultrafast AMO science in the US while the field is generally dominated by groups in Europe, Asia, and Canada, and (iii) human resources will be developed through involvement, training, and mentoring of junior researchers such as undergraduate and graduate students and postdocs. The PI, who is female, is also actively involved in mentoring women in physics, in particular at the undergraduate level.
The research in this proposal centers on the study of ultrafast quantum dynamics through the strong-field production of high order harmonics and attosecond pulses in different macroscopic non-linear media. In particular, the work proposed is to study (i) the time-domain effects of correlation in two-electron systems through the characteristics of high order harmonics in the vicinity of auto-ionizing resonances, and more generally the effects of resonances (in one- or two-electron systems) on the microscopic and macroscopic properties of the harmonic radiation, (ii) high harmonic generation (HHG) in periodic media (transparent solids) which has recently been experimentally demonstrated, and (iii) HHG driven by mid-infrared laser pulses, in terms of their potential as a source of kiloelectron volt attosecond pulses. Some of these studies center on characterizing the inherent dynamics of the strong-field interaction with the quantum system, such as correlation, or the nature of the coherent process that leads to harmonics in solids. Other studies aim to characterize or optimize the properties of the VUV/XUV light itself. The proposed work will be accomplished through the continued development and application of an ab-initio, non-adiabatic theoretical model for ultrafast laser-matter interactions. This "numerical non-linear medium" (NNLM) allows for the complete simulation of the most advanced short pulse laser experiments from first principles. The NNLM is based on the coupled solutions of the time-dependent Schroedinger equation and the Maxwell wave equation with sub-cycle precision, and requires large scale computations.
