Ph.D., 1997 - University of Copenhagen, Denmark

Louisiana State University

Department of Physics & Astronomy

215-B Nicholson Hall, Tower Dr.

Baton Rouge, LA 70803-4001

(225) 578-0889-Office

gaarde@phys.lsu.edu

My research program is centered around the production and application of ultrafast pulses of coherent VUV and XUV light, and lies in the interface between ultrafast AMO physics and extreme non-linear optics. Attosecond pulses, which are generated through the extremely nonlinear process of high harmonic generation (HHG), are the shortest bursts of light ever produced and allow for probing and controlling the dynamics of bound electrons on their natural time scales.

In the LSU attosecond theory group we do many different types of calculations to address these ultrafast dynamics. These are mainly centered around the solution of the time-dependent Schrödinger equation (TDSE), spanning from using the strong field approximation to essential states models to full ab-initio solutions for one and two-electron systems. We also study the interplay between the microscopic strong-field laser-matter interactions and the macroscopic effects resulting from the propagation of the strong driving field and the weaker XUV fields through a macroscopic nonlinear medium. To this end we solve the coupled TDSE and the Maxwell wave equation in real time, and at the sub-cycle level which requires large-scale computations. We are currently applying these different models to four different types of projects:

**HHG from transparent solids:** Recent experimental and theoretical results have shown that not only is it possible
to generate high order harmonics, and potentially attosecond pulses, from transparent
solids - we can also learn a lot about ultrafast electron dynamics in the condensed
phase by studying the HHG process. See press release about recent Nature paper, a
collaboration between LSU theory and experiments at SLAC National Accelerator Laboratory
and Stanford University: http://www.lsu.edu/physics/news/2016/06/20160606-frozen-argon.php and [1,2] below.

**Attosecond transient absorption** (ATA): ATA spectroscopy is a pump-probe scenario in which an attosecond XUV pulse
and a delayed but synchronized IR pulse can be used to study electron dynamics on
its natural time scale. We study a range of ATA phenomena in atoms, molecules, and
solids using a time-domain description that allows us to treat on an equal footing
all the different linear and nonlinear processes by which the medium can exchange
energy with the XUV and IR fields, see [3,4] below. Much of this work has been done
in collaboration with experimental groups in the US and Europe.

**HHG as probe of ultrafast charge migration:** Our goal is to measure and control the quantum dynamics of electrons and holes in
polyatomic molecules through the signatures that the hole motion imprints on high
order harmonics emitted by these molecules. This project is a multi-institutional
collaboration between theory at LSU, and experiments at the Ohio State University
and the University of Virginia.

**Filamentation of intense mid-infrared (MIR) laser pulses:** When intense pulses propagate through air, they can form a so-called filament in
which several highly nonlinear processes balance each other to allow for propagation
over long distances without collapse or substantial energy loss. As part of a multi-institutional
project spanning many labs in the US and Europe, we are currently investigating novel
effects in filamentation of few-cycle MIR pulses.

(1) G. Ndabashimiye, S. Ghimire, M. Wu, D. A. Browne, K. J. Schafer, M. B. Gaarde, and D. A. Reis, Solid-state harmonics beyond the atomic limit, Nature 534, 520 (2016).

(2) M. Wu, D. A. Browne, K. J. Schafer, and M. B. Gaarde, Multi-level perspective on high-order harmonic generation in solids, Phys. Rev. A 94, 063403 (2016).

(3) M. Wu, S. Chen, S. Camp, K. J. Schafer, and M. B. Gaarde, Theory of strong-field attosecond transient absorption, Topical Review, J. Phys. B 49, 062003 (2016).

(4) C.-T. Liao, A. Sandhu, S. Camp, K. J. Schafer, and M. B. Gaarde, Beyond the single-atom response in absorption line shapes: Probing a dense, laser-dressed helium gas with attosecond pulse trains, Phys. Rev. Lett. 114, 143002 (2015).

(5) S. B. Schoun, R. Chirla, J. Wheeler, C. Roedig, P. Agostini, L. F. DiMauro, K. J. Schafer, and M. B. Gaarde, Attosecond pulse shaping around a Cooper minimum, Phys. Rev. Lett. 112, 153001 (2014).

(6) D. S. Steingrube, E. Schulz, T. Binhammer, M. B. Gaarde, A. Couairon, U. Morgner, and M. Kovacev, High-order harmonic generation directly from a filament, New Journal of Physics 13, 043022 (2011).

(7) M. B. Gaarde, J. L. Tate, and K. J. Schafer, Macroscopic aspects of attosecond pulse generation, Topical Review, J. Phys. B 41, 132001 (2008). Selected as one of 50 most influential papers in Journal of Physics.

For a full list of publications see here