# Mette Gaarde

## Les and Dot Broussard Alumni Professor of Physics

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

mgaarde1@lsu.edu

### Research Interests

#### Ultrafast Atomic, Molecular and Optical Physics - Theory

My research program is centered around probing and controlling the ultrafast laser-matter interactions in atomic, molecular, and condensed-phase systems, involving a wide range of ultrafast dynamics. Our studies include both the production and application of attosecond and femtosecond pulses of coherent VUV and XUV light, and lies in the interface between ultrafast AMO science 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 ultrafast AMO theory group we do many different types of calculations to address these dynamics. At the microscopic level, studying the dynamics of the individual quantum system, we use the time-dependent Schrödinger equation, time-dependent density functional theory, or the semi-conductor Bloch equations. A long-time focus of my research has been the interplay between the microscopic (quantum) effects and the macroscopic (classical) effects that govern many intense laser-matter interactions. To this end we solve coupled equations for the microscopic response (as above) with 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:

**Ultrafast dynamics in transparent solids: **Recent experimental and theoretical results have shown that not only is it possible
to generate high order harmonics from transparent solids - we can also learn a lot
about ultrafast electron dynamics in the condensed phase by studying the HHG process.
Recent theory efforts in our group have shown that HHG in solids often involve imperfect
recollisions of electron-hole pairs [1], facilitated by the highly delocalized nature
of the electron and hole wave packets in the condensed phase. In previous works we
have demonstrated the influence of higher-lying conduction bands, in collaboration
with experimental collaborators at SLAC/Stanford [2]. For further theoretical understanding
of solid-state HHG, see our recent tutorial paper [3].

**Transient absorption and reshaping of attosecond pulses**: Attosecond transient absorption (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 [4]. In collaboration with several different
experimental groups in the US and Europe we have studied a range of ATA phenomena
in atoms, molecules, and solids using a description that allows us to treat both linear
and nonlinear ultrafast dynamics across temporal, spectral, and spatial dimensions
[5-7].

**Probing ultrafast charge migration**: Charge migration refers to the coherent electron dynamics taking place in a molecule
in the first few femtoseconds after ultrafast excitation or ionization. In collaboration
with the LSU Dept. of Chemistry, and experimental groups at the Ohio State University
and University of Virginia, we are investigating how to initiate, describe, and experimentally
measure charge migration in organic molecules [8-12]. At the theory level, we have
demonstrated robust, particle-like charge migration [8,9] and shown that it can probed
with high-harmonic spectroscopy [10].

**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 investigating novel effects
in filamentation of few-cycle MIR pulses, see [13-14] below.

### Current and Select Publications

- L. Yue and M. B. Gaarde, Imperfect Recollisions in High-Harmonic Generation in Solids, Phys. Rev. Lett. 124, 153204 (2020).
- 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).
- L. Yue and M. B. Gaarde, Introduction to Theory of High-Harmonic Generation in Solids: Tutorial, J. Opt. Soc. Am. B 39, 535-555 (2022).
- 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).
- A. P. Fidler, S. J. Camp, E. R. Warrick, E. Bloch, H. J. B. Marroux, D. M. Neumark, K. J. Schafer, M. B. Gaarde, and S. R. Leone, Nonlinear XUV Signal Generation Probed by Attosecond Transient Grating Spectroscopy, Nature Communications 10, 1384 (2019).
- S. Bengtsson, E. W. Larsen, D. Kroon, S. Camp, M. Miranda, C. L. Arnold, A. L’Huillier, K. J. Schafer, M. B. Gaarde, L. Rippe, and J. Mauritsson, Controlled free-induction decay in the extreme ultraviolet, Nature Photonics 11, 252 (2017).
- K. Li, M. Labeye, P. J. Ho, M. B. Gaarde, and L. Young, Resonant propagation of x-rays from the linear to the nonlinear regime, Phys. Rev. A 102, 053113 (2020).
- A. S. Folorunso, A. Bruner, F. Mauger, K. A. Hamer, S. Hernandez, R. R. Jones, L. F. DiMauro, M. B. Gaarde, K. Lopata, and K. J. Schafer, Molecular modes of attosecond charge migration, Phys. Rev. Lett. 126, 133002 (2021).
- F. Mauger, A. S. Folorunso, K. A. Hamer, C. Chandre, M. B. Gaarde, K. Lopata, and K. J. Schafer, Charge migration and attosecond solitons in conjugated organic molecules, Phys. Rev. Res. 4, 013073 (2022).
- K. Hamer, F. Mauger, A. Folorunso, K. Lopata, R. R. Jones, L. F. DiMauro, K. J. Schafer, and M. B. Gaarde, Characterizing Particle-Like Charge Migration Dynamics with High-Harmonic Sideband Spectroscopy, Phys. Rev. A 106, 013103 (2022).
- T. T. Gorman, T. D. Scarborough, P. M. Abanador, F. Mauger, D. Kiesewetter, P. Sandor, S. Khatri, K. Lopata, K. J. Schafer, P. Agostini, M. B. Gaarde, and L. F. DiMauro, Probing the Interplay between Geometric and Electronic-Structure Features via High-Harmonic Spectroscopy, J. Chem. Phys. 150, 184308 (2019).
- P. Sandor, A. Sissay, F. Mauger, M. Gordon, T. Gorman, T. Scarborough, M. B. Gaarde, K. Lopata, K. J. Schafer, and R. R. Jones, Angle-dependent Strong-Field Ionization of Halomethanes, J. Chem. Phys. 151, 194308 (2019).
- J. M. Brown, A. Couairon, P. Polynkin, and M. B. Gaarde, Analysis of the angular spectrum for ultrashort laser pulses, JOSA B 36, A105 (2019).
- R. Piccoli, J. Brown, Y.-G. Jeong, A. Rovere, L. Zanotto, M. B. Gaarde, F. Légaré, A. Couairon, J. C. Travers, R. Morandotti, B. E. Schmidt, and L. Razzari, Intense few-cycle visible pulses directly generated via nonlinear fibre mode mixing, Nature Photonics 15, 884 (2021).
- 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.