Dr. Esteban Goetz, Department of Physics, University of Connecticut

Interferometric Harmonic Spectroscopy for Electron Dynamics Imaging and Attosecond Pulse Train Phase Characterization

The advent of ultrashort light pulses has opened the possibility of investigating atomic and molecular processes on their natural time scales. In particular, Attosecond Transient Absorption Spectroscopy (ATAS) [1], a technique that allows to time-resolve the quantum dynamics of electrons by monitoring the absorption of extreme ultraviolet (XUV) radiation by an atomic or molecular system when the latter is dressed by an infrared (IR) laser source.

Motivated by recent experimental advances in self-referenced interferometric harmonic spectroscopy [2], we theoretically investigate an alternative approach to ATAS for electron dynamics imaging and attosecond pulse train (APT) phase characterization. In contrast to ATAS, which gives access to the imaginary part of the refractive index through an absorption measurement, an interferometric phase measurement gives information of its real part. In this talk, I will discuss the link between the XUV phase measurements of Ref. [2] and the different photoexcitation pathways occurring at the atomic level which are imprinted in the real part of the macroscopic refractive index. As an application, we show how such an interferometric approach can be used for phase retrieval of attosecond pulse trains based on two-arm harmonic spectroscopy and an optimization algorithm. Finally, I will highlight the impact of spin-orbit couplings and macroscopic and field propagation effects on the phase measurements and APT phase retrieval. Our theoretical description is based on numerical solution of the scalar Maxwell equations beyond Beer’s Law for the macroscopic field propagation coupled to the time-dependent Schroedinger equation for the quantum dynamics.

[1] M. Holler et al., Phys. Rev. Lett. 106, 123601 (2011)

[2] G. R. Harrison et al., arXiv:2305.17263 (2023)

Graduate student Geoff Harrison, Department of Physics, University of Connecticut

ITAS: A Technique for Complete Quantum Measurements on a New Timescale

Transient absorption spectroscopy is a well-established technique used to study electron dynamics in atomic and molecular systems but typically can only measure the magnitude of the electronic wavefunction. We have integrated interferometric methods into this technique to allow complete quantum measurements of both the magnitude and phase of electronic wavefunctions. A spatial light modulator (SLM) is used to separate the interferometric arms in an extremely stable way, enabling the measurement of effects on the zeptosecond timescale (with a jitter of 3zs). In this talk, I’ll describe how we’ve utilized SLMs to make these measurements possible and share some initial data we’ve taken looking at phase effects in argon.

Graduate student Debadarshini Mishra, Department of Physics, University of Connecticut

Imaging ultrafast dynamics in molecular systems

Imaging electronic and molecular dynamics at the attosecond and femtosecond timescales is crucial for understanding the mechanisms of chemical reactions, a fundamental aspect in fields ranging from materials science to biochemistry. This in-depth understanding of chemical processes may allow for precise control over reaction dynamics, thereby paving the way for advancements in technology and medicine, for example, by guiding the development of efficient catalysis, innovative materials, and targeted drugs. In this talk, I will describe our work on imaging time-resolved molecular dynamics using two distinct and complementary techniques.

In the first part of my talk, I will discuss the use of coincident Coulomb explosion imaging for the direct visualization of roaming reactions. These reactions represent unconventional pathways that allow fragments to remain weakly bonded, leading to the formation of unexpected final products. Typically, the neutral character of the roaming fragment and its indeterminate trajectory make direct experimental identification challenging. However, I will demonstrate that by leveraging the power of coincidence imaging, we can reconstruct the momentum vector of the neutral roamer and thus identify an unambiguous signature for roaming.

In the second part of my talk, I will discuss the imaging of UV-induced ring-opening and dissociation dynamics using ultrafast electron diffraction. I will demonstrate that by harnessing the superior temporal and structural resolution of this technique, we can explore the competition among different molecular pathways as well as their wavelength-dependent behavior.

Dr. François Légaré, Institut national de la recherche scientifique, Energy Materials Télécommunications center

Ultrafast IR/mid-IR laser technologies and their applications at ALLS

The Advanced Laser Light Source (ALLS) is a unique user facility located at INRS-EMT (Varennes, Canada) counting on 40M CDN$ of investment since 2002. Since 2019, this facility has jointed the LaserNetUS network and is now funded as a national research infrastructure by the Canada Foundation for Innovation – Major Science Initiatives. These fundings ease access to the facility for academic and government users. In the first part of my talk, I will give an overview of the facility’s capabilities including the most powerful laser in Canada with 750 TW. In the second part, I will discuss novel approaches developed by my team for the generation of ultrashort pulses in the IR and mid-IR spectral range. This includes multidimensional solitary states in hollow core fibers [1,2] as well as using the frequency domain optical parametric amplification for the generation of tunable CEP stable mid-IR laser pulses [3,4]. Pulse characterization in the mid-IR spectral range will be presented [5]. Finally, I will present recent results on the generation of high-dose MeV electrons from tight focussing in air [6].

References

[1] R. Safaei, G. Fan, O. Kwon, K. Légaré, P. Lassonde, B. E. Schmidt, H. Ibrahim, and F. Légaré (2020), High-energy multidimensional solitary states in hollow core fiber, Nature Phot. 14, 733-739.

[2] L. Arias, A. Longa, G. Jargot, A. Pomerleau, P. Lassonde, G. Fan, R. Safaei, P. Corkum, F. Boschini, H. Ibrahim, and F. Légaré, Few-cycle Yb laser source at 20 kHz using multidimensional solitary states in hollow-core fibers, Opt. Lett. 47, 3612-3615 (2022).

[3] A. Leblanc, G. Dalla-Barba, P. Lassonde, A. Laramée, B. Schmidt, E. Cormier, H. Ibrahim, and F. Légaré (2020), High-field mid-infrared pulses derived from frequency domain optical parametric amplification, Opt. Lett. 45, 2267-2270.

[4] G. Dalla-Barba, G. Jargot, P. Lassonde, S. Tóth, E. Haddad, F. Boschini, J. Delagnes, A. Leblanc, H. Ibrahim, E. Cormier, and F. Légaré, Mid-infrared frequency domain optical parametric amplifier, Opt. Express 31, 14954-14964 (2023).

[5] A. Leblanc, P. Lassonde, S. Petit, J.-C. Delagnes, E. Haddad, G. Ernotte, M. R. Bionta, V. Gruson, B. E. Schmidt, H. Ibrahim, E. Cormier, and F. Légaré (2019), Phase-matching-free pulse retrieval based on transient absorption in solids, Opt. Express 27, 28998.

[6] S. Vallières, J. Powell, T. Connell, M. Evans, M. Lytova, F. Fillion-Gourdeau, S. Fourmaux, S. Payeur, P. Lassonde, S. MacLean, and F. Légaré, High Dose-Rate MeV Electron Beam from a Tightly-Focused Femtosecond IR Laser in Ambient Air (2024), Laser Photonics Rev. 18, 2300078.

François Légaré is a chemical physicist who specializes in developing novel approaches for ultrafast science and technologies, as well as biomedical imaging with nonlinear optics (Ph.D. in chemistry, 2004 – co-supervised by Profs. André D. Bandrauk and Paul B. Corkum). Full professor (2013 - …) at the Energy Materials Telecommunications center of the Institut national de la recherche scientifique (INRS-EMT), he was the director of the Advanced Laser Light Source (ALLS) until 2023. Since 2022, he is the director of the INRS-EMT center and CEO of ALLS. Under his scientific leadership, INRS has received in 2017 a grant of 13.9M CDN$ from the Canada Foundation for Innovation and the Quebec government, with 11.9M CDN$ to upscale the ALLS facility with high average power Ytterbium laser systems and advanced instrumentation for time-resolved material characterization. He is a Fellow and senior member of OPTICA and Fellow of the American Physical Society. He is a member of The College of New Scholars, Artists and Scientists of the Royal Society of Canada (2017). He was awarded the Herzberg medal from the Canadian Association of Physics in 2015 and the Rutherford Memorial Medal in physics of the Royal Society of Canada in 2016. He has contributed to about 200 articles in peer reviewed journals including prestigious ones such as Nature, Science, Nature Photonics, Nature Physics, Nature Communications, and Physical Review Letters. According to Google Scholar, his h-index is 59 with more than 13,000 citations.

Dr. Thomas Weinacht, Stony Brook University

Long Lived Electronic Coherences in Molecules

I will discuss experiments and calculations that demonstrate long lived electronic coherences in molecules using a combination of measurements with shaped octave spanning ultrafast laser pulses, 3D velocity map imaging and calculations of the light matter interaction. Our pump-probe measurements prepare and interrogate entangled nuclear-electronic wave packets whose electronic phase remains well defined despite vibrational motion along many degrees of freedom. The experiments and calculations illustrate how coherences between excited electronic states survive even when coherence with the ground state is lost, and may have important implications for light harvesting, electronic transport and attosecond science.

Dr. Edwin Pedrozo, MIT

Quantum Sensors and the Search for New Physics

The continuously improving performance of quantum sensors is enabling the exploration of fundamental physics with unprecedented precision. Notable examples of these systems include optical atomic clocks and atom interferometers, which are among the most precise devices ever invented by humankind. As a result, they are increasingly utilized in the search for new physics. The application of Atomic, Molecular, and Optical (AMO) Physics techniques to such inquiries in the realm of nuclear physics has been gaining attention in the current decade. The level of control and precision achievable in AMO tabletop experiments, especially with ultracold atoms, enhances the measurement capabilities in complex experimental systems that pursue tests of fundamental physics and symmetries, the search for the electron electric dipole moment (eEDM), and physics beyond the standard model. In this talk, I will explain how incorporating entanglement into these systems can further improve their measurement capabilities. Additionally, I will discuss several proposals that employ laser-cooled atoms and molecules in the search for physics beyond the Standard Model.

Professor Chii-Dong Lin, Kansas State University

Post-Nobel Award on attosecond Science – Challenges and opportunities in the field going forward

It is an exciting month for the attosecond and strong-field physics communities after the announcement of the three Nobel Laureates earlier. How will this field evolve going forward? While it is very attractive to talk about the shortest light pulses to the general public, and even to the physical science community, the field still faces great challenges but also opportunities. I will “talk” about the challenges and will share with you some of the recent progress toward developing theories that can be compared to experiments.

Dr. Sandra Beauvarlet, UConn and PULSE, SLAC National Laboratory

Attosecond X-ray pulse pair generation at SLAC- LCLS X-ray Free Electron Laser and application to probe ultrafast electron dynamics in aminophenol

I will present in this AMO seminar general notions about how Free Electron Laser (FEL) work and will present some of the characteristics and more recent developments at the SLAC X-ray FEL light source. I will discuss the key advances to reach the sub-fs timescale enabling the production of isolated attosecond X-ray pulses but also the generation of attosecond X-ray pulse pairs with controllable delays. These pulses open the way to pump-probe measurements of ultrafast dynamics with attosecond temporal resolution and angstrom spatial resolution due to the X-ray nature of the light produced. Going from a more technical development perspective to an experimentalist vision, I will report on two examples of attosecond X-ray pump - attosecond X-ray probe measurements conducted on gas phase Aminophenol molecules (C ₆ H ₇ NO). The first one relies on carbon K-shell ionization and the effect of post-collision interaction (PCI). The second one relies on X-ray absorption spectroscopy to probe charge migration across the molecules on a sub-10 fs timescale.

Dr. Heide Ibrahim, INRS, Canada

Filming ultrafast chemical reactions in real-time – from coherent motion to roaming dynamics

Upon photoexcitation, molecules can undergo different types of transformations such as simple vibrations or rotations, but also migrations, or dissociations. Due to the required combination of ultrafast time-scales and ultra-small length-scales involved, specific tools are required to follow such reactions in real-time. Here, we are using time-resolved Coulomb explosion imaging (CEI) as the ultrafast tool to image such dynamics. With CEI, we can not only image coherent molecular dynamics such as proton migration in acetylene; it also works for various dissociation pathways, even if they occur differently from one molecule to another.

In the formaldehyde molecule, we can see fragments following the direct, conventional dissociation path, as well as fragments deviating from this minimum energy path. So-called roaming fragments or “roamers” explore the potential energy landscape in a statistical manner and were directly captured in real-time, despite the signal’s statistical character. This is possible due to the single-molecule sensitivity of CEI. In addition to the first direct observation of roaming fragments undergoing dynamics, we could show that the onset of roaming occurs actually several orders of magnitude earlier than previously expected. We thus show that CEI provides the means to extract new, unexpected pathways, which would otherwise remain hidden underneath a strong background.

Prof. George Gibson, Department of Physics, University of Connecticut

The influence of molecular structure on strong-field interactions and the problem of coupling three electrons

1D 1-electron models explain a surprising amount of phenomena in the strong-field interaction with atoms and molecules, such as ionization rates, enhanced ionization at a critical internuclear separation, and high-harmonic generation. 1D 2-electron models predict multielectron phenomena, such as enhanced multiphoton susceptibilities leading to strong multiphoton excitation. In this talk, we examine 1D 3-electron systems to address the possibility of direct inner-orbital ionization. Indeed, an old question of excitation through inner-orbital ionization, versus a two-step process, in which ionization is followed by excitation can finally be answered. Based on symmetry arguments, we show unambiguously that direct inner-orbital ionization is possible.

Graduate student Phi-Hung Tran, Department of Physics, University of Connecticut

An accurate semiclassical method for constructing photoelectron momentum distribution for atoms and molecules in intense lasers

We propose a version of the semiclassical method based on the Herman-Kluk propagator combined with the strong-field approximation for atoms and molecules in intense lasers. Here we illustrate the application of the method for calculation of photoelectron momentum distribution (PMD) for different atoms (H, Ar, Ne, and Xe) in intense few-cycle laser pulses. We show that the constructed PMDs agree very well with the exact numerical solutions of the time-dependent Schrödinger equation. We further show that our method provides a clear physical interpretation for the validity of the factorization of the recollision process. In contrast to the common belief that there are two paths, so-called long and short trajectories, we found that there are typically multiple trajectories that lead to a given final momentum in the high-energy region. Accurate phases of those trajectories are needed to obtain correct interference patterns in the PMD. Our method can be used to extend current capabilities of laser-induced electron diffraction and other ultrafast imaging and strong-field spectroscopic techniques.