Andy Maxwell

Andy Maxwell

Postdoctoral physicist working on quantum aspects of attoscience

Selected Talks

Entanglement of Orbital Angular Momentum in Non-Sequential Double Ionization

September 14, 2023

Invited Talk, AttoChem Young Scientist Symposium 2023, Vienna, Austria

I will discuss our new results on entanglement and orbital angular momentum (OAM) in non-sequential double ionization (NSDI). We demonstrate that there is entanglement in the OAM between the two photoelectrons in NSDI. Due to the quantization of OAM, this entanglement is easily quantified and has a simple physical interpretation in terms of conservation laws. Using the strong-field approximation, we quantify the entanglement for a large range of parameters, isolating the best targets for experimentalists. We also explore efficient methods to quantify and measure the entanglement, in particular by using an entanglement witness. Importantly, the methodology presented here could be applied to many other systems to help understand and exploit entanglement in attosecond processes.

Entanglement of Orbital Angular Momentum in Non-Sequential Double Ionization

August 05, 2023

Invited Talk, 22nd International Symposium on Correlation, Polarization and Ionization in Atomic and Molecular Collisions, Toronto, Ontario, Canada

I will discuss our new results on entanglement and orbital angular momentum (OAM) in non-sequential double ionization (NSDI). We demonstrate that there is entanglement in the OAM between the two photoelectrons in NSDI. Due to the quantization of OAM, this entanglement is easily quantified and has a simple physical interpretation in terms of conservation laws. Using the strong-field approximation, we quantify the entanglement for a large range of parameters, isolating the best targets for experimentalists. We also explore efficient methods to quantify and measure the entanglement, in particular by using an entanglement witness. Importantly, the methodology presented here could be applied to many other systems to help understand and exploit entanglement in attosecond processes.

Spin-Orbit Dynamics and a Relativistic Surprise

June 29, 2023

Invited Talk, Quantum Battles 2023, University College London

Spin is often ignored in strong-field physics, as it couples non-dynamically to the initial states, while spin-resolved measurements have only recently become possible. Spin-orbit dynamics are neglected in strong-field ionization, as the photoelectron energies appear too low. However, long wavelengths used in imaging process such as laser induced electron diffraction (LIED), means that spin dynamics could play a role, through high-energy rescattering. Using the flexible Coulomb quantum-orbit strong-field approximation (CQSFA) path-integral formalism, we include all terms from the fine-structure Hamiltonian. This enables a semi-classical treatment of spin, which is the first of its kind in strong-field physics. We confirm the validity by comparing the non-relativistic model without spin to a TDSE code, with exceptional agreement. Then we are able to show that the most energetically rescattered electrons, undergo huge momentum transfer and briefly reach relativistic velocities. We probe these effects and show that they yield significant differences at 1600nm wavelengths. We also explore dynamical spin and spin-orbit coupling effects and find they are vastly over estimated, if the relativistic corrections are not included, otherwise the effects are quite subtle. We make a key step in accurate modeling of strong-field ionization at longer wavelengths and highlight important implication for imaging processes such a LIED or photoelectron holography.

Relativistic and Spin-Orbit Dynamics at Non-Relativistic Intensities in Strong-Field Ionization

June 08, 2023

Talk, APS Division of Atomic Molecular and Optical Physics (DAMOP) 2023, Virtual Attendance [In person meeting held in Spokane, Washington USA]

Spin is commonly ignored in strong-field physics, given that it is only through spin-orbit coupling for states with high orbital-angular momentum and with an elliptically polarized field that it has been shown to play a role, while in experiment, spin-resolved measurements have only fairly recently become possible. Thus, theoretical models treat spin only through coupling to initial states and generally neglect the spin dynamics. However, the trend for longer wavelengths, e.g. in imaging process such as laser induced electron diffraction (LIED), means that spin dynamics may play an important role, through high energy rescattering. We explore spin, spin-orbit coupling, and relativistic corrections to the kinetic energy by modifying the path-integral model, the Coulomb quantum-orbit strong-field approximation (CQSFA). Spin is included into the path-integral formalism and solved exactly, while the remaining system is solved via the semi-classical saddle point method. We confirm the validity of the CQSFA method by comparing the non-relativistic model without spin-orbit coupling to a non-relativistic TDSE code, with exceptional agreement. At 1600 nm wavelengths, there are differences in the photoelectron momentum distributions when comparing with and without spin-orbit coupling or relativistic corrections, which are most apparent in the high-energy region of the photoelectron momentum distributions and centre around rescattered electron wavepackets. We demonstrate that these recolliding electrons undergo a very large momentum transfer, which warrants a relativistic treatment, and leads to large spin-orbit coupling. We demonstrate that this has an impact on both the phase and amplitude of these wavepackets. These results are a key step in accurate modelling of strong-field ionization at longer wavelengths and highlight effects that may have an impact on imaging processes such as LIED or photoelectron holography.

Entanglement of orbital angular momentum in non-sequential double ionization

October 05, 2022

Invited Talk, Lund University, Lund University, Sweden

I will discuss our new results on entanglement and orbital angular momentum (OAM) in non-sequential double ionization (NSDI). We demonstrate that there is entanglement in the OAM between the two photoelectrons in NSDI. Due to the quantization of OAM, this entanglement is easily quantified and has a simple physical interpretation in terms of conservation laws. Using the strong-field approximation, we quantify the entanglement for a large range of parameters, isolating the best targets for experimentalists. We also explore efficient methods to quantify and measure the entanglement, in particular by using an entanglement witness. Importantly, the methodology presented here could be applied to many other systems to help understand and exploit entanglement in attosecond processes.

Ultrafast imaging of molecular chirality with photoelectron vortices

June 08, 2022

Talk, Bryant University, Bryant University, Rhode Island, United States

We present a new highly enantio-sensitive effect that exploits the helicity in twisted photoelectrons ionized from a chiral target, dubbed photoelectron vortex dichroism (PEVD), which we proposed as a new ultrafast chiral imaging technique.

Entanglement of Orbital Angular Momentum in Non-Sequential Double Ionization

May 25, 2022

Invited Talk, Kings College London, Kings College London, United Kingdom

I will discuss our new results on entanglement and orbital angular momentum (OAM) in non-sequential double ionization (NSDI). We demonstrate that there is entanglement in the OAM between the two photoelectrons in NSDI. Due to the quantization of OAM, this entanglement is easily quantified and has a simple physical interpretation in terms of conservation laws. Using the strong-field approximation, we quantify the entanglement for a large range of parameters, isolating the best targets for experimentalists. We also explore efficient methods to quantify and measure the entanglement, in particular by using an entanglement witness. Importantly, the methodology presented here could be applied to many other systems to help understand and exploit entanglement in attosecond processes.

Ultrafast imaging of molecular chirality with photoelectron vortices

April 08, 2022

Invited Talk, Edinburgh University, Edinburgh University, United Kingdom

Motivated by advances in electron vortex states, which carry orbital angular momentum (OAM), we exploit conserved helicity inherent in photoelectrons carrying OAM from a chiral target to propose a new ultrafast chiral imaging technique, dubbed photoelectron vortex dichroism (PEVD). We theoretically demonstrate huge asymmetry in OAM-resolved photoelectron emission, sensitive to molecular chirality, for electrons ionized by strong linearly polarized fields.

Twisted electrons in attosecond physics: Unravelling photoelectron orbital angular momentum in interference spirals, entanglement and chirality.

February 04, 2022

Invited Talk, Atto Fridays Conference Series, Virtual Seminar

Twisted electrons may carry orbital angular momentum (OAM) as free particles, which manifest as a vortex of rotating phase. Recent advances have allowed the generation and, importantly, measurement of such particles. In this talk, I will explore the properties and applications of these twisted continuum states in attoscience. In particular, I will present our work showing general conservation laws for the OAM of photoelectrons after ionization by a strong circularly or linearly polarized laser field, as well as a new interpretation for the interference spirals formed by two time-delayed circularly polarized fields. I will also present, our new results on entanglement and OAM in non-sequential double ionization (NSDI). Where we demonstrate that there is entanglement in the OAM between the two photoelectrons in NSDI. Due to the quantization of OAM, this entanglement may be simply understood through conservation laws. We also explore efficient methods to quantify and measure the entanglement, in particular by using an entanglement witness. Importantly, the methodology presented here could be applied to other systems to help understand and exploit entanglement in attosecond processes. Finally, I will discuss ongoing work on using OAM and helicity of photoelectrons removed via a strong laser field to detect enantiomers from chiral targets.

Quantum estimation in strong fields: in situ ponderomotive sensing

March 16, 2021

Talk, APS March Meeting 2021, Virtual Conference

We present the first derivation of quantum and classical Fisher information for attoscience to characterize the laser intensity uncertainty for in situ measurements in strong field ionization. We demonstrate that interference effects greatly reduce the laser intensity uncertainty and suggest how to minimize uncertainties in a previous experiment.

Manipulating Twisted Electrons in Strong-Field Ionization

February 03, 2021

Talk, Faraday Discussions – Time-resolved imaging of photo-induced dynamics Faraday Discussion, Virtual Conference

The discussion to complement the paper with the same title, examining the orbital angular momentum in strong field ionization with a view to applying it for time-resolved imaging.

Battle 3: Numerical vs Analytical Methods

July 03, 2020

Invited Talk, Quantum Battles in Attoscience, Virtual Conference

A panel discussion in a debate format on the pros and cons of ab-initio, numerical and analytical methods.