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Tuesday, November 4, 2025
2:00 pm – 3:00 pm
Presenter: Prof. David Masiello, Department of Chemistry, University of Washington
Alongside the recent proliferation of 2D quantum and nanophotonic materials comes the companion need to develop new measurement techniques for their characterization at their natural response scales in energy, length, momentum, and pseudoangular momentum (PAM). Advances in monochromation and aberration-correction technologies in scanning transmission electron microscopes (STEMs) now enable the investigation of low-energy excitations through inelastic free electron scattering approaching 1 meV, 0.01 Å-1, and 1 nm energy, momentum, and spatial resolutions, via electron energy-loss spectroscopy (EELS). The broad objective of this talk will be to highlight recent work conducted in my group on the theoretical design of new classes of delocalized and localized electron probes endowed with precise symmetries to additionally characterize the PAM-dependence of collective excitations such as phonons, exciton polaritons, and lattice plasmon polaritons in 2D periodic quantum and nanophotonic materials exhibiting chiral and topological character [1-4].
Specifically, this talk will address the theoretical development of phase and amplitude structured free electron probes tailored for reciprocal-space excitation mapping using momentum- and PAM-resolved inelastic electron scattering measurements [1] in both forward and reflection geometries typical of TEM and high-resolution EELS (HREELS) measurements [2]. New classes of widefield, phase-structured ‘pinwheel’ free electron states possessing well-defined PAM associated with C3, C4, and C6 discrete rotational symmetries will be introduced theoretically and incorporated within our momentum-resolved EEL (q-EEL) scattering theory. Leveraging this formalism from our recent work [1-4], I will first highlight a fully-retarded theoretical framework for describing the inelastic scattering of wide-field electron beams [3] from 2D materials, applying it to investigate the complementarity in sample excitation information gained in the q-EELS measurement from a honeycomb plasmon lattice versus from monolayer graphene. Second, I will show that deficiencies of typical q-EELS measurements that make it impossible to distinguish the PAM of chiral valley phonons in monolayer hexagonal atomic crystals can be overcome by introducing pinwheel free electron states with well-defined PAM. Going forward, other quantum and nanophotonic material systems hosting chiral phonons or polaritons with various composition, mode symmetry, and dimensionality could be characterized using generalizations of the proposed inelastic scattering measurements discussed [5].
References:
1. Marc R. Bourgeois et al., Strategy for Direct Detection of Chiral Phonons with Phase-Structured Free Electrons, Physical Review Letters 134, 026902 (2025)
2. Andrew W. Rossi et al., Probing the Polarization of Low-Energy Excitations in 2D Materials from Atomic Crystals to Nanophotonic Arrays using Momentum-Resolved Electron Energy Loss Spectroscopy, Nano Letters 24, 7748-7756 (2024)
3. Austin G. Nixon et al., Inelastic scattering of transversely structured free electrons from nanophotonic targets: Theory and computation, Physical Review A 109, 043502 (2024)
4. Marc R. Bourgeois et al., Optical Polarization Analogs in Inelastic Free Electron Scattering, Science Advances 9, eadj6038 (2023)
5. All work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES), Materials Sciences and Engineering Division, under Award No. DOE BES DE-SC0022921.
Bio:
Originally from New England, David J. Masiello went to college and graduate school at the University of Florida where he earned a B.S. degree in Mathematics in 1999 and a Ph.D. in Chemical Physics in 2004 under the advisement of Professor Yngve Öhrn at the Quantum Theory Project (QTP). As an undergraduate, David conducted research in nonlinear optics at the University of Bordeaux’s Centre de Physique Moléculaire Optique et Hertzienne (CPMOH) and in photothermal refractive glasses at the University of Central Florida’s Center for Research and Education in Optics and Lasers (CREOL). At the QTP, David’s dissertation work explored a nonperturbative treatment of the interaction between molecules and the electromagnetic field, accounting for the redistribution of energy not only between different internal molecular degrees of freedom but also for its liberation to the dynamical electromagnetic field. He then took two postdoctoral positions, one with Professor William P. Reinhardt at the University of Washington (2004-2006) and the second with Professor George C. Schatz at Northwestern University (2006-2009). Subsequently, David was hired back to the University of Washington in 2010 as an assistant professor in theoretical chemistry. In 2016 he was promoted to associate professor with tenure and in 2019 he was promoted to full professor and Bernard and Claudine Nist Endowed Scholar in Chemistry. Currently, Professor Masiello’s research focuses on the theoretical understanding of nanoscale light-matter interactions in nanophotonics, plasmonics, and quantum optics probed using advanced electron beam and optical spectromicroscopies. Particular emphasis is placed upon bringing insight and understanding to experiment through the formulation of simple yet rich theoretical models. David is also an adjunct professor of Applied Mathematics and Materials Science and Engineering at UW. David is a recipient of an NSF CAREER Award in 2013 and a Presidential Early Career Award (PECASE) awarded by President Obama in 2016.