Stéphane Kéna-Cohen
Polytechnique Montreal

 

Abstract

In this talk, we will discuss two recent observations from our group that have challenged widespread assumptions held (by us included!) about the optical response of commonly used optical materials: that material polarization can safely be considered to respond locally to the electric field and that the second-order nonlinear response of amorphous films should vanish due to centrosymmetry. In the first part of the talk, we will describe our proposal for a new type of optical antenna dubbed a "photonic gap antenna", and our realization of its extreme version where an epsilon-near-zero (ENZ) material is enclosed within the gap. Such antennas can provide electric field enhancements of >100 and large Purcell factors without requiring stringent nanofabrication. To our surprise, when measurement third harmonic generation as a proxy for field enhancement, sharp peaks emerge in the response that are completely absent in our full wave electromagnetic calculations. We find that the appearance of these peaks can only be explained when including nonlocality in the dielectric response of the ENZ material. Nonlocal simulations show that the volume averaged field enhancement can be 4–6 greater than that predicted by the local model, which becomes an important consideration when designing optical devices. In the second part of the talk, we will describe our recent discovery that amorphous thermally evaporated organic thin films of small molecules can have second-order optical nonlinearities on par with those of state-of-the-art nonlinear materials (c(2)31, c(2)33 >50 pm/V), with the important advantage that they can be deposited on arbitrary photonic platforms. We will show that by harnessing the interplay between the permanent dipole moment and surface energy minimization, it is possible to spontaneously break centrosymmetry during thermal evaporation, without the need for special alignment procedures. In addition to its applications in photonics, this observation has allowed us to better understand molecular alignment beyond the mean molecular orientation angle.

 

Biosketch

Stéphane Kéna-Cohen is a Full Professor of Engineering Physics at Polytechnique Montréal, where he heads the Light-Matter Group and is the Canada Research Chair in Light-Matter Photonics. His group works both on the development of advanced optoelectronic components and in better understanding and harnessing light-matter interaction in novel materials. His group is widely recognized for pioneering advances in understanding and exploiting the strong light-matter coupling regime in optical microcavities at room-temperature. Other recent achievements include the development of record efficiency near infrared organic light-emitting diodes, the first 2D material-based mid-infrared light-emitting diodes, the observation of superfluidity of light and the realization of photonic XY Hamiltonian lattices using polariton condensates. He obtained his PhD in 2010 as a Gordon Wu scholar at Princeton University under the supervision of Stephen R. Forrest and was a Junior Research Fellow at Imperial College London, working closely with Stefan A. Maier and Donal D.C. Bradley before joining Polytechnique Montréal.

Timbits, coffee, tea will be served in STI A before the colloquium.

 

 

Mark Chen
Queen's University

 

Abstract

In this colloquium, I’ll talk about liquid scintillator neutrino detectors. SNO+ is the Sudbury Neutrino Observatory (SNO) filled with liquid scintillator. I’ll present recent (preliminary) results from SNO+ including measurements of the oscillation of neutrinos from nuclear power reactors in Ontario and the detection of geoneutrinos. I’ll also describe the upcoming 3rd phase of SNO+ that will dissolve tellurium in the liquid scintillator to search for neutrinoless double beta decay, seeking to understand the origin of neutrino mass and their matter-antimatter properties. If time permits, I’ll also talk about some of my recent research on “opaque” liquid scintillators.

Timbits, coffee, tea will be served in STI A before the colloquium.

 

 

Benjamin Broerman
Queen's University

 

Abstract

Understanding the properties of dark matter is one of the most important goals of modern physics. Noble liquids, like argon, can be used as a target to search for dark matter over a wide range of masses from the GeV-scale through the TeV-scale. As a member of the Scintillating Bubble Chamber (SBC) collaboration, I am involved in developing liquid-noble bubble chambers sensitive to the sub-keV nuclear recoils expected from dark matter-nucleus scattering of GeV-scale dark matter. These detectors combine the excellent electron-recoil insensitivity inherent in bubble chambers with the ability to reconstruct energy based on the scintillation signal for further background suppression. The targeted nuclear recoil threshold of 100 eV is made possible by the high level of superheat attainable in noble liquids while remaining electron-recoil insensitive. In order to verify this reduced threshold, the SBC collaboration is building two functionally-identical 10 kg liquid argon detectors. The first, SBC-LAr10, to be operated at Fermilab, will be used for engineering and calibration studies. The second detector, SBC-SNOLAB, for a low-background dark matter search will be operated underground at SNOLAB. An overview of scintillating liquid-noble bubble chambers, the physics potential of SBC-SNOLAB, and new ideas for detector materials will be presented along with the physics reach of a proposed future, large volume dark matter search with liquid argon to cover masses through the TeV-scale.

 

Timbits, coffee, tea will be served in STI A before the colloquium.

 

 

Kim Berghaus
Caltech

 

Abstract

Dark matter comprises 85% of the matter density of our Universe, yet all of its known evidence lies in its gravitational imprint. One of the most important scientific goals of the next decade in fundamental physics is to reveal the nature of dark matter by measuring its non-gravitational interactions in the laboratory. To accomplish this goal, the experimental landscape must delve deep and search wide. Ensuring success requires overcoming emerging challenges in low threshold dark matter detection, such as quantifying how dark matter signals and backgrounds manifest in detectors. My research bridges the critical intersection of theoretical prediction and experimental verification of signals and backgrounds. As an example, I will discuss my work on quantifying the Migdal effect, a rare atomic ionization process that occurs when a neutron or dark matter scatters off a nucleus, in semiconductors, with relevance for experiments such as SENSEI and DAMIC. I will also discuss the role of phonon backgrounds from gamma-rays for sub-GeV dark matter experiments such as EDELWEISS and SuperCDMS. Lastly, I will show that phonons efficiently couple to paramagnetic qubits, opening up potential new pathways for single phonon detection. 

 

Timbits, coffee, tea will be served in STI A before the colloquium.

 

 

Michela Lai
University of California Riverside

 

Abstract

Since the 1930s, many astrophysical and cosmological observations have motivated the search for dark matter particles, building a wide variety of experiments specifically designed to perform their very first direct detection. Liquid argon is one of the most sensitive targets for GeV-scale candidates, such as Weakly Interacting Massive Particles (WIMPs), as demonstrated so far by the DEAP-3600 experiment and DarkSide-50 experiment, featuring respectively a 3.3 tonne single-phase and a 50 kg double-phase Time Projection Chamber design. In the meantime, unique R&D has led to the design of the first experiment within the Global Argon Dark Matter collaboration, DarkSide-20k, currently under construction at LNGS. Its 50-tonne ultra-pure argon target, together with the extraordinarily low background level, will allow for investigating for the very first time in argon dark matter-nucleon cross-section as low as 7.4 x 10^{-48} cm^2 for a WIMP mass of 1 TeV/c^2 in a 200 t yr run.

DarkSide-20k, and after it, ARGO, will give the ultimate answer to whether WIMPs exist and whether we can detect them. But there is more to investigate with noble liquids. Argon detectors are indeed not only the favorite for ultra-heavy dark mark candidates but are also at the very center of a multi-messenger physics program, including the detection of neutrinos released by core-collapse supernovae, by neutron star binary mergers as well as by the accretion disk on black holes. While extending the physics program of argon detectors, we will also investigate how we can push their sensitivity to sub-GeV dark matter candidates by just adding a few part-per-millions of xenon or other photosensitive dopants. If scaled to a tonne-scale experiment, this innovative technology will pave the way for searching unexplored dark matter candidates while opening a new window into our Universe.

 

Timbits, coffee, tea will be served in STI AUD before the colloquium.

 

 

 

Prof. Stefanie Czischek
University of Ottawa

 

Abstract

Large language models, like transformers, have recently demonstrated immense powers in text and image generation. This success is driven by the ability to capture long-range correlations between elements in a sequence. The same feature makes the transformer a powerful wavefunction ansatz that addresses the challenge of describing correlations in simulations of qubit systems. In this talk I consider two-dimensional Rydberg atom arrays to demonstrate that transformers reach higher accuracies than conventional recurrent neural networks for variational ground state searches. I further introduce large, patched transformer models, which consider a sequence of large atom patches, and show that this architecture significantly accelerates the simulations.

 

Timbits, coffee, tea will be served in STI A before the colloquium.

 

 

 

Dr. Ilsa Cooke (UBC),
Harrison-MacRae Lecture

 

Abstract

Dr. Ilsa Cooke will present "The unusual chemistry of interstellar molecules" to the Queen’s University community with funding from the Harrison-MacRae Lectureship in the Department of Chemistry.

Interstellar space is not empty, it contains a huge variety of molecules including some that have been found in space and never on Earth! In this talk, I will discuss how astrochemists find these molecules in space and what clues they can give us about other stars and planets. In addition, I will discuss how interstellar space provides an opportunity to study strange and exotic reactions that can defy chemical intuition.

About the Speaker

Dr. Ilsa Cooke is an Assistant Professor in the Department of Chemistry at the University of British Columbia. She leads the UBC Astrochem Lab, which studies how complex organic molecules form in the interstellar medium. Dr. Cooke is also a co-Principal Investigator with the GOTHAM collaboration and uses radio telescopes to probe aromatic molecules and their precursors in star-forming regions of space. Dr. Cooke was a lead researcher for the recent discovery the largest molecule ever detected by radioastronomy (and the third-largest in space!) published this fall in Science. She has won many awards for her research, most recently the Laboratory Astrophysics Division of the American Astronomical Society 2024 Early Career Award.