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Author ORCID Identifier


Open Access Dissertation

Document Type


Degree Name

Doctor of Philosophy (PhD)

Degree Program


Year Degree Awarded


Month Degree Awarded


First Advisor

Scott Hertel

Subject Categories

Condensed Matter Physics | Elementary Particles and Fields and String Theory | Instrumentation


Dark matter, one of the greatest mysteries in physics, continues to elude direct detection even after decades of effort. Physicists, in more recent years, are looking toward smaller mass ranges (sub-GeV), and a slew of new detector ideas have emerged. The HeRALD experiment seeks to use superfluid helium as a target to detect low-mass dark matter through the production of phonons and helium excimers as well as the resulting photons and quantum-evaporated helium atoms. HeRALD and many other experiments across particle physics use transition edge sensors (TESs) to detect small energy deposits - for HeRALD, such events are characteristic of a collision between a low-mass dark matter particle and a target He atom. Energy from impinging photons and He atoms are funneled to the TES through athermal phonons in a silicon substrate followed by quasiparticles (broken Cooper pairs) in thin-film superconducting aluminum fins. For all such calorimeters, energy efficiency is reduced by quasiparticles (QPs) becoming trapped by impurities in the Al. Additionally, immersed sensors could potentially lose QP energy to the surrounding superfluid He. In this thesis, I present studies of quasiparticle diffusion in superconducting Al fins using a laser-scanning microscopy-based technique. The characteristic QP trapping length (or diffusion length) is measured with the TES-fin device both in vacuum and immersed in superfluid He, to measure the QP energy lost to the superfluid. QP are produced at a localized origin in the Al film using a focused 1550nm laser coupled to a single-mode optical fiber mounted on piezoelectric nanopositioners. The resulting QP propagation is then monitored using a TES, and described using a simple 1D diffusion model. The measurements of 100µm-scale quasiparticle diffusion determine that the Al fins - fabricated at Argonne National Laboratory - work sufficiently well to be used in TES-based detectors. Additionally, no significant drop in QP collection efficiency due to device immersion in superfluid helium was measured. I conclude this thesis with the detailed design considerations for - and some preliminary testing results of - the upgrade to the HeRALD cryogenic photodetector (CPD), which attempts to eliminate the low energy excess background.


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Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

Available for download on Saturday, February 01, 2025