In recent years, the dark matter direct detection community has become increasingly interested in dark matter below the mass scale of the WIMP. Often called `low mass dark matter' or `light dark matter', this refers to a collection of models for fermionic dark matter with masses typically in the range eVGeV. Due to the slow relative velocity of the dark matter in the local halo, the dark matter is poorly kinematically matched to typical detector targets, leaving a detectable amount of energy that is orders of magnitude below the current state-of-the-art detector technology.

In this thesis, I will discuss the details of the athermal phonon sensor mediated detector technology used by the SuperCDMS and SPICE/HeRALD collaborations. I will motivate how this technology can be used to reach detector baseline energy thresholds of order meV. I then use these concepts in the design of the SPICE MELANGE detectors - the initial prototype dark matter detectors for the SPICE/HeRALD collaboration, with baseline energy resolutions expected to be sub-100 meV. I present the characterization from the testing of the initial fabrication on Si substrates. These Si versions are expected to be able to explore nuclear recoil dark matter parameter space for masses of order MeV - GeV. A future fabrication on Sapphire is planned, which will extend this mass range down easily into the keV range.

A major hurdle in the realization of ultra-low noise detector technology is the fact that the sensitivity to noise and backgrounds also increases. As such, much of this thesis is also dedicated to the characterization of ultra-sensitive cryogenic calorimeters and analysis of noise - from both intrinsic and environmental sources.