Multi-level focal plane design for future high-density line intensity mapping experiments

Millimeter/sub-millimeter spectroscopy enables access to a vast cosmic volume through measurements of redshifted CII and CO rotation lines. This provides unprecedented opportunities to study large-scale structures and their evolution. As a recently advanced technology, the on-chip filter-bank spectrometer offers the advantage of reducing overall instrumental complexity. However, one challenge of the current design arises from the large footprint occupied by the filter bank, significantly diminishing pixel packing density on the telescope's focal plane. As the success of next-generation line intensity measurements relies on higher spectral resolution and improved array sensitivity, this presentation introduces a multi-level focal plane design aimed at substantially increasing pixel packing density. We build a prototype using feedhorn-coupled OMTs and optimize the design for typical filter bank geometries like those used for SPT-SLIM. The coupling between feedhorns and OMTs on different layers is simulated in COMSOL. We will present testing results of the prototype as a validation of the concept.