Year: 2026
Type: preprint
Abstract: Total internal reflection traps light inside high-index emitters, and in laser-cooled crystals the trapped fluorescence limits how far purification lowers the parasitic heat load. The symmetry of the crystal shape is known to govern this trapping. Here we show that the dielectric tensor supplies a second, intrinsic symmetry, and we unify the geometric and dielectric symmetries in a single conservation law. A fluorescence photon stays trapped when every facet operation conserves a wave-vector component that keeps it inside total internal reflection. Mirror facets of a cuboid conserve three components and trap 17.2% of all directions. Brewster end facets, or mode conversion between ordinary and extraordinary waves at facets containing the optic axis of uniaxial Yb:YLF, leave one component and reduce the trapped set to a universal shell of 3.45% of directions. Sub-degree misorientation of the optic axis, subnanometer facet roughness or an asymmetric cut releases this shell, and the release rate determines how strongly purification lowers parasitic loss. Released Yb:YLF crystals keep nearly linear purity leverage at 77 K and tolerate about ten times more cold background and up to 240 times more facet absorption than the same crystals with mode-conserving reflection. Birefringence releases trapped light on the smoothest facets, where surface absorption is lowest. Crystal symmetry, set jointly by shape and dielectric tensor, thereby becomes a design parameter for optical refrigeration toward liquid-nitrogen temperature and for uniaxial scintillators, light-emitting diodes and other facet-bounded emitters that trap light by total internal reflection.
Source: ChemRxiv
Authors: Daqian Bian, Yuzhi Xu
Institutions: University of Chicago, New York University
Language: English
Subfield: Electrical and Electronic Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7221066499