Bottom-up approach to scalable growth of molecules capable of optical cycling

TL;DR

This study demonstrates that increasing the size of hydrocarbon ligands attached to alkaline-earth-phenoxides does not significantly impair their optical cycling capabilities, supporting scalable molecular designs for laser cooling.

Contribution

It provides a systematic experimental investigation of ligand size effects on vibrational closure in molecules for optical cycling, suggesting scalability without loss of performance.

Findings

Varying ligand size from 1 to over 30 atoms maintains ~90% cycle closure.

Vibrational branching fractions are not systematically reduced by larger ligands.

Theoretical models indicate potential for maintaining optical cycling in larger systems.

Abstract

Gas-phase molecules capable of repeatable, narrow-band spontaneous photon scattering are prized for direct laser cooling and quantum state detection. Recently, large molecules incorporating phenyl rings have been shown to exhibit similar vibrational closure to the small molecules demonstrated so far, and it is not yet known if the high vibrational-mode density of even larger species will eventually compromise optical cycling. Here, we systematically increase the size of hydrocarbon ligands attached to single alkaline-earth-phenoxides from (-H) to -C$_{14}$H$_{19}$ while measuring the vibrational branching fractions of the optical transition. We find that varying the ligand size from 1 to more than 30 atoms does not systematically reduce the cycle closure, which remains around 90%. Theoretical extensions to larger diamondoids and bulk diamond surface suggest that alkaline earth phenoxides may maintain the desirable scattering behavior as the system size grows further, with no indication of an upper limit.