Franck-Condon Tuning of Optical Cycling Centers by Organic Functionalization

Laser induced electronic excitations that spontaneously emit photons and decay directly to the initial ground state (“optical cycling transitions”) are used in quantum information and precision measurement for state initialization and readout. To extend this primarily atomic technique to large, organic compounds, we theoretically investigate optical cycling of alkaline earth phenoxides and their functionalized derivatives. We find that optical cycle leakage due to wave function mismatch is low in these species, and can be further suppressed by using chemical substitution to boost the electron-withdrawing strength of the aromatic molecular ligand through resonance and induction effects. This provides a straightforward way to use chemical functional groups to construct optical cycling moieties for laser cooling, state preparation, and quantum measurement.

Figure 1

(a) Substitution positions investigated on the metal (Sr, Ca)-oxygen phenyl ring. (b) Global minimum structures and molecular orbitals (isosurface value of 0.03) for $\mathrm{Sr}{\mathrm{OC}}_6{\mathrm{H}}_5$ show the atomlike character of the electron distribution for the ground and first three excited states. The lack of electron density on the ligand suggests very little structural change will be involved in the electronic transitions.

Figure 2

The calculated vibrationless $\tilde{A}\to \tilde{X}$ Franck-Condon factors for substituted $M{\mathrm{OC}}_6{\mathrm{H}}_5$ derivatives show a strong correlation with the total of the Hammett $\sigma$ constants of their substituents. CaOR (SrOR) species are shown in blue (red). Solid curves are fits to Gaussians centered at the $x$ intercept of the bond length change vs Hammett total trends (Table 1).

Figure 3

Photon cycling scheme with an excitation (red) to the first excited electronic state ($\tilde{A}$), and decay (blue) to the ground electronic state ($\tilde{X}$). The FCFs are shown along with each decay. The rotational constants of these species are sufficiently large that individual rotational transitions will be spectroscopically resolved from the natural linewidth on $\tilde{A}\leftarrow \tilde{X}$.