Fig. 2.

Making two-photon emission dominant. (A) Purcell spectra for a z-polarized dipole above 10-nm-thick cBN at atom-surface separations of 5, 10, and 25 nm. (B–D) Two-photon Purcell spectra for an s $\to$ s transition as a function of photon frequency $\omega$ for the same set of atom-surface separations. Blue denotes the Purcell spectra with losses accounted for, and orange denotes the Purcell spectra for cBN with 100× weaker losses. In B–D, we note both the overall two-photon transition rate between the 5s and 4s states of hydrogen and the corresponding radiative ratio (${\mathrm{\Gamma }}_R/\mathrm{\Gamma }$). The permittivity of the substrate is taken to be 2, and the damping constant for cBN is taken to be 5 cm⁻¹ (∼10¹² s⁻¹).