Efficient Electronic Excitation Transfer via Phonon-Assisted Dipole-Dipole Coupling in Fe2+:Cr2+:ZnSe

${\mathrm{Cr}}^{2+}$ - and ${\mathrm{Fe}}^{2+}$ -doped $\mathrm{Zn}\mathrm{Se}$ crystals are laser materials with phonon-broadened absorption and emission spectra, which provide broadband laser gain in the 2- to 5-$\text{\ensuremath{\mu}}\mathrm{m}$ wavelength range. While ${\mathrm{Cr}}^{2+}\text{:ZnSe}$ can be directly pumped with high-power Er, Ho, or Tm lasers, no such possibility exists for ${\mathrm{Fe}}^{2+}\text{:ZnSe}$. To this end, electronic excitation transfer between ${\mathrm{Cr}}^{2+}$ and ${\mathrm{Fe}}^{2+}$ ions in codoped ZnSe is investigated as an alternative excitation process in photo luminescence (PL) experiments with sub-10-ns temporal resolution. For a wide range of ion concentrations, we observe nonexponential decays of ${\mathrm{Cr}}^{2+}$ PL on a microsecond time scale, a 60-ns rise time of ${\mathrm{Fe}}^{2+}$ PL at high doping densities, and a prolonged decay of ${\mathrm{Fe}}^{2+}$ PL due to the temporal characteristics of excitation transfer over a range of interionic distances. Using a multirate equation model, the transfer process is analyzed on length scales up to 30 nm and compared to the established continuum model approach. The analysis reveals an unexpectedly efficient excitation transfer from ${\mathrm{Cr}}^{2+}$ to ${\mathrm{Fe}}^{2+}$ ions with an enhancement of the excitation transfer rates by up to a factor of 5 in comparison to resonant dipole-dipole coupling. The enhancement is assigned to (multi)phonon-assisted excitation transfer, in analogy to the phonon-mediated efficient radiationless decay of the excited ${\mathrm{Fe}}^{2+}$ state. As nonradiative losses and excitation transfer show different temperature scaling, a cryogenic temperature regime is found that promises overall efficiencies above 50%, making ${\mathrm{Fe}}^{2+}:{\mathrm{Cr}}^{2+}\text{:ZnSe}$ a much more viable alternative to parametric conversion schemes in the midinfrared range.