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

Cr2+ -and Fe2+-doped ZnSe crystals are laser materials with phonon-broadened absorption and emis-sion spectra, which provide broadband laser gain in the 2-to 5-& mu;m wavelength range. While Cr2+:ZnSe can be directly pumped with high-power Er, Ho, or Tm lasers, no such possibility exists for Fe2+:ZnSe. To this end, electronic excitation transfer between Cr2+ and Fe2+ 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 Cr2+ PL on a microsecond time scale, a 60-ns rise time of Fe2+ PL at high doping densities, and a prolonged decay of Fe2+ PL due to the temporal characteristics of excitation transfer over a range of interionic distances. Using a multi -rate 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 Cr2+ to Fe2+ 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 Fe2+ state. As nonradiative losses and excitation transfer show different temperature scaling, a cryogenic tem-perature regime is found that promises overall efficiencies above 50%, making Fe2+: Cr2+:ZnSe a much more viable alternative to parametric conversion schemes in the midinfrared range.