Unusual optical emission from microcrystals containingEu2+: Experiment

Experimental observations and inferred properties are reported for an optical emission near 368 nm which is unusual in that it occurs selectively from microcrystals of optical wavelength dimensions and is anomalously fast and efficient. The properties of principal interest are: (i) the emission exhibits a large quantum efficiency (>0.1) in microcrystals of optical wavelength dimensions, but becomes unobservable in crystals with significantly larger dimensions. (This requires a decrease in quantum efficiency of more than five orders of magnitude with increasing microcrystal size.) (ii) The emission lifetime (≲80 psec) and the emission quantum efficiency (>0.1 in the more efficient crystals) set a conservative lower limit on the emission oscillator strength (1.3±0.7) which is seven orders of magnitude greater than that expected on the basis of the assignment of the transition determining the spectral properties [${}_{}{}^6{}_{}{}^{}I-{}_{}{}^8{}_{}{}^{}S_{\frac{7}{2}}^{}{}_{}{}^{}$ ($4f^7-4\mathrm{}{f}^7$) transition in ${\mathrm{Eu}}^{2+}$]. (iii) The emission occurs from hosts providing a divalent cation site of $m3m$ symmetry (Ca${\mathrm{F}}_2$: Eu, Sr${\mathrm{F}}_2$: Eu, Ba${\mathrm{F}}_2$: Eu, Cd${\mathrm{F}}_2$: Eu, EuO, EuS, EuSe, and EuTe), but not from similar hosts not having cation sites which are both divalent and of $m3m$ symmetry (e.g., KCl: Eu, CaW${\mathrm{O}}_4$: Eu, Eu${\mathrm{Cl}}_2$). (iv) The emission spectrum appears sensitive to the microcrystal dimensions, exhibiting variations in relative component intensity and component frequency from one microcrystal to another, both below and above threshold for nonlinear behavior. (v) The emission quantum efficiency is large and the excited-state populations nonthermal, despite the presence of competing pathways for rapid nonradiative relaxation, both below and above threshold for nonlinear behavior. (vi) A threshold for nonlinear behavior is observed at ∼1 kW/${\mathrm{cm}}^2$ and a nonlinear competition between emission components is observed with increasing pump intensity neither of which can be ascribed to sample heating or laser action.