The oscillator strength of extended exciton states and possibility for very fast scintillators

Streak camera measurements of luminescence from ZnO single crystals as a function of temperature show that the spontaneous radiative lifetime of the “free exciton” band at 380 nm is 440 ps at room temperature, decreasing to 290 ps at 85 K. The donor-bound exciton lifetime is 50 ps from 85 K down to 16 K, the lowest temperature we measured. This is significantly faster than is consistent with the dipole matrix element of any single atom transition scaled to the same wavelength. We discuss the oscillator strength of extended exciton states based on the theory of defect-bound excitons, and examine its limit for free excitons. In that regard, we ask what physically limits the coherent dipole size and how a material might be engineered for “giant” exciton oscillator strength. The phenomenon applies to single excitons, not just coherently produced multiple excitations.