Unambiguous determination of spin dephasing times in ZnO by time-resolved magneto-optical pump-probe experiments

Abstract

Time-resolved magneto-optics is a well-established optical pump probe technique to generate and to probe spin coherence in semiconductors. By this method, spin dephasing times T2 can easily be determined if their values are comparable to the available pump-probe-delays. If T2 exceeds the laser repetition time, however, resonant spin amplification (RSA) can equally be used to extract T2. We demonstrate that in ZnO these techniques have several tripping hazards resulting in deceptive values for T2 and show how to avoid them. We show that the temperature dependence of the amplitude ratio of two separate spin species can easily be misinterpreted as a strongly temperature dependent T2 of a single spin ensemble, while the two spin species have T2 values which are nearly independent of temperature. Additionally, consecutive pump pulses can significantly diminish the spin polarization, which remains from previous pump pulses. While this barely affects T2 values extracted from delay line scans, it results in seemingly shorter T2 values in RSA.