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

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.