Traditionally, spin lifetimes are obtained from pump-probe Faraday rotation measurements by varying the pump-probe temporal delay with a conventional mechanical delay stage, which typically provides delays up to a few nanoseconds. Spin lifetimes may be difficult to obtain with this technique if the observed Faraday rotation experiences little decay over the accessible range of the delay line.

To measure spin lifetimes longer than the pulsed laser repetition rate, we exploit the fact that in our measurements the pump and probe pulses arrive at the sample at a fixed frequency established by the length of the Ti:sapphire laser cavity. When the magnetic field is adjusted so that an integral number of precession cycles occurs between successive pump pulses, the spin injection associated with each pump pulse reinforces the preceding spin polarization. The result is an amplification of the optically-excited magnetization and a corresponding increase in the Faraday rotation. Moving away from this "resonance field", the amplified spin response disappears, as shown below:

By scanning the magnetic field at a fixed pump-probe delay, we obtain a periodic series of resonances at fields for which all the successive pulses add constructively. This technique has proven useful in studying extremely long spin lifetimes in n-type GaAs crytstals. Shown below is the resonant Faraday response of a GaAs bulk crystal with a doping concentration of 1e16 electrons per cubic centimeter, taken at a temperature of 5 Kelvin.

The resonant peaks grow sharper and higher in amplitude as the field approaches zero. Qualitatively, this is due to longer lifetimes near B = 0, which require an increasing number of successive pump pulses to be in phase within the resonance peak. While non-pulsed optical pumping would yield a single resonance at zero magnetic field, the pulsed operation of the Ti:sapphire laser yields a sequence of resonances that give detailed information about transverse spin lifetimes over a wide range of fields.