Effect of pump-probe detuning on the Faraday rotation and ellipticity signals of mode-locked spins in (In,Ga)As/GaAs quantum dots

Abstract

We have studied the Faraday rotation and ellipticity signals in ensembles of singly-charged (In,Ga)As/GaAs quantum dots by pump-probe spectroscopy. For degenerate pump and probe we observe that the Faraday rotation signal amplitude first grows with increasing the time separation between pump and probe before a decay is observed for large temporal separations. The temporal behavior of the ellipticity signal, on the other hand, is regular: its amplitude decays with the separation. By contrast, for detuned pump and probe the Faraday rotation and ellipticty signals both exhibit similar and conventional behavior. The experimental results are well described in the frame of a recently developed microscopic theory [Phys. Rev. B 80, 104436 (2009)]. The comparison between calculations and experimental data allows us to provide insight into the spectral dependence of the electron spin precession frequencies and extract the electron g factor dependence on energy.

Spin coherence generation in negatively charged self-assembled (In,Ga)As quantum dots by pumping excited trion states

“…The ellipticity signal intensity is maximum for resonant conditions and drops continuously with increasing separation between probe photon energy and probed resonance energy. 37 However, the behavior is more involved: The amplitude of the ellipticity signal in Fig. 5(a) does not drop continuously when moving the probe energy across the p shell to higher values, but it instead shows a maximum on the high energy flank of the s shell, and a dip at about 1.42 eV, followed by another maximum.…”

Generation and detection of mode-locked spin coherence in (In,Ga)As/GaAs quantum dots by laser pulses of long duration

“…It is also worth mentioning that the corresponding ellipticity traces look qualitatively similar to the Faraday rotation traces, even though there are quantitative differences concerning signal amplitudes and in particular the ratio of signals before and after pump pulse application. 8 In all cases a dephasing of the signal is seen on time scales of a few nanoseconds. There are two reasons for this dephasing: the excitation of an inhomogeneous spin ensemble with varying g factors and therefore also varying precession frequencies and the spin precession about the randomly oriented nuclear magnetic field in the ensemble.…”

“…8 Hence, the observed spin coherence signal should be averaged over the spin distribution. The result of this averaging depends strongly on the possible asymmetry of the spin distribution as well as on the details of spin coherence excitation and mode locking.…”

“…9(b) shows the theoretical result for S + z as a function of reduced magnetic field, calculated after Eq. (8). The electron spin z component value at the moment of pump pulse arrival is normalized by its value at L τ pump = 0.…”

Non-resonant optical excitation of mode-locked electron spin coherence in (In,Ga)As/GaAs quantum dot ensemble

“…The FR sensitivity has a dispersive profile with a width beyond the spectral width of the probe laser. 6 Figure 1 shows FR traces at B ¼ 1 T for different pump photon energies. The pump hits the sample at time zero, where it induces spin coherence, which subsequently precesses about the magnetic field, as seen from the lowest trace with the pump resonant to the probe energy.…”