Observation of magnetic field sweep direction dependent dynamic nuclear polarization under periodic optical electron spin pumping

Abstract

Optical pump-probe techniques are used to generate and measure electron spin polarization in a gallium arsenide epilayer in which the electron spin coherence time exceeds the mode-locked laser repetition period. Resonant spin amplification occurs at magnetic fields at which the electron spin polarization excited by successive laser pulses add constructively. Measurements of Kerr rotation as a function of applied magnetic field reveal nuclear spin polarization that aligns either with or against the external magnetic field depending on whether the applied magnetic field is being decreased or increased. Furthermore, the nuclear spin polarization magnitude varies in proportion to the perpendicular net electron spin polarization as the latter changes due to resonant spin amplification and other causes. We also report an experimental technique that reveals a minutes-long memory of precise field history in the electron-nuclear spin system.

Dynamic Nuclear Polarization by optical Stark effect in periodically-pumped gallium arsenide

“…3(f), with pump laser wavelength 819.4 nm, we observe a prolonged rising edge, relative to the data in Fig. 3(d), with the same peak warping as our previous experiments [12]. However, for positive detuning, as shown in Fig.…”

“…We demonstrate experimentally that the model describes our physical system before showing that it reproduces the magnetic field sweep results from Ref. [12]. Finally, we give an intuitive explanation for understanding this sweep direction dependence.…”

“…We use our model to explore the hysteretic transient solutions which describe and qualitatively replicate the previously unexplained nonlinear magnetic field sweep dependent behavior in Ref. [12]. We justify this theoretical model by performing additional optical pump-probe experiments investigating the nuclear spin polarization dependence on pump wavelength and external magnetic sweep speed.…”

“…The Overhauser field can be extracted by performing field sweep scans at multiple time delays between pump and probe pulses [12]. This results in two-dimensional Kerr rotation data as a function of both time delay and external field.…”

“…This enhancement in electron spin polarization is periodic with respect to the transverse magnetic field amplitude. Measurements in fluorine-doped ZnSe [11] and GaAs [12] have both revealed a DNP that is periodic in magnetic field. The former was attributed to electron spins optically rotated onto the magnetic field axis via the optical Stark effect (OSE) [13], while the cause of the latter was not conclusively identified.…”