Electric Field-Driven Coherent Spin Reorientation of Optically Generated Electron Spin Packets in InGaAs

Full electric-field control of spin orientations is one of the key tasks in semiconductor spintronics. We demonstrate that electric-field pulses can be utilized for phase-coherent $\pm \pi$ spin rotation of optically generated electron spin packets in InGaAs epilayers detected by time-resolved Faraday rotation. Through spin-orbit interaction, the electric-field pulses act as local magnetic field pulses. By the temporal control of the local magnetic field pulses, we can turn on and off electron spin precession and thereby rotate the spin direction into arbitrary orientations in a two-dimensional plane. Furthermore, we demonstrate a spin-echo-type spin drift experiment and find an unexpected partial spin rephasing, which is evident by a doubling of the spin dephasing time.

Figure 1

(a) Schematic setup. dc or pulsed $E$ fields are applied along the $[01\overline{1}]$ direction of an $n$-InGaAs transport channel. Electrically or optically generated spins are probed by static and TRFR in polar geometry. (b) $T_2^{*}$ vs dc $E$ field. (c) (upper panel) Schematics of total effective magnetic field ${\mathbf{B}}_{\mathrm{tot}}$ for positive and negative $B_{\mathrm{ext}}$. (lower panel) Asymmetric Hanle curve taken at $E=+5\frac{\mathrm{mV}}{\mu \mathrm{m}}$ and $T=30\mathrm{K}$. The green line is a fit to Eq. (1). (d) $B_{\mathrm{int}}$ determined from asymmetric Hanle signal vs dc $E$ field.

Figure 2

TRFR after optical spin orientation in InGaAs ($T=30\mathrm{K}$). Electron drift in an $E$ field induces $B_{\mathrm{int}}$, which results in spin precession about ${\mathbf{B}}_{\mathrm{tot}}={\mathbf{B}}_{\mathrm{int}}+{\mathbf{B}}_{\mathrm{ext}}$, shown for (a) $B_{\mathrm{ext}}=0\mathrm{mT}$ and (b) $B_{\mathrm{ext}}=15\mathrm{mT}$. TRFR scans are plotted in the lower parts at selected $E$ fields. An offset is added for clarity. The resulting parameters (c) $T_2^{*}$ and (d) $B_{\mathrm{int}}$ are in good agreement with the values extracted from Hanle measurements [Figs. 1b, 1d].

Figure 3

Spin manipulation by unipolar and bipolar $E$ field pulses. All $E$ field pulses start at $\Delta t=0\mathrm{ns}$. (a) TRFR measurements of optically created spin packets ($T=30\mathrm{K}$), which precess at $B_{\mathrm{ext}}=0$ during $E$ field pulses of different width $\delta w$, which is visualized in the lower panel. Spatiotemporal evolution of spin packet after spin manipulation by (b) unipolar and (c) bipolar pulse. The temporal spacing is 0.4 ns. The final spin distribution at $\Delta t=3.6\mathrm{ns}$ in (c) has been reversed and added as a dashed line in (b). (d) Spatiotemporal drift of spin packet after spin manipulation with unipolar (blue) and bipolar (red) pulses of equal pulse width and magnitude.

Figure 4

False color plots of TRFR measurements for spin manipulation by (a) unipolar and (b) bipolar $E$ field pulse with variable pulse width $\delta w$. The end of the pulses is marked by solid lines. (c) ${\theta }_F(\Delta t=\delta w+200\mathrm{ps})$ vs $\delta w$ from traces along dashed lines in (a) and (b). (d) $T_2^{*}$ taken from fits to curves in (c). $T_2^{*}$ values during unipolar pulses agree with results from dc $E$ fields [cf. to Fig. 2c], while $T_2^{*}$ after bipolar pulses shows partial rephasing.